volume 2 issn: 1857-839x issue 2 december...

Volume 2Issue 2December 2013

ISSN: 1857-839X

Scientific Journal of Civil Engimeering, Volume 2, Issue 2, December 2013

EDITORIAL - Preface to the Second Issue of Volume 2 of the Scientific Journal of Civil Engineering (SJCE)

Darko Moslavac EDITOR – IN - CHIEF Dear Readers,

It is an honour to introduce the Second Issue of VOLUME 2 of the Scientific Journal of Civil Engineering (SJCE). Only very few professional and scientific journals are devoted to civil engineering issues in the region, so this is the reason why we are trying to devote at least two issues of SJCE to these questions yearly.

At the end of the second year of publication of our journal we are now able to think with justification about the realization of the ideas which led us to lunch the journal, the positive results achieved together with authors and readers, but also about new challenges for the future.

The second issue of volume 2 presents a special issue with selected articles from the latest several Congresses on Dams, organized by the Macedonian Association on Large Dams.

This issue includes 6 articles in the field of in-situ testing of dams, monitoring of dams, effects from construction of large dams and creation of water storage reservoirs on the environmental and social surroundings, numerical modelling of dam behaviour etc.

It’s our honour to publish several articles which are specially selected as

most interesting, from the number of quality articles published in the proceedings from the latest congresses on large dams. Special thanks to my colleague prof. Ljupco Petkovski who help me in the selection of the articles, and also special gratitude to all authors whose papers are published in this issue.

We continue to invite all researchers, practitioners and members of the academic community to contribute through their articles to the development and maintenance of the quality of the SJCE journal. We are particularly pleased to publish the results of research, best practice, case studies, ideas for solutions of complex problems, proposals of innovations and the results of experience on important projects.

Sincerely Yours,

Prof. Ph.D. Darko Moslavac

December, 2013

Impressum

FOUNDER AND PUBLISHER

Faculty of Civil Engineering -Skopje Partizanski odredi 24, 1000 Skopje

EDITORIAL OFFICE

Faculty of Civil Engineering -Skopje Partizanski odredi 24, 1000 Skopje Rep. of Macedonia tel. +389 2 3116 066; fax. +389 2 3118 834 Email: [email protected]

EDITOR IN CHIEF

Prof. Ph.D. Darko Moslavac University Ss. Cyril and Methodius Faculty of Civil Engineering -Skopje Partizanski odredi 24, 1000 Skopje Rep. of MACEDONIA tel. +389 71 368 372; Email: [email protected]

ISSN: 1857-839X

EDITORIAL BOARD

Prof. Ph.D. Darko Moslavac University Ss. Cyril and Methodius, Rep. of Macedonia Prof. dr. sc. İbrahim Gurer Gazi University, Turkey Prof. dr Miodrag Jovanovic University of Belgrade, Rep. of Serbia Em.O.Univ.Prof. Dipl.-Ing. Dr.h.c.mult. Dr.techn. Heinz Brandl Vienna University of Technology, Austria Prof. dr. sc. Zalika Črepinšek University of Ljubljana, Slovenia Prof.dr.ir. J.C. Walraven Delft University of Technology, Netherland univ.dipl.ing.gradb. Viktor Markelj University of Maribor, Slovenia PhD, Assoc. Prof. Jakob Likar University of Ljubljana, Slovenia PhD,PE,CE Davorin KOLIC ITA Croatia Prof. dr. sc. Stjepan Lakušić University of Zagreb, Croatia Marc Morell Institut des Sciences de l’Ingénieur de Montpellier, France Prof. Ph.D. Miloš Knežević University of Montenegro Prof. Ph.D. Milorad Jovanovski University Ss. Cyril and Methodius, Rep. of Macedonia Prof. Ph.D. Cvetanka Popovska University Ss. Cyril and Methodius, Rep. of Macedonia Prof. Ph.D. Ljupco Lazarov University Ss. Cyril and Methodius, Rep. of Macedonia Prof. Ph.D. Goran Markovski University Ss. Cyril and Methodius, Rep. of Macedonia

Prof. Ph.D. Zlatko Srbinovski University Ss. Cyril and Methodius, Rep. of Macedonia Prof. Ph.D. Radojka Donceva University Ss. Cyril and Methodius, Rep. of Macedonia ORDERING INFO

SJCE is published semiannually. All articles published in the journal have been reviewed. Edition: 200 copies SUBSCRIPTIONS Price of a single copy: for Macedonia (500 den); for abroad (10 EUR + shipping cost). BANKING DETAILS (MACEDONIA) Narodna banka na RM Account number: 160010421978815 Prihodno konto 723219 Programa 41

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IN-SITU TESTING OF DAMS IN CANADA - IZIIS EXPERIENCE

CONTENT

Tina Dašic, Branislav Djordjevic INCORPORATION OF WATER STORAGE RESERVOIRS INTO THE ENVIRONMENT 7

Lidija Krstevska, Ljubomir Tashkov, Mihail Garevski 17

Ljupcho Petkovski, Ljubomir Tanchev, Stevcho MitovskiCOMPARISON OF NUMERICAL MODELS ON RESEARCH OF STATE AT FIRSTIMPOUNDING OF A ROCKFILL DAM WITH AN ASPHALT CORE 27

Igor Planinc, Matija Gams, Nina Humar, Simon SchnablTHE EFFECT OF ACCUMULATION RESERVOIR ON THE DYNAMIC RESPONSE OF GRAVITY DAMS 37

53

Violeta MircevskaDAM - FLUID INTERACTION EFFECTS INDUCED 43

ADVANCED MONITORING OF ROMANIAN DAM PORTFOLIODan Stematiu,Adrian Popovici, Radu Sârghiuta, Altan Abdulamit, Catalin Popescu, Daniel Gaftoi

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SCIENTIFIC JOURNAL OF CIVIL ENGINEERING •Volume 2 •Issue 2•December 2013 Page

AUTHORS

Tina Dašić Ph.D. Assistant Professor University of Belgrade Faculty of Civil Engineering [email protected]

Branislav Djordjević Ph.D. Professor University of Belgrade Faculty of Civil Engineering [email protected]

INCORPORATION OF WATER STORAGE RESERVOIRS INTO THE ENVIRONMENT The construction of large dams and creation of water storage reservoirs have significant effect on the environmental and social surrounding. Although these impacts have been recognized in the first stage of modern dam construction period (period after Second World War), in the last decades they have become among the most debatable and for public opinion significant effects of the large dams. Water storage reservoirs are irreplaceable element of complex water resources systems, particularly having in mind climate changes - shorter and more intense rainy periods with floods threatening the man and his systems and long dry periods, when again, due to lack of water, whole ecosystem is endangered, as well as man. Reasons why water storage reservoirs are necessary are described in the paper. Main positive and negative impacts of reservoirs on the environment are analyzed and measures for neutralization, mitigation or compensation of negative impacts are summarized.

Keywords: large dams, water storage reservoirs, water as a resource, ecological aspects, social impacts, environment

INTRODUCTION In the Master Plans of all levels two space users have special priority: mines, especially ones with surface exploitation, and water resources systems. These users have very specific requirements for location, as their resources are located in specific and limited areas. So, the structures for their exploitation must be at those exact locations. If locations are not timely reserved and ensured for that specific utilization, they can be permanently lost. That is why these two users must prepare studies and designs, and precisely define areas necessary for realization of future systems.

In recent times water storage reservoirs became the object of contestation, supposedly

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SCIENTIFIC JOURNAL OF CIVIL ENGINEERING •Volume 2 •Issue 2•December 2013Page Page

Tina Dašic, Branislav Djordjevic

because they have negative impact on the surrounding area and the environment. This is an incorrect conclusion! The main purpose of this article is to emphasize next:

(a) Integral water resource systems, with water storage reservoirs as their important part, are the key element for regulation and protection of the area (water supply, sanitary of settlements, flood protection, improvement of river banks, protection of catchments areas, improvement of conditions for better urban prosperity of settlements, etc.),

(b) Water storage reservoir redistributes water in time in the best possible manner for the environment. By adequate management they can increase water regimes downstream from the dam or gate. That is especially important in the low flow periods. In that way the actual concept of water protection is realized: Conduction of water resources management to help eco-system and preserve biodiversity in the best possible way.

In the process of water storage evaluation a few facts should be taken into account.

(1) Development or stagnation of water resources directly influences the conditions and development of all other systems. Some countries found their way out form big economical crises in realization of complex water resources systems, especially realization of water storage reservoirs (American "New Deal", polders, water storage reservoirs in Spain, Iran, Turkey, China, South Africa, etc.). It is well known that investment in water resources and hydro energy cannot be miss investment, and those are investments that start up different industries of a country.

(2) In line with basic principles of maintenance development there is very strong connection between development and environmental protection. That is pointed out in a well-known document: The Report of the World Commission on Environment and Development (1987). Summarizing that document in only one sentence, would state: “Environment could not be properly protected without adequate economical development.” And economical development is not possible without adequate water resources systems, with water storage reservoirs as their key element.

(3) When analyzing the environmental impact of water storage reservoirs some important categories of impacts must be considered: soil as an area and resource, water as a resource

and biotope, air as an area that should be protected, pollution with solid waste, pollution with liquid waste, thermal pollution, noise as pollution of the air, radiation, impact on biocenose, aesthetic of landscape. All alternatives should be compared and evaluated, including the "do nothing" choice, which often has a negative impact on the environment (considering the stated 10 impacts). If all defined impact categories are analyzed, hydropower plants - as renewable and clean energy source - has an incomparable advantage over all other energy sources.

WHY ARE WATER STORAGE RESERVOIRS NECESSARY? There are numerous reasons why water storage reservoirs are a necessary element of water resource systems in Serbia. The most important ones are stated below:

Irregular temporal distribution of water flow in rivers

Lots of rivers in Serbia have torrent type flow. Very often discharge runs in short flood period, succeeded with long dry periods. Average annual flow of all domestic water is around 509 m3/s. In low flow period it decreases ten times and is around 50 m3/s. That is not enough even for ecological needs of water ecosystem.

Ratio between minimal monthly discharge with occurrence probability of 95% and maximal annual discharges with occurrence probability of 1% are often greater than 1:1000. Variation coefficients of annual discharges are rivers in Serbia), indicating variation of mean annual discharges, which are usually higher than 3:1.

Analysis of the coefficient of autocorrelation of annual discharges and spectral functions of those values indicates one unfavourable phenomena: accumulation of dry years joining in one long dry period, when discharges are very low on all rivers (catchments) and all water users are endangered as well as the rivers as ecosystems. Those extremely dry periods affect a wider region and without water storage reservoirs it would be impossible to provide water for normal human activities (settlements and economy). Those low flow periods are very dangerous for flora and fauna in the river.

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INCORPORATION OF WATER STORAGE RESERVOIRS INTO THE ENVIRONMENT

Only way to mitigate it is to discharge water from water storage reservoirs in the upstream part of the river.

Spatial irregularity of water flow

Water flows in Serbia are also characterized by extreme spatial irregularity. Specific water flows of domestic water vary in wide range, from only around 1

lowest specific flows are in the lowland areas with the highest density of population and with the fertile land which should be intensively irrigated. Taking all this into account it can be concluded that water storage reservoirs are only structures that can deal with temporal and spatial water irregularity. Without them it would be impossible to transfer water from the water source area to the consumer area.

Underground water mainly from river alluvium

Around 2/3 of underground water in Serbia is in river alluvium, meaning that quantity and quality of that water directly depends on the river flow. That is the reason why settlements providing water from wells in river alluvium have huge problems with water supply. Many settlements (Vranje, Kruševac, Kragujevac, Užice, Čačak, Aleksinac, Leskovac, etc.) changed their water supply system from groundwater to more reliable supply - from water storage reservoirs. Source of water supply system for Belgrade has changed similary. Water demand could not be satisfied by using wells near two big rivers, so the source was changed, and now it uses water from Sava lake - the special form of water storage reservoir.

Difference between water resource and water existing on the catchment area

There is an important difference between two categories of water: water existing on the catchment area and water that can be treated as a water resource. Unnoticing difference between those categories can lead to big mistakes in evaluation of water resources of some catchments areas or regions.

Water existing on the area/catchment (V) is exclusively geophysical category, and it can be defined as: V = L,Q,K, with matrix structure defining location (L), quantity (Q) and quality (K) of water [3].

Water resource (VR) is social, economic and ecological category, because beside previously mentioned three attributes it has to possess one more, very important one - existence of conditions for catching, utilization and protection of water (US). So, when defining the water resource matrix, the structure of "existing water" has to be enlarged with the conditions for utilization, and it can be defined by the relation:

VR = V,US (1)

Following the same system logic water demand on some area/catchment can be defined by the matrix Vz = Lz,Qz,Kz, where Lz - location where water is demanded, demanded quantity Qz and demanded water quality Kz. Now, planning of water resources systems can be presented by the logical structure S:

S: V VR VUS VS Uz , (2)

Relation (2) means that water existing on the catchment (defined by the matrix V) can be considered as a resource only after including utilization conditions (US). Through the appropriate water resource system (VS) and appropriate management (U) it can be transformed into matrix structure of water demand (Vz).

Conditions for water utilization (US) are of multidimensional structure, with lots of components on which the realization of water utilization depends. In each water resource alternative some conditions has to be analyzed: geotechnical conditions (GU), hydrotechnical conditions (HU), economic conditions (EU), conditions of interaction with social and urban environment (SU), interactions with cultural-historical and other properties (KU), conditions for ecological environmental protection (ZU) and conditions that results from international obligations (MU). US can be decomposed in following structure:

US = GU, HU, EU, SU, KU, ZU, MU (3)

Some parameters in equation (3) can be defined with appropriate quantitative or qualitative valuations. With those parameters it can be emphasized practicability, impracticability or practicability only under particular circumstances of water utilization on some area. If only one of the mentioned parameters in matrix (3) gets a value defining impracticability of the design for water utilization (for example GU=0, because

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karstified valley can't provide water tightness), whole design becomes impracticable because appropriate water resources system (VS), necessary for transformation from "existing water" in "water demand" (equation (2)) can't be achieved. In that case water existing on the area can't be considered as water resource and it shouldn't be accounted for future use. For all mentioned reasons real water resources are considerably lower than those estimated by analyzing water existing on the catchment area, or analytically:

VR << V (4)

Equation (4) is simple but fundamentally important. Fact that the quantity of water that can be defined as water resource is much lower than the water existing on the catchment area (sign <<) is the main reason for misunderstandings between the public and designers. Usualy, public opinion about water is much more optimistic. That is why public do not understand that key element for water utilization are water storage reservoirs.

Reliability of water delivery

Reliability of water delivery is the main reason why water storage reservoirs became such an important element in water supply systems. Expansion of settlements into the big urban centres requires the increase in reliability of water supply. Presently, reliability of water supply systems is required to be over 97%. Similar situation is in providing water for technological needs for heavy industry and thermoenergetics. Those are the reasons why water storage reservoirs are absolutely necessary: only water storage reservoirs with large relative volume can provide high reliability of water supply. Analyses performed with simulated synthetic series of flows [4] demonstrate one characteristic relation: for flow series with stochastic parameters close to those common for rivers in our country (variation coefficient Cv=0.5, relation between coefficients of asymmetry and variation Cs/Cv = 2, autocorrelation coefficient of annual flows r=0.3) if demanded relative water supply is = 0,7 (delivered quantity of water is 70% of average multiyear flow), then increase of reliability from P=80% to P=90% requires increase of water storage volume of 2.5 times! (Figure 1) For higher reliabilities situation is even more drastic. Enormous increase of necessary water storage volume is that high price must be paid for increased water supply reliability. However, reliability of around P=97% is the area of system saturation, and

further increase of reliability practically cannot be realized without system of reservoirs with multiyear regulation.

Figure 1. Relation between relative water storage volume β, relative water supply α and reliability P

Requirements in the area of flood protection are more severe

Requirements in the area of flood protection are very strict and often cannot be satisfied without active use of water storage reservoirs. Namely, modern flood protection systems require high protection level (for example protection from flood water with probability occurrence of 1%). Such demands can be satisfied only with combination of passive protection measures (linear systems - embankment and regulatory works) and active measures (mitigation of flood water in especially reserved areas (volumes) of multipurpose reservoirs). High level of protection for settlements with urban parts entirely defined in relation to the river cannot be accomplished without active retention effects of water storage reservoir. One of the examples is Leskovac, town reliably protected only after completing water storage reservoir Barje on Veternica River. Another example is town Skoplje whose high required protection (P 0,3%) was accomplished after completing water storage reservoir Kozjak at Treska River. Similar situation can be found in numerous towns in the world. High urban development was realized after managing flood water in the reservoirs in the catchment area. Sena River used to flood Paris (Figure 2)

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until those flood water were mitigated by the water storage reservoir.

Figure 2. Paris downtown (Rue de Lion) during flood in 1910. before realization of reservoirs in River Sena catchment area

Water quality protection

Water quality protection can be properly accomplished only by the following measures: technological, water resources and organization-economical measure. Water resource measure are very important protection measure as without them it is almost impossible to accomplish the required high quality levels in rivers, especially during the low flow periods. Those measures consider so called improvement of low flows, by discharging water during the low flow periods. During that period river biocenoses and ichtiofauna are endangered as a result of synergetic influence of few ecological factors each of them near its pessimum (simultaneous effects of low flow, high temperature and low concentration of dissolved oxygen in the water). In such conditions if water can be discharged from the water storage reservoir, water of high quality, desirable temperature, with high concentration of dissolved oxygen - water quality can be managed and maintained within the boundaries favourable for existence of river biocenoses. To accomplish this structures for discharge of guarantied ecological flow are built as a selective inlet, with possibility to take water from the layer with the most appropriate temperature (Figure 4). It is an active way to utilize thermal stratification of lake.

POSITIVE IMPACTS OF WATER STORAGE RESERVOIRS

From the previous part of the article a number of positive impacts can be perceived. They indicate that water storage reservoirs are irreplaceable structures for human survivor. As ecological impact of water storage reservoirs are often subject of discussion, their role from the ecological point of view will be discussed. It should be analyzed comprehensively, in time and space in which all their impact must be evaluated. Only the most important positive ecological impacts, important for water storage reservoir valuation are discussed.

- Healthy drinking water is provided, hydric epidemics are prevented - an important ecological impact.

- Production of hydro energy - ecologically the cleanest energy - decreases pollution with solid, liquid, gas, thermal and radioactive waste from alternative thermal and nuclear power plants (these should not be built at the expense of hydro power plants).

- Production of food is intensive in the irrigation condition. That is one of the most important ecological impacts. At the same time, ecological pressure on the land of lower quality is weaker and it can be used for reforestations and other purposes.

- Flood water flows are decreased and flood risk is smaller. Human population is free of fear of floods, but the environment is also protected from floods as the biggest ecological destruction.

- River flows increase in warm part of the year (improvement of low flows), when conditions for survival of biocenoses in the river are limited as a result of synergetic influence of low flows, high temperature, low concentration of dissolved oxygen.

- A water regime becomes managed: low flows can be increased and flood water decreased, with positive impact on ecological state downstream from reservoir. With better water regimes, regulation and organization of river banks settlements (earlier suffered from floods and low flows) can come down to the river banks and incorporate them in the urban city framework. Through the settlements river regulation should be done on the bases of natural regulations - one of the most important measures for harmonious

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incorporation and arrangement of river banks part of the settlements.

- Construction of water storage reservoirs are accompanied by anti erosion works, especially rehabilitation of erosion areas of I and II category (excessive and strong erosion). Biological protection measures are particularly important (reforestation, restoration of degraded forests, drainage of meadows, etc.) - ecologically important contribution to the areas eco-environment.

- Construction of water storage reservoirs are always followed by improved sanitary arrangement, sewage system and waste water treatment plants, to protect reservoir and river from process of eutrophication. Those water quality protection measures are financed from the dams and reservoirs designs.

- Finally, big water storage reservoirs create favourable conditions for tourism and recreational valorisation of the area.

N E G AT I V E I M P AC T S O F W AT E R S TO RAG E R E SE R VOI R S AN D M E AS U R E S FO R T H EI R M AI N T E N AN C E

Construction of each water storage reservoir is followed by some negative impacts. Most of those negative impacts could be maintained, mitigated or completely eliminated by adequate design solutions. The most important negative impacts are:

- Impact on riparian area as a result of changed groundwater regimes. That impact is specially seen at reservoirs on alluvial rivers, with low riparian area. It can be successfully neutralized by constructing suitable drainage system. Those systems become inseparable part of the area and enable managing groundwater regimes - maintenance of groundwater levels within defined boundaries appropriate for urban systems and agricultural production. Those systems can be of two purposes - drainage and irrigation, when negative impact transforms into the positive. It was performed in riparian area of HPP Đerdap, and the same principles of managing groundwater regimes is planned for riparian area of Velika Morava, Mačva and Semberia after construction of integral Systems on Morava and Drina River.

- Reservoir sedimentation as a result of disturbed regimes of deposit flow is negative impact that cannot be neutralized, but can be mitigated and maintained by adequate anti erosion works and selection of adequate discharge objects.

- Change of ecological factors can endanger survivor and development of some biocenoses in the backwater zone. Changed water regimes in backwater zones change living conditions for biocenoses in that zone. Conditions for development of reobionits - species adopted living in fast waters, change very unfavourable. Survival of those species can be provided if some parts of the river, out from the backwater zones, remain in their natural condition.

- Dams are barriers for fish migration. That negative impact can be successfully solved if special structures for fish migration are provided: for lower barriers - fish paths, and for higher - fish navigation lock and fish elevators. In some cases disturbance in fish reproduction can be solved by special spawning zones in backwater.

- Eutrophication of lakes is one of the most serious problems causing water quality degradation if protection measures are not implemented. Those negative impacts can be neutralized and controlled if control of inflow water quality is performed. There are mathematical models for predicting water quality. Those models, with appropriate investigation, can predict changes of water quality. That enables designer to make some changes in reservoir design and to predict adequate protection measures [3].

- Change of aesthetic values of some spatial natural characteristic. Some reservoirs, especially ones in deep gorges, after forming reservoir becomes different kind of biotope and also is experienced as different aesthetic ambience. That change can not be mitigated, and that is the most important problem facing construction of some very attractive water resources systems in canyon parts of some rivers (Tara, Morača, Studenica). But, that new aesthetic view is not unpleasant, further more for many people it is of special aesthetic value. That is the matter of personal experience of some elements in space. It can be demonstrated by the fact that the biggest problem after filling the reservoir is, most of the time, how to prevent building of settlements on its coastal areas.

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- Change of microclimate conditions in the narrow zone around reservoir is another impact of reservoir. Analyses performed in resent years in lots of countries indicate that microclimate changes are of much lesser degree than previously considered (thought). In the case of Studenica reservoir, for which the most detailed analysis were done, indicate that all impacts in the aspects of temperature and humidity were negligibly small and measurable only in the distance of 600 m to 800 m from the reservoir coast.

- Oscillations of reservoirs water levels have few negative impacts. One is aesthetic, because bare coasts in the backwater zones are unpleasant view. Second impact is ecological: changes of water levels can cause destruction of fish spawn laid in the shallow zones. Third is from the point of view of tourist and recreational usage of reservoir: lowering the water levels decrease possibility for that use. Those negative impacts can not be neutralized but can be mitigated if an additional criterion is implemented in management rules - criteria for maintain water level in some periods of the year (periods of fish spawning, summer period when reservoir is used for recreation and tourism). Furthermore, in lots of cases, especially in the case of reservoirs for hydro energy generation, reservoirs are full and levels are stable during that period of the year.

- Changed water regimes downstream from the dam and its impact on biocenoses is another important impact. It can be neutralized by designing adequate guarantied ecological flow. Methodology for defining guarantied ecological flow in Serbia exist [9]. According to that methodology downstream parts of river are permanently maintained in the state needed for undisturbed development of aquatic ecosystem. During periods of year intentional additional discharge from reservoir may create better conditions than it would be in natural conditions (without reservoir).

DESIGN MEASURES FOR INTEGRATION (FITTING) OF RESERVOIRS IN THE ENVIRONMENT

From the master planning point of view wider question is asked: can water storage reservoirs be harmoniously incorporated in social and ecological environment with adequate design and management measures?

Answer is affirmative and some of the measures will be tabled.

- Reservoir parameters, especially water levels, should be defined in line with ecological criteria, considering behaviour of the reservoir as a biotope in the period of exploitation. Dispositions with wide shallow zones should be avoided, because such reservoirs are very prone (likely) to develop of submerged plants and intensify eutrophication processes in the lake.

- Design of all infrastructures of the system (dam, intakes, valves, powerhouse, etc.) should be architecturally implemented and horticulturally arranged in such a way that they fit as harmoniously as possible into the environment. At rivers with special ambiental values all those structures, except dam, can be placed under ground. Example of harmonious integration of dam into the ambience is Marathon dam made for water supply of Athens (Figure 3).

- Excavations and borrow pits should be subsequently submerged, or, if impossible, these areas should later on be shaped and "cured" by biological measures, or even used for improvement of ambient values.

- Each water resource design has to be accompanied by detail ichthyologic studies. Those studies should define if there is a need to incorporate structures for providing fish migration (fish paths, navigation locks and elevators). Water storage reservoirs are new aquatic biotope in which the desired development of fish population can be realized by anthropologically guided successions. In line with this fact all activities on stocking reservoir with fish and disposition of structures for fish protection should be planned.

Figure 3. Marathon dam for water supply of Athens

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- Dynamism of first filling of the reservoir should be planned and performed in line with ecological demands. Submerged areas of the reservoir should be carefully cleaned just before the filling to avoid unfavourable effects on eutrophication process.

- The design of outlets (capacity, number of inlets, elevation, etc.) should respond to ecological requirements. To provide the best quality of guarantied ecological flow - structures for discharging that flow should be designed as a selective intake

structure with possibility to manage quantity and quality of discharged water. Discharged water should be accommodated to needs of downstream biocenoses (discharge from adequate water level, the one most appropriate for the specific development phase of downstream biocenoses, Figure 4). Valves have to be of regulatory type to enable management of water flow. Dispositions and types should also envisage the best possible aeration of the stream (the best are Howler-Bunger valves, Figure 5) to enable controlling the oxygen content in the water.

Figure 4. Discharge of guarantied ecological flow from adequate water level

Figure 5. Howler-Bunger valve

- Bottom outlets should be of high discharge capacity to allow pre-emptying of the reservoir for efficient mitigation of flood waves.

- Groundwater levels in the affected land should be controlled by drainage systems. Those systems should be designed as management systems to enable better water regimes comparing to those in natural conditions. Those systems should also be adapted to water resources and ecological objectives (irrigation, touristic valorisation of the area). Good example of such system is design of Srebreno jezero on Danube River, as a part of protection of riparian area of HPP Djerdap, who become popular touristic and recreational centre thanks to managed water regimes. Systems for protection of riparian area should be designed as multipurpose systems, so they can be used for drainage as

well as for irrigation, control of salt regimes, etc.

- Antierosion protection of reservoir should be considered as wider measure of catchment area cultivation. Especially important are biological measures (afforestation, drainage of pastures). It should be treated not only as ecological parameter, but also as stabilizing economical parameter for survivor of people on catchment areas with low soil quality.

- Water levels management should be adapted to ecological and touristic requirements. For example maintaining stable water levels in periods of fish spawning to prevent destruction of fish spawn laid in shallow zones, or maintaining stable water levels during the summer in the reservoirs with touristic utilization.

- All biological intervention in the system (afforestation, stocking reservoir with fish) should be done only after detailed ecological studies, to prevent destabilization of some already established ecological balance.

- Guarantied ecological flow should be defined in line with ecological requirements, considering it as dynamic category. It should be adaptable to development stage of biotope downstream from the dam (discharging more water in warm part of the year [9]).

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- To maintain reservoir water quality in the best possible level (state) quality of inflow water should be protected. Adequate observation of reservoir water quality, with mathematical models for water quality prediction enables predictions of processes of degrading water quality. In that case some measures for water quality protection could be undertaken

- Envisage protective forest corridors, in the areas of new reservoirs, for the migration of animals and to provide safe cross over the water barrage.

- Hydraulic engineering structures in towns and settlements should be planned especially carefully from the viewpoint of the harmonious functional and aesthetical incorporation into the urban framework. Building of reservoirs in the settlements area should be utilized for harmonious connection of settlements and water body (examples are some parts of Belgrade, which are urbanisticaly adequate connected with Sava and Danube - those rivers are part of Djerdap reservoir, Kladovo, Gloubac, Bečej in central parts of those towns).

CONCLUSION

Summarizing water resources, economic, social and other aspects of water storage reservoirs it can be concluded that there is unambiguously clear answer to question whether they should be built. They have to be built because the economic and social prosperity and even the survivor of civilization depend on water storage reservoirs. The main question to be asked is: what protection measures should be implemented to harmoniously incorporate water storage reservoir into the environment. Harmonious integration of reservoirs into the environment is not technical matter. It was pointed out that technically the majority of negative impacts can be neutralized, mitigated or compensated, and some of the other components of eco-system (environment), in process of building water storage reservoirs, significantly improved. Criteria for developing optimal solution must be extended. Optimization of integral solutions must be performed, with complex structure of objectives, in which the technical solution is reached through defining sets of social, economical, ecological, urban and other objectives, criteria and constraints. Future water resource systems should be built only as a part of integral systems, meaning

complex solutions optimally incorporated in requirements of other users of space.

REFERENCE

[1] S. Bruk: „Scientific Management of a Vital Resource“, Report of the IAHS, UNESCO, Paris, 1990.

[2] S. Dyck: “Integrated Planning and Management of Water Resources”, IHP, UNESCO, Paris, 1990.

[3] T. Dašić, B. Đorđević: “Mathematical modeling of ecological processes in reservoirs of hydroenergetic systems” (in Serbian), Journal “Elektroprivreda”, 2009/2, 2009, pp. 38-51

[4] B. Đorđević: “Water resources systems” (in Serbian), Naučna knjiga, Beograd, 1990.

[5] B. Djordjević: Cybernetics in Water Resources Management. WRP, Fort Colins, Co, USA, 1993.

[6] B. Đorđević: “On the economic evaluation of water and water activities” (in Serbian), Journal “Vodoprivreda”, Beograd, 141-146, 1993.

[7] B. Đorđević: “Principles, criteria and constraints for the planning of flood protection systems” (in Serbian), Journal “Vodoprivreda”, 158, Beograd, 1995, pp.211-222

[8] B. Đorđević: “Key ecological principles - essential for planning water resources systems” (in Serbian), Journal “Vodoprivreda”, 175-176, 1998.

[9] B. Đorđević, T. Dašić: “Guaranteed flows downstream of hydropower plants” (in Serbian), Journal “Elektroprivreda”, Beograd, 2007, pp. 3-12

[10] M. Felkenmark, G. Lindh: “How can we cope with the Water Resources Situation in the Year 2015?”, Ambio, 3 (4), 1984.

[11] V. Klemeš: “The Science of Hydrology: Where have we been? Where should we be going? What do hydrologists need to know?”, Commemorative Symposium "25 Years of IHD/IHP", Paris, 1990.

[12] A. T. McDonald, D. Kay: “Water Resources: Issues and Strategies”, John Wiley & Sons, New York, 1988

[13] T. Milanović (Dašić), B. Đorđević B.: “Dynamical processes in the reservoirs and their modeling for the planning and operation of water resources systems” (in Serbian), Journal ”Vodoprivreda”, 1999, pp.1-6.

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[14] I. Orloci, K. Szesztay, K. Varkonyi: “National Infrastuctures in the Field of Water Resources”, IHP-UNESCO, Paris, 1985.

[15] P. J. Parsons: “Methods of Projection of Water Use Industrial and Public Water Demand”, In: Long-term Planning of Water Management, United Nation, ECE /Water/15, 1976.

[16] United Nations: “The United Nations Water Conference”, Mar del Plata, Argentina, 1977.

[17] WMO: “Water Resources and Climatic Change”, WMO / TD, 247, 1987.

[18] V. Yevjevich: “Tendencies in Hydrology Research and Its Applications for 21st Century”, Water Resources Management, 1991, pp.1-23

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SCIENTIFIC JOURNAL OF CIVIL ENGINEERING •Volume 2 •Issue 2•December 2013 PagePage Page

AUTHORS

Lidija Krstevska Ph.D. Professor University Ss. Cyril and Methodius IZIIS - Skopje [email protected]

Ljubomir Tashkov Ph.D. Professor University Ss. Cyril and Methodius IZIIS - Skopje [email protected]

Mihail Garevski Ph.D. Professor University Ss. Cyril and Methodius IZIIS - Skopje [email protected]

IN-SITU TESTING OF DAMS IN CANADA - IZIIS EXPERIENCE In the last 6 years three dams have been tested in Quebec, Canada by ambient vibration testing method. The testing was a part of complex activities within cooperative projects between Hydro-Quebec, Quebec, Canada and the Institute of Earthquake Engineering and Engineering Seismology - UKIM-IZIIS, Skopje, Republic of Macedonia. The main objective of testing was to obtain their dynamic characteristics. The recorded ambient vibrations were post-processed and analysed by ARTeMIS software. The resonant frequencies for all dams were well expressed. The mode shapes showed clearly the way of deformation and the locations where the max deformations can be expected. The obtained experimental results have been further used for verification and improving of the numerical models for non-linear analyses and evaluation of the seismic stability of the dams.

Keywords: Natural frequencies, mode shapes, damping, ambient vibrations, dams.

INTRODUCTION Within the cooperative projects realized in the period 2006-2010 between the Institute of Earthquake Engineering and Engineering Seismology (IZIIS), "Ss. Cyril and Methodius" University, Skopje, Republic of Macedonia and Hydro-Quebec, Quebec, Canada, three dams have been tested applying ambient vibration testing method in order to define their dynamic characteristics: natural frequencies, mode shapes and damping coefficients. The obtained experimental results have been further used for verification and improving of the numerical models for non-linear analyses and evaluation of the seismic stability of these structures.

TESTED DAMS In-situ testing applying ambient vibration testing method for definition of the structural dynamic characteristics was performed on three concrete gravity dams in Quebec:

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Lidija Krstevska, Ljubomir Tashkov, Mihail Garevski

1. The dam of Beauharnois power plant, Fig. 1;

2. The dam of La Tuque power plant, Fig. 2;

3. Sartigan Dam in St. George, Fig.3.

The testing equipment applied for the measurement of La Tuque dam consisted of Ranger type seismometers and signal conditioning system produced by Kinemetrics, USA. High-speed data acquisition system and special software for on-line data processing and plotting the time histories and Fourier amplitude spectra of the response at any recorded point was used, while for post-

processing and analysis of the recorded vibrations at all the measuring points ARTeMIS software was used.

In case of Sartigan dam and Beauharnois dam Kinemetrix equipment, consisting of two three-axial force-balance accelerometers (epi-sensors ES-T, acceleration range 0.25g) and 12 channels digital recorder Model Granite were used.

Post-processing and analysis of the results was performed by ARTeMIS software.

Figure 1. The dam of Beauharnois power plant and its evacuation part, down-stream part

Figure 2. The dam of La Tuque power plant

Figure 3. Sartigan dam near St. George

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EXPERIMENTAL RESULTS Dynamic in-situ testing of evacuation part of Beauharnais powerhouse

The Beauharnais power plant is located on St. Lawrence River near Montreal and it is one of the biggest power plants in the world. It harnesses the powerful waters of the St. Lawrence to drive 38 generating units that are spread out over nearly one kilometer, Fig. 1. Experimental testing of a selected part of Beauharnais powerhouse evacuation part was carried out at the end of August 2010 by

ambient vibration testing method. The main objective of the measurements was to define the dynamic characteristics - natural frequencies, mode shapes and damping coefficients for the selected part of the structure measuring the micro-vibrations at selected points. The measurements were performed in three orthogonal directions in selected points of the evacuation part of the powerhouse. Five profiles were selected, as presented in Fig. 4. The total number of measured points was 69, both on up-stream and down-stream part, including the reference point R on the highest level. Presentation of the test set-up is given in Fig. 5.

Figure 4. Measured profiles - cross sections and generated geometry by Artemis software

Figure 5. Measured points on the selected part of the structure – spatial presentation

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The peak picking of the dominant frequencies is presented in Fig. 6, while the values of these frequencies and the damping coefficients are given in Table 1. In Fig. 7 presented is the shape of vibration at a

frequency of 9.47 Hz. It was evident that at some frequencies the upper part (up-stream side) vibrates more intensively than the lower part (down-stream side), f=9.96Hz and f=16.2 Hz.

Figure 6. Peak-picking of the dominant frequencies, evacuation part of Beauharnois powerhouse

Table 1. Dominant frequencies and damping

Frequency [Hz] Damping coef. (%) 1.56 3.6 3.12 2.06 4.98 1.1 6.34 1.05 7.91 0.72 9.47 0.49 9.96 1.4 16.2 0.35 18.95 0.25

Considering the mode shapes of vibration, it is obvious that the upstream part and the downstream part are vibrating at some frequencies more intensively, in phase and out of phase, in both longitudinal and transversal direction. The most characteristic shape for the upper part is expressed for the frequencies of 9.47Hz, 9.96Hz and 16.2 Hz.

Figure 7. Shape of vibration at a frequency f=9.47 Hz, evacuation part of Beauharnois powerhouse

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Dynamic in-situ testing of La Tuque dam

Centrale de La Tuque is located on St. Maurice River. The measurements were performed on the machinery building (A) and on the dam. The dam is composed of four parts - right part (B), evacuation part (C), water-intake part (D) and left part (E), Fig. 8.

The measurements on the dam were performed in two directions. On the evacuation part of the dam measured were transversal and vertical vibrations at 22 points - on the columns and on the bridge deck. For the water in-take part of the dam only transversal vibrations were measured in 8 points along the length. Presented schematically on Fig. 9 are the measured points together with the reference one.

Figure 8. Plan of Centrale de La Tuque

Figure 9. Measuring points on La Tuque dam

The peak-picking of the dominant frequancies is presented in Fig. 10, as well as in Table 2. The resonant frequencies of the massive dam body are 7.52 Hz, 12.99 Hz and 16.89 Hz,

while the frequencies of the evacuation part are 5.08 Hz, 11.04 Hz and 13.87 Hz.

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The mode shape at a frequency of 7.52 Hz is given on Fig. 11.

Figure 10. Peak-picking of the dominant frequencies, La Tuque dam

Table 2. Dominant frequencies and damping coefficients

frequency [Hz] damping (%) 1.855 8.6 3.711 7.3 5.078 2.5 7.52 4.3 11.04 0.8

Figure 11. Mode shape of vibration of the dam at a frequency of 7.52 Hz

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Dynamic in-situ testing of Sartigan dam

The Sartigan dam is located on Chaudiere River, near St. George in Quebec, Fig. 12. It is a concrete gravity dam composed of 4 parts -

left part (A), closure part (B), evacuation part (C) and right part (D). Along the dam there are several construction joints. During the experimental testing of the dam 52 tests have been performed, including the dynamic calibration tests.

Figure 12. Sartigan dam

Test set-up is presented in Fig. 13. The measurements were performed in 49 point, out of which 29 at the level of the dam crest (including 15 points on the bridge deck) and

20 at selected level on the columns. Time duration of each particular record (test) was 300 seconds and the sampling frequency was 200 samples/sec.

Figure 13. Distribution of measured points on Sartigan dam

The dominant frequencies of the dam in transversal and in longitudinal direction are presented in Figs. 14 and 15, as well as in Tables 3 and 4, respectively, while characteristic mode shapes are given in Fig.

16. Considering the obtained mode shapes, some discontinuity can be noticed, probably because of the presence of the vertical joints along the dam.

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Figure 14. Peak-picking of the dominant frequencies, Sartigan dam, transversal direction

Table 3. Dominant frequencies and damping coefficients in transversal direction

Frequency [Hz] [%] 13.5 7.2 20.5 9.4 25.9 - 27.5 - 36.7 -

Figure 15. Peak-picking of the dominant frequencies - Sartigan Dam, longitudinal direction

Table 4. Dominant frequencies and damping coefficients in longitudinal direction

Frequency [Hz] [%] 7.8 4.5 10.9 5.5 17.2 5.8 20.2 5.9 28.7 6.5

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Figure 16. Shape of vibration in transversal direction for frequencies f=20.5Hz and f=25.9Hz

CONCLUSIONS In the last several years experimental testing of three dams within power plants in Quebec, Canada have been performed by ambient vibration testing for definition of their dynamic characteristics: natural frequencies, mode shapes and damping coefficients. The obtained results have been further used for verification and improving of the mathematical models for non-linear sofisticated analysis of the dams and evaluation of their seismic stability.

REFERENCES [1] LJ. Taskov, L. Krstevska. (2007). “In situ testing of "Centrale de laTtuque", Canada,

by ambient vibration measurements”, Report IZIIS 2007-39

[2] L. Krstevska, LJ. Taskov, M. Garevski, V. Gocevski. (2010). "Experimental investigation of seismic stability of power plants in Canada", 9th US National and 10Th Canadian Conference on Earthquake Engineering: Reaching Beyond Borders, Proceedings, Paper ID 1285, Toronto, July 25-29, 2010

[3] L. Krstevska, M. Garevski. (2010). "In situ testing of Sartigan Dam, Canada, by ambient vibration measurements", Report IZIIS 2010-18.

[4] L. Krstevska, LJ. Taskov. (2010), “Ambient vibration testing of selected part of Beauharnois power plant evacuation system”, Report IZIIS 2010-43/1

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SCIENTIFIC JOURNAL OF CIVIL ENGINEERING • Volume 2 • Issue 1 • July 2013

Violeta Gjesovska

16 | Page


AUTHORS

Ljupcho Petkovski Ph.D. Professor University Ss. Cyril and Methodius Faculty of Civil Engineering - Skopje [email protected]

Ljubomir Tanchev Ph.D. Professor University Ss. Cyril and Methodius Faculty of Civil Engineering - Skopje [email protected]

Stevcho Mitovski MSc, Teaching Assistant University Ss. Cyril and Methodius Faculty of Civil Engineering - Skopje [email protected]

COMPARISON OF NUMERICAL MODELS ON RESEARCH OF STATE AT FIRST IMPOUNDING OF A ROCKFILL DAM WITH AN ASPHALT CORE

The response of a rockfill dam with an asphalt core on static loading state is complex issue, in highest part not describable by physical laws, but is estimated with various numerical models. Including of models instead of laws implies that for analysis of same dam type, the models (based on different approximations) are not mutually excluded, but in contrary, are contributing on better understanding of the prototype behaviour. The dam stress state at reservoir first impounding stage can be analysed by two numerical models. The basic difference between these models is the boundary condition on application of the hydrostatic pressure. The first model relies on the correspondence of the effective stresses from the displacements, approximated by superposition of three effects: softening of the submerged granular material, its alleviation and hydrostatic pressure on the core wall. The second model is based on the fact that effective stresses are difference of the total stresses and water pressure in the cavities of the rock material (according to hydrostatic laws). In this case, the change of the total stresses results from the additional load from increase of the volume weight and the external hydrostatic pressure along the upstream face, while the displacements are influenced by the rock material stiffness reduction due to decreasing of the effective stresses. Comparison of the two different numerical models on research of the stress state at embankment dams at reservoir first impounding stage is illustrated by results of the static analysis of dam Konsko in Republic of Macedonia – a rockfill dam with asphalt core with height of 80 m, currently in design stage.

Keywords: rock fill dams, asphalt core, effective stresses, displacements.

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INTRODUCTION – CRITICAL LOADING STATES AT EMBANKMENT DAMS Static stability of the upstream and downstream slope of embankment dams is assessed under critical loading conditions during construction and service stage. Critical loading states at earth and rockfill dams for the upstream slope are states after dam construction and rapid drawdown of the water level, while for the downstream slope, state after dam construction and at full reservoir. The shape of the potential sliding surfaces at rockfill dams cannot be predicted. In matter of fact, at these types of dams the slopes are stabile even if they are constructed with slope equal to the repose slope of the rock material, because there is no occurrence of pore pressure [1]. The safety of the thin water impermeable element of artificial material, and by that and safety of a rockfill dam as a whole, depends on the deformations. Therefore for estimation of the stability of rockfill dams, the deformations in the specified critical loading states are of crucial meaning.

The state after dam construction is short lasting state of loading and necessary safety coefficient of the slope is F=1.3. This state is especially important at earth and earth-rock dams, where analysis with effective stresses in time domain should be applied, respectively, by following the realistic loading chronology, while at rockfill dams because there is no occurrence of pore pressure, the time domain is irrelevant, thus a more simplified analysis is applied, using total stresses. During dam construction, the loadings from machinery for compaction, as well as the loads from the upper layers, presses the medium air-water, which is filling the pores. In that way, during construction of embankment dams pore pressure develops in the cohesive material. The pore pressure value during construction is variable, because parallel with growth of the pore pressure, there is also a process of its dissipation [2]. The development of the pore pressure at earth materials during construction depends of the following factors: (a) humidity at embedment, (b) compressibility, which is in dependence of the loading, (c) water permeability, (d) construction schedule and (e) drainage structure type.

The state of full reservoir has two typical phases: (1) state after rapid impounding and (2) state of long-term full reservoir. The first phase is short lasting loading state of the

structure with hydrostatic pressure along upstream face of the low permeable earth material (or water impermeable element of artificial material) and necessary slope safety coefficient is F=1.3. The second phase is long lasting loading state of the structure with established steady seepage flow through the earth material (or hydrostatic pressure at rock material) and necessary slope safety coefficient is F=1.5. The state of reservoir rapid impounding takes place before the seepage process through the earth cohesive material has started. Then the hydrostatic pressure from the upstream face acts as additional external load, that causes increase of the consolidation pore pressure, generated during dam construction. At long lasting of constant high headwater level, through the earth material (core of an earthfill dam and shell of an earth dam) it is established steady seepage flow. The material below the seepage line is fully saturated and exposed to seepage pore pressure. The pore pressure (h) at any point of the seepage zone is determined as difference of the potential (H) and elevation head (Z). The potential pressure is determined from the flow net equipotential lines, according to the interpolated hydrodynamic flow net. The state of full reservoir is especially important because it is pre-earthquake state with highest potential hazard on the downstream valley and it is used for confirmation of the dam seismic resistance [3, 4].

The state of rapid reservoir drawdown is short lasting loading state and accepted design criteria for the necessary safety coefficient is F=1.2÷1.3. Initial state for the stage of rapid emptying of the reservoir is stage of steady seepage, for which in the earth material was generated seepage pore pressure. Drawdown of the water level causes change in the pore pressure of the earth material. It should be noted that lowering of the reservoir level in period of few days to few weeks is rapid compared with the slow hydrodynamic process of water leaching from the saturated earth low permeable material. Similarly, as the stage during construction and after construction (before reservoir first impounding) occurs pore overpressure, that initiates consolidation process (manifested by decreasing of the pore pressure, increasing of the effective stresses and settlements). The difference of the consolidation processes at these two stages relies on the factor that causes the pore overpressure. During construction, the pore overpressure is caused by the loading of the upper layers (phase progressing of the embankment), while the change of the

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COMPARISON OF NUMERICAL MODELS ON RESEARCH OF STATE AT FIRSTIMPOUNDING OF A ROCKFILL DAM WITH AN ASPHALT CORE

boundary hydraulic conditions at the rapid drawdown in the reservoir are generating the pore overpressure.

CONTEMPORARY METHODS ON STATIC ANALYSIS OF EMBANKMENT DAMS

Methods on static analysis of stress-deformation state of embankment dams are divided on classical and contemporary (also the term “advanced” is used) [5]. According to the classical methods, started to develop at beginning of XX century, the dam stability is evaluated by slope safety coefficients against sliding (Ks). Methods of this group are called Limit Equilibrium Methods and their common lack is that they don’t calculate the stress-deformation state.

The contemporary methods are composed of structure and foundation modelling (discretization) and application of numerical methods of finite differences or finite elements. The development and application of the contemporary numerical methods dates from the seventh decade of the XX century, connected with informatics development and with the pioneer work in this area by Zienkiewicz and Clough. By application of the contemporary methods, satisfactory results of the stress-deformation state are obtained and the dam stability is estimated through stability factor (Fs), depending on the realized stresses. The improvement of the computation technology (hardware and software) towards end of the XX century, conditioned these methods more and more to supersede classical methods in application in the engineering practice. So, today the classical methods are actual from educational point of view and eventually at some preliminary analysis of embankment dams.

The essence of application of methods for estimation of the dam stability, consisting of determination of safety coefficient (Ks) or stability factor (Fs) against slope sliding is to calculate the minimum value of these coefficients. The sliding surface of certain dam loading state, for which is obtained the minimum value of KS or FS is called critical sliding surface. It should be noted that there is no exact methodology, by which geometry of the critical sliding surface will be determined. Therefore, great number of potential sliding surfaces are analysed (200÷300), for the both slopes and for each typical loading state. The choice of number, shape and geometric parameters of the potential sliding surfaces in

the engineering practice is still intuition problem for the designer of the embankment dam.

First attempts to solve the problem of determination of stress-deformation state of embankment dams by application of theory of elasticity and plasticity are made in the third decade of the XX century. These attempts had serious lacks due to the inability in the calculations to introduce the improper geometry, stage construction in layers and non-linear behaviour of local materials. Specified lacks are overcome by structure modelling (discretization with finite elements) and application of numerical (approximate) method of finite elements.

At prototype modelling is done physical approximation of the continuum that is being replaced by finite elements connected in nodes. From mathematical aspect, the application of finite element method comprises of numerical solving of the algebra equations. It is a great number of equations that can only be solved by use of calculation processors. Therefore, the appearance, development and application of finite element method at embankment dams emerges in same time with development of the computer technique. By analysis of the embankment dam with application of finite element method are determined stresses and deformations in the model, thus explaining the behaviour of prototype with complex geometry and heterogeneous composition.

SPECIFIC ASPECTS OF NUMERICAL MODELS FOR STATIC ANALYSIS OF EMBANKMENT DAMS The powerful numerical finite element method (or finite differences method) is a strong tool applied by engineers for solving of the tasks in the field of continuum mechanics. However, we should have in consideration that application of contemporary numerical methods has purpose to explain certain phenomena and processes and to define more precisely the solution, but the designer must take responsibility on results interpretation. If the results do not match with values obtained by engineering reasoning, then it should be discarded and the mistake should be detected in order to be eliminated due to its repetition. The mistakes in application of numerical models can arise due to: (a) improper input data, (b) not enough precise discretization of

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the real system, (c) non-adequate constitutive law and (d) numerical problem (if iteration procedure is applied).

At modelling of embankment dams and conveying of numerical experiments on structure response in critical loading states, a general rule should be followed – the task should be solved in several stages, by gradual increase of the model complexity. In the static analysis of embankment dams the application of this general rule would mean to start with model of initial stage (simplified geometry, more zones with similar material parameters should be replaced with one zone, linear constitutive law, analysis with total stresses). When the result from the initial model will be obtained, a transition to the following stage can be done, by gradual increase of the complexity, and the most important – by interpretation of the variations of the results, caused by each level of increased complexity. In that way a simulation model for behaviour of the embankment dam at the end stage can be done, composed of complex geometry, heterogeneous medium, non-linear plastic constitutive law and analysis with effective stresses [6].

The embankment dams are structures constructed in layers. The size of the cumulative settlements in the layers (state after dam construction) depends primarily on: (a) load of the upper layers, and (b) distance of the stiff non-deformable foundation [7]. If we analyse state after construction of embankment dam on rock foundation, then engineering reasoning refers on the fact that zero vertical displacements will appear at: (a) foundation, that is non- deformable, and (b) dam crest, due to non-existence of loading on the highest layer. It means that for this loading state of the structure, it is obvious that maximal cumulative settlements will appear at intermediate part of the structure and the necessity of modelling with finite element method is to define more precisely the isolines values and distribution of the vertical settlements. However, if by finite element method dam stage construction in layers is not simulated, but is simulated instantaneously construction (typical for numerous civil engineering structures), then it will obtained that maximal settlements appear at dam crest – not corresponding with the engineering reasoning.

Independently that on first glance, the water seems as simple element in the nature, the theory indicated, and the practice confirmed, that the most complex phenomena at

embankment dam behaviour are due to the water effect. For dam construction state, the water effect is actual only in case of zones of coherent earth material, so at rockfill dams for this state there is no structural problem influenced by this effect. Cohesive materials are embedded in layers at optimal humidity and by their compaction, the pores are fully filled with water. At loading increase (construction of upper layers) in first moment the full load is accepted by the pore water, and pore pressure develops. During time, by leaching of the pore water, its dissipation takes place. This slow hydrodynamic process of freeing of the pore pressure, followed by growth of the effective stresses and material settlement is called consolidation. Simulation of the development (raise and dissipation) of the pore pressure by application of the finite element method is carried out according to two approaches: analysis with total stresses and analysis with effective stresses [8]. According to first (simplified) approach, by application of total stresses or non-drained conditions, in low-permeable (cohesive) materials can be generated consolidation pore pressure, caused by change of the total stresses. The stress-deformation state and development (generation and dissipation) of the pore pressure, most realistically are determined by analysis with effective stresses that is when cohesive material is treated in drained conditions. In such a way, in structure response, according to the second approach, are included three components: (a) mechanic and elastic properties, (b) hydraulic properties (seepage coefficients and volume content of water), and (c) factor of time (schedule of dam construction).

At state of reservoir rapid impounding, the water effect in simplest way is manifested at embankment dams with facing of artificial material. It is a case when the water is outside of the dam body and acts as external pressure. The consequences of the water action in this case are: (a) increase of the normal stresses in upstream dam shell, thus conditioning growth of the material shear strength, contributing to dam stability, (b) in the dam body occurs horizontal displacements (directed downstream) and vertical displacements (maximal settlements in the intermediate part of facing and eventual raising with maximal value at downstream slope in the upper part), and (c) normal deflection in the facing (important for the estimation of the stability and dimensioning of the water impermeable element). Far more complex is the response of rockfill dams with

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internal core and earth-rock dams with cohesive core on the water effect. For the state of reservoir rapid impounding, the water rapidly fills the rock material cavities in the upstream dam shell and causes the following impacts: (a) softening of the submerged non-cohesive material in the upstream shell; as consequence of this additional settlements occurs, (b) alleviation of the permeable material (increase of the total stresses and pore pressure and decrease of the effective stresses), and (c) action of a force due to hydrostatic pressure along the upstream core face.

At full reservoir state, with established steady seepage flow through the earth material, the stress distribution depends on the following factors: dam composition and geometry, material parameters and dam type. At embankment dams stress transfer regularly appears. As a consequence, some elements are unloaded and other overloaded. If in certain zone the pore pressure value exceeds the value of the total normal stresses, a zone with negative effective pressure is obtained. It can cause hydraulic fracturing, usually manifested with occurrence of cracks in the cohesive material. The most common reason for occurrence of hydraulic fracturing is non-uniform settlement of material zones with different stiffness properties, where the softer material are “hanging” on the stiffer material (arch effect), thus transferring them a part of their stresses. Next potential reasons are violations and different slopes in the foundation of the cohesive material, that can cause non-uniform settlements and zone of bulk and unloaded material. According to Penman, the appearance of hydraulic fracturing at earth materials is possible if pore pressure is in the interval between maximal and minimal total normal stress σ3max < Pw < σ1max. According to other authors, on the occurrence of cracking of the cohesive material, beside the negative effective normal pressure, is necessary and adequate material non-homogeneity, referring on the statement that this phenomena is still not fully clarified.

ANALYSIS OF THE IMPOUNDING STATE AT ROCKFILL DAMS WITH AN ASPHALT CORE From the specified review on modelling of embankment dams under action of static loads it can be concluded that so far research on this issue introduced standard numerical models

applied for typical loading states for different embankment dam types. Only in the case of reservoir impounding of rockfill dams with a core made from artificial material, there are two approaches, whose advantages and lacks are analysed in the further text. The stress state of the dam at first impounding stage can be analysed by two numerical models. The basic difference of these models is the boundary condition on application of the force of hydrostatic pressure. The comparison of the different numerical models on research of the stress state at rockfill dams at first impounding stage is illustrated with the results of the static analysis of dam Konsko (Fig. 1) in Republic Macedonia, a rockfill dam with asphaltic core with height of 80 m, elevation of rock foundation 470.0 m.a.s.l. and dam crest at 550.0 m.a.s.l., currently in design stage.

In the model no. 1, reservoir Konsko impounding up to normal water level is applied in three increments: up to elevation 496.0 m.a.s.l., 520.0 m.a.s.l. and 546.0 m.a.s.l. appropriately. The numerical analysis is done with application of the program Sofistik [9]. By the applied approach in this model, the effective stresses should be in correspondence with the displacements, obtained with superposition of three effects: softening of the granular material in the upstream shell, alleviation and force of hydrostatic pressure on the core. The first effect is simulated by reduction of the angle of internal friction (up to 1.5 degrees) and stiffness material parameters in the saturated zones, respectively with reduction of the elasticity modulus approximately for 20% and by application of additional load in 5% of the self weight of the zones above saturation line.

reservoir impounding are obtained with application of the second effect (alleviation simulated with Archimedes force directed upwards), but with reduced intensity and full horizontal hydrostatic pressure (the third effect), that leads to horizontal displacements along the asphalt core height, displayed on Fig. 3, and partial vertical displacements, displayed on Fig. 4.

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Figure 1. Cross section of dam Konsko; (1) asphalt core, (2) fine transition, (0-60) mm, (3) coarse transition, (0-250) mm, (4) rockfill, grains to 700 mm, (5) removed humus layer, (6) cofferdam filter from river gravel, (7) cofferdam clay facing, (8) river deposit, (9) protection of the downstream slope, (10) rip-rap for protection of the upstream slope, (11) rock foundation (12) grouting gallery.

Figure 2. Isolines of maximal main effective normal stresses σ`1 at reservoir impounding: [MPa], equidistance

0.1 MPa; (–) denotes compression

Figure 3. Horizontal displacements per asphalt core axis, at reservoir impounding; dimensions [mm], (at core

axis, maximal value of 134 mm at 60% of the dam height, in the crest value of 95 mm)

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Figure 4. Isolines of partial vertical displacements in dam cross section, at reservoir impounding; dimensions [mm], equidistance of 5 mm; (+) displacements downward (settlements) ; (–) displacements upwards (raising)

In the model no. 2 (Fig. 5), the impounding of reservoir Konsko is simulated in ten increments. The numerical analysis is done with application of program Sigma [10]. In this case, the change of the total stresses results from additional load of the increase of the volume weight and the external hydrostatic pressure along the upstream slope, and the displacements are influenced by rock material stiffness reduction due to reduction of the effective stresses. By this approach, the effective stresses (Fig. 6) are difference of the total stresses and water pressure (according on hydrostatic laws) in cavities of the submerged materials upstream of the core. The change in stresses results from the additional load by growth of the volume weight (from natural to saturated) and from material softening caused from elasticity modulus decrease at reduction of effective stresses (far bigger then value of 20% adopted in model no. 1).

The reason why in realistic case by saturation of the upstream shell (or by reduction of the effective stresses) does not occur rising

(elastic response), displayed on Fig. 7, is superposition of at least three effects: (a) increased stiffness at unloading, (b) downstream displacements under action of the basic load – hydrostatic pressure (Fig. 8), and (c) occurrence of so called “collapse settlement” (previously term “material softening” was used). The third effect is manifested by settlement of the coarse material after submersion with water due to reduction of its strength properties, crushing of the coarse edges and etc., phenomena intensively researched in the last two decades [11, 12, 13]. The reason why in the model no. 2 are obtained higher downstream horizontal displacements of the core, compared with the upstream slope of the dam, although the hydrostatic load is applied along the upstream slope, are the reduced effective stresses in the upstream slope that conditioned stiffness reduction of the saturated materials. This effect does not appear at rockfill dams with facing and therefore at these type of dam the maximal horizontal displacements (by application of the same model) are obtained at the upstream dam slope.

Distance [m]100 150 200 250 300 350 400 450

Ele

vatio

n [m

]

450460470480490500510520530540550560

Figure 5. Model no. 2, discretization of dam Konsko by 534 elements and 588 nodes, first impounding at

elevation 546 m.a.s.l.

33



200

200

400

400

600

600

800

1000 1200

Distance [m]100 150 200 250 300 350 400 450

Ele

vatio

n [m

]

450460470480490500510520530540550560

Figure 6. voir impounding up to level 546 m.a.s.l., value from 21.8

to 1,545.0 kPa -0.02

0

0

0.02

0.02

0.04

0.06

0.08

Distance [m]100 150 200 250 300 350 400 450

Ele

vatio

n [m

]

450460470480490500510520530540550560

Figure 7. Vertical displacements at reservoir impounding up to level 546 m.a.s.l., from - 0.034 m (settlement) up to + 0.086 m (raising)

1 upstr-slope : 16sec

2 diaphragm : 16sec

Y (m

)

X-Displacement (m)

470

480

490

500

510

520

530

540

550

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Figure 8. Horizontal displacements [m] along height of: (1) upstream slope and (2) asphalt core, at reservoir impounding up to elevation 546.0 m.a.s.l., (at core axis maximal value of 15.4 cm at 60% of the height, in the crest 11.0 cm)

34



CONCLUSION The response of the embankment dam under action of static loading is complex issue, in largest amount not describable by physical laws, but is estimated by numerical models. Inclusion of models instead of laws implies that for analysis of same dam, the models (based on various approximations), should not be mutually excluded, but in contrary, they should contribute on better explanation of the prototype behaviour. By comparison of the results of the two applied models for behaviour of rockfill dam with asphalt core at first impounding stage, three following facts can be outlined. First, by analysis of the horizontal displacements along the core axis is noticed that in both models is obtained identically deformed shape, with maximal displacements around 0.2% of the dam height, located above the foundation at approximately 60% of the dam height, and reduced downstream displacement in the crest, estimated at 70% of the maximal horizontal displacement. Second, in case of the vertical displacements, in both models is obtained same maximal settlement, located in the upstream shell, at nearby of the reservoir normal water level, value of 0.05% of the dam height. And third, with model no. 2 is obtained more realistic display on distribution of the maximal main effective stresses, which is the basis for all future structural (static and dynamic) analysis of the dam. The degree of accuracy of the two different numerical models on distribution of the effective stresses in the upstream shell of rockfill dams with core wall at first impounding stage most appropriately can be confirmed by monitoring data for the total stresses and pore pressure at testing and service period of the dam.

REFERENCES

[1] TANČEV L., 2005. “Dams and appurtenant hydraulic structures”, Taylor & Frances, London, UK, 2005

[2] NUMERICAL ANALYSIS OF DAMS, ICOLD, 1994. Volume III "Evaluation of pore pressure and settlements of an embankment dam under static loadings", September, Paris, France

[3] PETKOVSKI L., 2007. “Seismic Analysis of a Rock-filled Dam with Asphalitic Concrete Diaphragm“, 4th International Conference on Earthquake Geotechnical Engineering, 25-28 June 2007, Thessaloniki, Greece, paper #1261, CD-ROM

[4] PETKOVSKI L., ILIEVSKA F., 2010. “Comparison of Different Advanced Methods for Determination of Permanent Displacements of Tailings Dams in Earthquake Condition“,14th Europian Conference on Earthquake Engineering, 30-03 August 2010, Ohrid, R.Macedonia, paper #1511, CD-ROM

[5] NOVAK P., MOFFAT, NALLURI, NARAYANAN, 2001. "Hydraulic structures", London, UK

[6] PETKOVSKI L., TANČEV L., MITOVSKI S., 2007. "A contribution to the standardization of the modern approach to assessment of structural safety of embankment dams", 75th ICOLD Annual Meeting, International Symposium “Dam Safety Management, Role of State, Private Companies and Public in Designing, Constructing and Operation of Large Dams”, 24-29 June 2007, St.Petersbourg, Russia, Abstracts Proceedings p. 66, CD-ROM

[7] PETKOVSKI L., TANČEV L., 2004. "Basic principles of modern approaches to static analysis of earth filled dams", I Congress of the Macedonian Committee on Large Dams, October, Ohrid, Proceedings, p. 25-36

[8] PETKOVSKI L., TANČEV L., 1998. "Hydraulic and mechanical response of an earth dam during construction", VI International Symposium on water management and hydraulic engineering, Dubrovnik, Croatia, Proceedings Vol.2, p.239-248

[9] SOFiSTiK, 2010, Analysis Program, manual

[10] Geo-Slope SIGMA/W, 2012. "Stress/deformation analysis", GEO-SLOPE International Ltd., Calgary, Alberta, Canada

[11] ALONSO, E.E., S. OLIVELLA AND N. M. PINYOL. 2005. A review of Beliche Dam, Géotechnique 55, No. 4, p.p. 267–285.

[12] OLDECOP, L.A., ALONSO, E.E. 2007. Theoretical investigation of the time-dependent behaviour of rockfill. Géotechnique, 57 (3), pp.289-301.

[13] ROOSTA M. R., AND ALIZADEH A. 2012. “Simulation of collapse settlement in rockfill material due to saturation”, International Journal of Civil Engineering, Vol. 10, no. 2, June 2012, pp.93-99.

35

61 | PageSCIENTIFIC JOURNAL OF CIVIL ENGINEERING • Volume 2 • Issue 1 • July 2013


AUTHORS

Igor Planinc Ph.D. Professor Universtity of Ljubljana Faculty of Civil and Geodetic Engineering [email protected]

Matija Gams Ph.D. Professor Universtity of Ljubljana Faculty of Civil and Geodetic Engineering Jamova 2, Ljubljana Slovenija

Nina Humar Ph.D. Professor Universtity of Ljubljana Faculty of Civil and Geodetic Engineering Jamova 2, Ljubljana Slovenija

Simon Schnabl Ph.D. Professor Universtity of Ljubljana Faculty of Civil and Geodetic Engineering Jamova 2, Ljubljana Slovenija

THE EFFECT OF ACCUMULATION RESERVOIR ON THE DYNAMIC RESPONSE OF GRAVITY DAMS

In the analysis of the dynamic response of gravity dams, the effect of accumulation reservoir must be taken into account. The use of computer programs is obligatory to quantitatively determine these effects. One such program is our finite element based computer program DIN3D. The speciality of this program is that it uses what is called addition matrix principle, in which the interaction can be taken into account (H. Šolinc, 1983, Use of Boundary Elements in dynamic analysis of elastic reservoirs, PhD Thesis (in Slovene), University of Ljubljana, Faculty of Civil and Geodetic Engineering). Finally, a simple parametric study of the effect of accumulation reservoir size on a period of a gravity dam is presented.

Keywords: gravity dams, dynamic response, finite element method, accumulation reservoir.

INTRODUCTION In literature, a number of computational procedures for the dynamic analysis of concrete gravity dams are found. In general all these procedures can be classified into two major groups which are closely related to each other. In the “first group”, there are so called simplified computational procedures for a dynamic analysis and design of concrete gravity dams. The main advantage of these simplified procedures is their mathematical simplicity. On the other hand, the disadvantages of above mentioned procedures are (i) inaccuracy of the results, which can result in a non-economical construction and, (ii) undetermined boundaries of the application of such methods (Fenves in Chopra, 1986). With the intensive progress of computer science in the last decades, the limitations of simplified methods can be suppressed to some extent or these methods

37

555.856


Igor Planinc, Matija Gams, Nina Humar, Simon Schnabl

are replaced with more sophisticated methods and procedures – computer programs. Thus, the “second group” is composed of these sophisticated procedures for the dynamic analysis of concrete gravity dams. Recently, a great interest is shown in developing of these sophisticated methods and as a result a lot of papers have been published in the literature. The common principal property of these so-called “exact” computational procedures is a higher accuracy of the results and consequently a higher mathematical complexity of the derived mathematical models. As a result, these procedures are based on numerical methods such as finite element method (Cook et al., 1989, Prelog, 1975, Zienkiewicz in Taylor, 1989). The basic theory of the finite element method is well known and it will not be described in the paper. Therefore, in what follows, the emphasis will be placed on what someone should pay attention if commercial programs for the dynamic analysis of concrete gravity dams are used. The commercial programs have to be judged by two essential aspects. The first is called a “verification” of the mathematical model. With the term “verification” the numerical accuracy is denoted which means how accurate the model equations are solved, while “exact” solution of the stress-strain state of the particular structures is unknown. The second aspect to assess the suitability of the numerical procedure and developed computer program is called “validation”. With the validation someone could estimate how accurate or how well the numerical model describes the real/physical behaviour of the structure. This could be done with a comparison of the numerical results with the experimental ones. If the experimental and numerical results agree in principal characteristics then the conclusion could be made that the model or the computational procedure is accurate. Nevertheless, the verification and the validation of the numerical model usually are not sufficient for practical use of computer software. For engineer, program speed and simplicity of use is very important. In other words, in the estimation of the program quality, it is very important how quick the accurate results are obtained.

Nowadays, there exist many commercial computer programs for analysing the civil engineering structures (e.g. LUSAS, ABAQUS, and so forth). The qualitative computer programs are usually very expensive, general and designed for specific problems that demand an extensive specific knowledge

about the behaviour of civil engineering structures and pretending mathematical procedures. Their generality is often also their defectiveness which means that general commercial programs often do not enable to find the solution by simple way which is especially true for a dynamic analysis of concrete gravity dams. The development of such sophisticated programs serves as an effective application of new knowledge in the field and a corresponding adjustment and completion of the mathematical procedure to the experimental results. One such program is also a computer program DIN3D for a dynamic analysis of concrete gravity dams where the influence of accumulation reservoir is taken into account. The program is based on a finite element method. In order to have a short calculation times, the interaction between a dam and an accumulation is modelled by the addition matrix procedure (Šolinc, 1983). For modelling the concrete dam and the accumulation reservoir well known trilinear brick finite elements are used (Cook et al., 1989, Prelog, 1975, Zienkiewicz in Taylor, 1989). It is well known that the addition matrix procedure is not the most accurate procedure for analysing the gravity dams but certainly accurate enough to model the behaviour accurately if the following conditions about the water properties and its circulation in the accumulation are full-field: (1) fluid/water is ideal and incompressible, (2) the water flow is irrotational, (3) in the fluid there are no sinks nor sources, (4) the amplitude of the surface wave is small and, (5) the displacements of the mid-surface of the dam finite element are small in the contact dam-water.

The behaviour of a concrete gravity dam is influenced by various geometric and material parameters of the dam and accumulation reservoir, the interaction between the dam and reservoir and, the interaction between the dam and surrounding soil. But in general the analysis which takes all these parameters into account is extremely difficult. As a result, in the paper, only the influence of the size and shape of the accumulation reservoir on the dynamic behaviour of a concrete gravity dam is analysed. This way we will try to demonstrate the applicability of the computer program DIN3D for the dynamic analysis of the concrete gravity dams and also to show to the engineers how important is the influence of size and shape of the accumulation reservoir on dynamic behaviour of gravity dams.

38



DYNAMIC ANALYSIS OF CONCRETE GRAVITY DAM

Figure 1 shows geometric data of the concrete gravity dam, while Figure 2 shows a scheme of the dam and accumulation reservoir. Because the dam is long we can consider that during the deformation only plane deformation state is occurred in the dam.

0.00

116

Figure 1. Geometric data of dam cross section

Figure 2. Scheme of the dam and accumulation reservoir

In the dynamic analysis of dams and structures in general, the frequency of the structure is one of the most important parameters of all. The computer program DIN3D was used to estimate the influence of the accumulation depth and length on the frequency modes of the dam. The comparison for two different mathematical models is presented by Figure 3.

Table 1. Comparison of the frequency modes regarding to different mathematical models used: (a - left) a

mathematical model without accumulation reservoir and, (b - right) a mathematical model with accumulation reservoir

frequency mode no accumulation accumulation

1

2

39



3

4

5

In both cases the accumulation reservoir with dimensions 100 × 200 m is used. The reservoir depth is 122 m. It is seen from the Table 1. that the influence of the accumulation reservoir is more on the frequency times than on frequency modes which is also clearly seen from the Figures 3-7. As expected, the influence of the size of the accumulation

reservoir is the greatest for first frequency mode, see Figure 3. The self-frequency times are higher for larger accumulation reservoirs. Similarly, the frequency times increase with the increasing of accumulation depth. This result is expected since the cumulative mass (dam + accumulation) is increased.

Figure 3. Dependence of the first frequency time of the dam against the depth and length of the accumulation reservoir

The influence of size of the accumulation reservoir on the second frequency mode is not significant except for the depth around 102 m. This result is

expected since the frequency mode is in the transverse direction with regard to the dam (see Figure 4).

40



Figure 4. Dependence of the second frequency time of the dam against the depth and length of the accumulation reservoir

The third frequency mode is torsional (see Figure 5). In this case, the size of the accumulation reservoir decreases the self-frequency times of the dam. On the other hand, the depth of the accumulation reservoir increases the self-frequency

times. The fourth frequency mode is again in principal frequency direction of the dam but the influence of the accumulation reservoir on this frequency mode is negligible (Figure 6).

Figure 5. Dependence of the third frequency time of the dam against the depth and length of the accumulation reservoir

Figure 6. Dependence of the fourth frequency time of the dam against the depth and length of the accumulation reservoir

The fifth frequency mode is in vertical direction which is again different compared to the principal frequency direction (first frequency mode). As a result, the accumulation reservoir

in this case does not have any important influence on frequency modes and times of the dam.

41



Figure 7. Dependence of the fifth frequency time of the dam against the depth and length of the accumulation reservoir

CONCLUSION In the paper the influence of the accumulation reservoir on the dynamic behaviour of concrete gravity dams is analysed. The analysis is performed using the computer program DIN3D which was developed especially for this case. This program is based on finite element method and takes into account the influence of accumulation reservoir on the dynamic behaviour of gravity dam using the so-called addition matrix principle. The results showed that the length and depth of the accumulation reservoir have dominant influence on first frequency mode only. For higher frequency modes this influence is negligible.

Nevertheless, the results of DIN3D are good so far, but they should also be compared (verificated and validated) to the numerical and experimental results for the real dams in Slovenia.

REFERENCES [1] Cook, R. D., Malkus, S. D., Plesha, E. M., 1989. Concepts and Applications of Finite Element Analysis. John Wiley & Sons.

[2] Fenves, G., Chopra, A. K., 1986. Simplified analysis for earthquake resistant design of concrete gravity dams, Report No. UCB/EERC-85/10.

[3] Prelog, E., 1975. Finite Element Method. University of Ljubljana, Faculty of Civil and Geodetic Engineering (in Slovene).

[4] Šolinc, H., 1983. The use of boundary element method in the dynamic analysis of elastic reservoirs. Doctoral dissertation, University of Ljubljana, Faculty of Civil and Geodetic Engineering (in Slovene).

[5] Zienkiewicz, O. C., Taylor, R. L., 1989. The Finite Element Method. McGraw-Hill Book Company.

42


AUTHORS

Violeta Mircevska Ph.D. Professor University Ss. Cyril and Methodius IZIIS - Skopje [email protected]

DAM - FLUID INTERACTION EFFECTS INDUCED Dam–fluid interaction occurs at the interface between a fluid and a deformable dam structure immersed in the fluid. The magnitudes of the hydrodynamic effects at the interface are proportional to the amount of energy transmitted to the fluid by the vibration of the dam and the surrounding terrain. The amount of transmitted energy depends on several factors, most notably on the dam deformability, direction and intensity of the excitation, water level in the reservoir, compressibility and viscosity of the fluid (water), energy dissipation of the pressure waves by the bottom deposits, radiational damping of outgoing waves, along with the shape of the canyon and the dam. In this paper the influence of these significant factors on the hydrodynamic effects is discussed. The BEM-FEM coupling time-domain method embedded in the ADAD-IZIIS software has been used to solve the interaction of the fluid impounded in a canyon with a complex topography and a130m high arch-dam. The conducted analyses are based on an original and simple FEM–BEM fluid–structure interaction solution embedded in the ADAD–IZIIS software developed for design and static and dynamic Analysis and Design of Arch Dams.

Keywords: dam–fluid interaction, BEM-FEM, hydrodynamic effects, added mass

INTRODUCTION The complexity of the dynamic analysis of arch dams is arisen by the undeniable need for treating dams as three-dimensional systems that recognize the semi-unbounded domains of the reservoir and foundation rock. Dynamic analysis of arch dams should consider the dam-water interaction, dam-foundation rock interaction, spatial variations in ground motion around the canyon and during intense earthquake motions, slipping or opening of the vertical construction joints. The objective of this paper, restricted to linear analysis, is to discuss the influence of the

43

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Violeta Mircevska

significant factors on the magnitudes and distribution of the hydrodynamic effects and therefore their impact on the dynamic response of the entire system.

It has long been recognized that hydrodynamic forces generated during earthquake shaking have important effects on the dynamic behaviour of arch dams. The magnitude of these effects is proportional to the amount of energy that is transmitted to the fluid domain. It depends on a variety of parameters such as intensity and direction of the excitation with respect to the dam and surrounding terrain, deformability of the dam, water compressibility and viscosity, water level, shape of the reservoir domain, reservoir topography, attenuation due to motion of the dam in the fluid environment, energy dissipation due to partial absorption of the hydrodynamic pressure waves by the bottom deposits, radiational damping of the outgoing waves, etc. The paper alerts that the topographic site effect despite its impact on the accuracy of the calculated hydrodynamic pressure (HDP) so far has been unfairly ignored in BE and FE modelling of the reservoir. Despite the use of non-reflecting TBC, if TS is located closer to the dam, the FSI interaction effect could be under or overestimated which inevitably leads to impingement of the calculation accuracy. Actually the engineering analysis for prove of the seismic stability of high dams in seismic prone regions should consider the presented strategy for detection of the “most adequate” truncation surface location, if dams are situated in narrow steep and twisted canyons. For the presented BE model, the elapsed CPU time for performing such analysis is approximately 15 min.

FACTORS INFLUENCING DAM–FLUID INTERACTION EFFECTS

Water viscosity

Viscosity is a measure of resistance of the fluid to deform by shear or tensile. Kinematic viscosity of water as Newtonian fluid is: , while its dynamic viscosity is According to the parametric sensitivity study [1], the impact of water viscosity on the hydrodynamic effects and sloshing phenomenon is negligible. However when the natural vibration frequency of the dam (empty reservoir) is close to the natural frequency of the interaction between sloshing fluid and the structure, the fluid motion in the reservoir creates high impact of

the generated dynamic loads at the interface. In this case a non-linear movement of the free surface should be solved using Eulerian eq. or Navier-Stocks eq. depending on other fluid assumptions. Assuming the inviscid property of the fluid and neglecting the surface wave effect in the analysis, the surface boundary condition is expressed as . Numerous studies are supporting this assumption. Chwang [2], has done a pioneer work reporting the first free surface flow in nonlinear convective acceleration analysis. Hung and Chen [3], have studied the viscous effects on the non-linear convective acceleration using implicit finite-difference algorithm and iterative scheme with corresponding boundary conditions. Chen with co-workers derived two dimensional finite difference numerical solutions [4-5], of non-linear hydrodynamic pressure. All these analyses are showing that the maximum water surface rising occur near the dam, with the amount of less than 5% of the impounded water depth and is primarily caused by the ground displacement. The magnitude of water rising near the dam is significant when the ground displacement is large and it is not affected by the vibration of the dam face and by the vertical component of ground acceleration.

Water compressibility

Compressible flow refers to the wave propagation phenomenon with wave speed higher than the speed of sound in the fluid. The Navier-Stocks equations are used for determination of pressure variation under assumption that water is compressible. Numerous studies have shown that the key parameter which determines the water compressibility impact on hydrodynamic effects during strong earthquakes is the ratio of the fundamental response frequency of the infinite reservoir to the natural vibration frequency of the dam alone (empty reservoir).

damempty

reservoir

ff

(1)

If the ratio ≥ 2 in equation 1, the effects of the water compressibility become insignificant.

≈1, then the water compressibility has to be considered in the dynamic response of the coupled dam-reservoir system. In general, the natural vibration frequency of an infinite reservoir with impounded water depth of some 50 to 100m is in the frequency range between 3 and 6 Hz. Likewise, the physical implication of the

44


DAM - FLUID INTERACTION EFFECTS INDUCED

criterion ≈1 is that in the case of a sufficiently flexible dam, the compressibility effect will significantly affect the dynamic response. Note that the range of natural vibration frequencies of an infinite reservoir depends mainly on the canyon shape. The topography of the reservoir itself may thus provoke the compressibility of the impounded water. Therefore, simplistic analyses that ignore the water compressibility may lead to non negligible under or over estimation of the hydrodynamic effects, which can be even higher in case of inability to treat the energy dissipation of the pressure waves by the bottom sediments.

Absorption of the bottom sediments

Along the reservoir bottom, the boundary condition depends on whether the foundation bottom is of flexible or rigid type. Hall and Chopra [7] have derived the boundary condition for flexible type of reservoir-bottom interaction, whereat the propagating pressure waves towards normal direction to the boundary could be partially absorbed and partially reflected

ttzyxWqtzyxa

ntzyxW n

g

),,,(),,,(),,,( (2)

Fenves and Chopra (1984) [8] defined the damping coefficient of the reservoir bottom,

q , as b

b

Cq

1

1, where, b is a

measure of the wave reflection given by the ratio of the amplitude of the reflected hydrodynamic pressure wave to the amplitude of incoming vertically propagating pressure wave. In the case when 01 qb , full reflection from the reservoir bottom occurs, whereas in the case when 10 qb , the bottom sediments fully absorb the pressure wave.

Radiational damping of the outgoing waves

Non-reflecting boundary condition enables radiational damping of the outgoing waves, replacing in that manner a large portion of the excluded far reservoir field from the analysis. Any reflection of the outgoing waves backward in the near reservoir field is prevented by enforcing the correct relation between the imposed pressure distribution at the truncated surface on one hand, and its calculated derivative according to BEM or FEM

formulation of the fluid motion on the other hand. The established equilibrium of the magnitudes of both variables can be simulated using various analytical solutions. Somerfield rigid and reflective type of truncated boundary conditions causes reflection of the acoustic waves back to the near reservoir field, impinging the upstream BC and resulting in spurious and increased values of the calculated hydrodynamic effects. In addition, it has been proved that rigid Somerfield boundary conditions could simulate the radiational damping with satisfactory accuracy only if the fictitious truncated surface is positioned sufficiently far from the upstream dam face, i.e., > 3~5 H. This inevitably leads towards robust models and considerable CPU time when FEM-FEM numerical coupling method is used. Therefore, the efficiency of the non-reflecting boundary conditions remarkably reduces the finite elements mesh representing the reservoir domain and consequently the CPU time required for solving the fluid motion if it is based on FEM-FEM formulation. Non-reflecting boundary conditions provide accurate and efficient solutions only when simple and regular shape reservoirs with constant transversal cross section are considered. The complex topography of the terrain in general requires sufficiently large extent of reservoir to be included in the model, which is feasible if BEM technique is used.

Terrain irregularities

Fluid-structure interaction is strongly affected by the irregularity of the terrain in the vicinity of the dam-fluid interface [9]. The topography of the terrain dictates the ‘most adequate’ location of the truncation surface where non-reflecting truncated boundary conditions should be imposed. The truncated surface should be located in a way that its further displacement away from the dam has only negligible impacts on the calculated hydrodynamic pressure intensities as it is presented in figures (1-4). These figures depict set of analyses that have been performed to study the impacts of the location of the truncated boundary conditions. As an assumption, the upstream dam face vibrates together with both banks as a rigid assemble following the applied seismic ground motion. In the simulations, the dam-reservoir system was subjected to horizontal PGA of 1g acting in downstream direction, figures (1) and (2) and at 45 degrees in respect to the downstream direction, figures (3) and (4). The acoustic waves were generated as a result of the

45


Violeta Mircevska

vibration of the considered rigid upstream dam face and the rigid canyon’s walls. The expansion of generated acoustic waves and the way of their propagation as compressive or dilatation waves depends not only on the specified boundary conditions but on the shape of the reservoir boundaries in respect to the direction of the seismic excitation. Three different types of truncation boundary conditions were considered: stationary type of truncated boundary conditions, i.e., perpendicular acceleration at all the points on the truncation surface is set to zero; hydrodynamic pressure at all points on the

truncation surface set to zero; and non-reflecting boundary condition such as Westergaard’s truncated boundary conditions. Sixteen different location of the truncation surface were considered and analyzed. It can be concluded from the obtained results that for a specific configuration of the terrain the type of the imposed truncated boundary conditions at the “most adequate” location is not of crucial importance, figures (1-4). In this particular case it is most probably caused by the configuration and the hairpin turn of the terrain at a distance of approximately 140m upstream from the dam.

Figure 1. Variation of the normalized hydrodynamic pressure magnitudes as a function of the considered 16 locations of the truncated surface and different truncation boundary conditions (selected node at the bottom and at the middle of the crown cantilever)

Figure 2. Variation of the normalized hydrodynamic pressure magnitudes as a function of the considered 16 locations of the truncated surface and different truncation boundary conditions (selected node at the bottom of the dam, close to the left and right bank side)

46



Figure 3. Variation of the normalized hydrodynamic pressure magnitudes as a function of the considered 16 locations of the truncated surface and different truncation boundary conditions (selected node at the bottom and at the middle of the crown cantilever)

Figure 4. Variation of the normalized hydrodynamic pressure magnitudes as a function of the considered 16 locations of the truncated surface and different truncation boundary conditions (selected node at the bottom of the dam, close to the left and right bank side)

Direction of the excitation

The seismic energy transmitted to the fluid by the vibration of the dam and the surrounding terrain generates hydrodynamic forces in the form of compressive and dilatation waves. Figure (5) and figure (6) present snapshots of hydrodynamic pressure distribution at time T=4.93 sec at the left hand side of the dam. However, the insight in the developed hydrodynamic effects is not possible when added mass method is used.

It can be observed in these figures that the magnitudes and distribution of the hydrodynamic effects depend primarily on the direction of the excitation. The results from the various simulations presented herein were

obtained using the ADAD-IZIIS software, specially written for analysis of arch dams. A double-curved arch dam with a structural height of 130 m was considered with a crown length of 405 m, and composed of 27 monolith blocks. The dam was subjected to the first 7 seconds of the 1940 El Centro earthquake record scaled to the peak ground acceleration of 0.3g. This duration of the input motion is jugged as sufficiently long to observe hydrodynamic shaking effects as well as the impacts of the terrain complexity effects. The acceleration record was applied firstly in the downstream direction, figure (5) and then at 45 degrees towards downstream direction, figure (6). Figures 5 and 6 show that the amount of energy transmitted to the fluid is a function of the intensity of the excitation, while for the

47


Violeta Mircevska

same PGA it depends on the direction of the excitation with respect to the dam and the surrounding terrain.

Dam flexibility and impounded water level

If flexible structure is considered, like an arch dam, added mass principle, according to Westergaard does not give accurate resultants, because Westergaard theory is based on straight rigid dam assumption. Since software ADAD-IZIIS have the possibility for calculation of hydrodynamic pressure according to Westergaard added mass method and according to coupled FE-BE method, the discrepancies in obtained resultants using both methods are presented below. Figure 7, presents the time history responses of relative displacement, velocity and acceleration for selected node in Y direction, at dam crest (crown cantilever) where the extremes of the response occurred. Obviously, the response acceleration of the dam is modified by 37% when BEM-FEM numerical method is used for solving FSI. Westergaard added mass method gives 28% modification of the dynamic response of the dam. The flexibility property of the structures and the influence of the reservoir domain alter the behaviour of the fluid significantly and consequently the coupled system has a stronger response. Figure 8 presents the time histories of the three principal stresses at the nodes where the extreme of each stress is achieved when FSI effect is omitted. It also

presents the impact of the FSI over the time histories of these stresses and its extremes. According to presented results herein, it is evident that added mass method is not applicable to flexible systems, because the discrepancies are significant, which is best shown in figures 10-12. These figures show diagrams of stress distribution with and without included hydrodynamic effects, whereat hydrodynamic effects are calculated according to added mass method and coupled BE-FE method. The stress extreme is increased by 15% if added mass method is used and 49% if coupled BE-FE method is used. This comparison is made in respect to the analysis with empty reservoir. Therefore, the expected inaccuracy in calculation will be 23% if added mass method instead FEM-FEM is used. There is a common belief that in case of a rigid structure, the magnitude of the hydrodynamic effects becomes high, which cannot be generalized. In order to confirm or deny this statement, other analyses have been done for 70m depth of impounded water. The dam is more rigid in the lower part and consequently the absolute accelerations at the dam-fluid interface are lower. It appears that Westergaard added mass method gives higher dynamic response of the dam, figure 9. Therefore, the above statement is almost valid for this particular case. However, despite the analyses in case of 100m water depth, as presented in figures 7 and 9, the mismatching of the response according to Westergaard added mass method and the coupled FE-BE method, given on figure 9, is almost negligible.

Figure 5. Snapshot of hydrodynamic pressure distribution at time T=4.93 sec (excitation in downstream direction)

48



Figure 6. Snapshot of hydrodynamic pressure distribution at time T=4.93 sec (excitation at 45°)

Figure 7. Modification of the dam response in Y direction due to the FSI effect

49


Violeta Mircevska

Figure 8. Time histories of extreme principal stresses with and without FSI effect

Figure 9. Modification of the dam response in Y direction due to the FSI effect

50



Figure 10. Stress distribution on dam extrados G3, T=2.42 s, withot HDE

Figure 11. Stress distribution on dam extrados G3, T=2.42 s, with HDE using BEM-FEM

Figure 12. Stress distribution on dam extrados G3, T=2.42 s, with HDE using added mass method

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Violeta Mircevska

CONCLUSION This paper shows the importance of including in dam-reservoir interaction analysis all the factors with significant impacts on the dam response. Currently, there is no commercial software available that treats all these factors at a same time. Among the most important findings of this study we will mention that a) the amount of energy transmitted to the fluid is a function of the intensity of the excitation, while for the same PGA it depends on the direction of the excitation with respect to the dam and the surrounding terrain; b) the detection of the ‘most adequate’ location of the truncated surface is important task in development of reservoir model due to the fact that the hydrodynamic effects on the dam-fluid interface are sensitive to the extend and type of generated waves by the boundaries; c) the hydrodynamic effect is stronger for more flexible structures. The paper especially emphasizes the importance of the irregularity of the terrain in the near surrounding of the dam-fluid interface.

REFERENCES [1] ADAD-IZIIS SOFTWARE: Analysis and Design of Arch Dams - User’s Manual. Institute of Earthquake Engineering – IZIIS, University of “Ss. Cyril and Methodius”, 2008.

[2] D.H. Lee, M.H. Kim, S.H. Kwon, J.W. Kim, Y.B. Lee, A parametric sensitivity study on LNG tank sloshing loads by numerical simulations. Ocean Eng; Vol.34, 2007, pp 3–9

[3] A. T. Chwang ,,Nonlinear hydrodynamic pressure on an accelerating plate’’, Physics of Fluids, Vol.26 (2), 1983, pp. 383-387

[4] T. K. Hung, B. F. Chen ,,Nonlinear hydrodynamic pressure on dams’’, Journal of Engineering Mechanics, Vol.116 (6), 1990, pp. 1372-1391

[5] B. F. Chen ,,Nonlinear hydrodynamic pressure by earthquakes on dam faces with arbitrary reservoir shapes’’, Journal of Hydraulic Research, Vol.32, 1994, pp. 401-413.

[6] B. F. Chen ,, Nonlinear hydrodynamic effects on concrete dam’’, Journal of Engineering Structures, Vol. 18, No. 3, 1995, pp. 201-212

[7] J. F. Hall and A. K. Chopra (1980). Dynamic response of embankment concrete

gravity and arch dams including hydrodynamic interaction, Report No. UCBJEERC-80139, Earthquake Engineering Research Center, University of California, Berkeley, CA

[8] G. Fenves, A. K. Chopra ,.Earthquake Analysis of Concrete Gravity Dams Including Reservoir Bottom Absorption and Dam-Water-Foundation Rock Interaction’’, Earthquake Engineering and Structural Dynamics, Vol. 12, 1984 , pp. 663-680

[9] V. Mircevska, V. Bickovski, I. Aleksov, V.Hristovski ,, Influence of irregular canyon shape on location of truncation surface’’, Engineering Analysis with Boundary Elements 37, 2013, pp. 624–636.

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AUTHORS

Dan Stematiu1 Adrian Popovici 2 Radu Sârghiuţă 3 Altan Abdulamit 4 Catalin Popescu 5 Daniel Gaftoi 6 1,2,3,4,5,6 Technical University of Civil Engineering Bucharest, Bv. Lacul Tei, 124, Sect. 2, Bucharest [email protected]

ADVANCED MONITORING OF ROMANIAN DAM PORTFOLIO At present, in Romania there are registered 427 dams included in importance classes A (exceptional) or B (special) that, according to the law, require a special monitoring. The dam monitoring system well established nowadays is the result of an evolution that in Romania lasted for half a century. The tracing of this evolution can be done by analysing the changes recorded in the course of time both in dam instrumentation with measuring devices and in the data processing and normal behaviour assessment as well. In the recent years new advanced monitoring approaches were promoted in order to enhance the monitoring activity. The paper presents in details two such techniques, namely the infrared image techniques employed in defining the seepage phenomena in embankment dams and systematic in situ dynamic measurements of vibration modes of concrete dams that may reveal the aging phenomena by changes in the dynamic characteristics.

Keywords: monitoring, seepage, aging, infrared, natural vibration

INTRODUCTION The dams and their associated reservoirs undoubtedly bring advantages and direct and indirect benefits to a large part of population. But, at the same time, the dams induce an additional risk to the population situated downstream of them that could be affected by the uncontrolled spillage of water from the reservoir in the case of failure or technical accidents.

The recent inventory of dams has identified 2366 dams on Romania’s territory that correspond to the definitions stipulated by the Water Law and the Dam Safety Law. Among them, 427 dams are included in importance classes A (exceptional) or B (special) that, according to the law, require a special monitoring [11].

53

773.442


Dan Stematiu,Adrian Popovici, Radu Sârghiuta, Altan Abdulamit, Catalin Popescu, Daniel Gaftoi

The role of dam monitoring within the safety management, well established nowadays, is the result of an evolution that in Romania lasted for half a century. The tracing of this evolution can be done by analyzing the changes recorded in the course of time both in dam instrumentation with measuring devices and in the data processing and normal behaviour assessment as well. The dam monitoring is made by observations and measurements. Usually, dam monitoring systems employ conventional instruments based on electrical, hydraulic or pneumatic principles, which yield important information on the changes of different physical quantities such as pressure, stress, strain, displacement or temperature. Nevertheless, the values measured by these instruments refer only to their location. Between the positions of the instruments the distribution of the measured physical parameters has to be estimated. It is unavoidable that the use of these data for the assessment of the condition of the overall structure contains uncertainties, especially in the case of lateral dams or dikes that have a length development.

Effective monitoring of seepage within embankment dams is essential in regards to management of dam safety and prevention of failure. Effective, early detection of seepage in embankment dams has been difficult as it originates and develops in the subsurface. Infrared Thermal Imaging is an appropriate technique that is non-contact, non-intrusive, simple and flexible.

On the other hand, in the case of concrete dams the aging phenomena are usually present but can not be directly identified by regular instrumentation. The decrease of concrete strength, the fissure development, the foundation weathering has as the end result the change of the overall stiffness of the dam structure. This change has to be early detected in order to be a starting point for further investigation concerning the actual dam safety.

The dam structure stiffness is directly reflected by its dynamic properties since the natural vibration properties are depending on the mass (assumed constant) and stiffness. The in situ measurements of the natural vibration modes (shape and periods) are a very convenient approach of the dam aging investigation.

These new dam monitoring techniques - infrared thermal imaging and in situ dynamic measurements - are presented in the followings based on two case studies.

INFRARED INVESTIGATION TO DEFINE SEEPAGE CONTROL FOR OSTROVUL MIC DAM The Problem

Uncontrolled seepage can progressively erode soil from the embankment or its foundation in an upstream direction towards the reservoir and develop a flow conduit (pipe) to the reservoir. If the seepage forces are large enough, soil will be eroded from the foundation and be deposited in the shape of a cone around the outlet. If these "boils" appear, professional advice should be sought immediately. Seepage flow which is muddy and carrying soil particles may be evidence of "piping" and complete failure of the dam could occur. If a seepage problem has already been identified investigation is required in order to determine the probable cause of the seepage and the remedial action needed.

The normal approach is to define an adequate numerical model and to examine the actual causes by defining several scenarios. The correctness of the investigation is dependent on the accuracy of the model that in its turn depends on the model calibration [3], [5]. The current field data available are the piezometric measurements. However, for long dikes the extensive seepage may occur in zones not covered by readings. One of the most convenient methods in terms of costs and duration that can furnish the needed information is seepage detection using infrared thermal imaging [6], [8], [10].

The present paper section deals with the seepage problems encountered at the Ostrovul Mic hydropower plant dikes.

Osrovul Micdam

Ostrovul Mic development consists in a gated dam and a hydropower station closing the river bed area and dikes (lateral dams) that create the reservoir contour (figure 1). The power station has an installed discharge of 90 m3/s and a head of 20 m the power being 15.9 MW.

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ADVANCED MONITORING OF ROMANIAN DAM PORTFOLIO

1. Gated dam, 2. Stilling basin I, 3. Stilling basin II, 4. River bed, 5. Power station, 6. Outlet channel, 7. Tailrace, 8. Concrete face, 9. Vegetaded slope

Figure 1. Plan and panoramic view of Ostrovul Mic reservoire and dam

The dikes are made of ballast (sand and gravel). The watertightening system provides concrete slabs (4.0 x 5.0 m) on the upstream face and a cutoff wall 5 to 6 m deep in the foundation (Figure 2). The cutoff wall was performed in an open cut excavation of

trapezoidal shape 4 m in the upper part and 1m at the basis. To allow the closure of the cut-off wall into the marle bed rock the upper layer of the river aluvium (3 to 5 m) was removed in front of the dike upstream toe.

466.80 465.00

Concrete slabs

Natural ground

Bedrock Groutings Cut-off wall Sand and gravel

Figure 2. Dike cross section

The investigations conducted during the low level reservoir operation have revealed several faults in the actual condition of foundation watertightening: - the cutoff wall is not fully closed into bedrock; - there are gaps beteen the concrete face support beam and the cut-off wall; several windows were created in the cut-off wall by oversized blocks in the foundation ground.

Infrared Seepage Investigation And Mathematical Model

Large leakage through the dikes and esspecial through the left bank one was noticed since the reservoir commissioning in 1986. This was the cause for restricted water level operating mode as a prime measure. Over the time, various remedial works have been performed, but they failed to effectively control the seepage flow. The total seepage was up to

55



900 l/s. The largest defective seal behaviour was found between km 1 + 230 and 1 + 370 at the left bank dike. In the last years, in spite of a significant lower level in the reservoir (5 to 6 m) the wet area on the downstream slope has extended, the number of springs has raised and the level of emergence on the slope has also raised.

The main causes of the seepages were investigated based on a calibrated mathematical model. The 2D finite element model was calibration based on the seepage line pozition within the dike body. Since the existing piezometric prophiles were far from the seepage affected zone it was decided to have as calibration data the level of seepage emergence on the downstream face. A simple visual evaluation was considered very inaccurate due to slope vegetation and the superficial soil layer. Consequently, the infrared image techniques was emploid.

Infrared light or thermography is the use of an infrared imaging and measurement camera to "see" and "measure" thermal energy emitted from an object. Seepage flows affect the

temperature field within the dams and their foundations. Leakage detection by means of infrared images allows detecting the presence and movement of water by evaluating the thermal field. In the Romanian dam monitoring practice the FLIR cameras are used [4] (FLIR took its name from the acronym for Forward-Looking InfraRed imaging systems). The analysis draws on the temperature behaviour and the heat capacity of materials within the body of the dam and consequently allows the user to identify and isolate temperature variations along the surface of interest.

Infrared images of the downstream face of the dike were taken. From the same location digital photos were also taken (figure 3). The images were taken in summer. The body of dike was warmer than the water in the outer slope of dike, therefore it is shown with red tone in the picture in compression with the yellow one above the saturated zone. By a simple calculation the elevation of the border between the saturated and unsaturated zones was determined as 455.90 mASL for a reservoir water level of 462.50 mASL.

Seepage exit on the downstream face

Reservoir level 462.50 Emergence level 455.90

455.90

a

b

Figure 3. Model calibration: a) infrared and digital images; b) flow net after calibration.

Starting from the permeability values determined in laboratory and having as calibration target the emergence of the seepage line on the downstream face, the actual permeability coefficients were defined by an iterative process. The calibration was

performed for two distinctive cases, one considering the cut-off wall closed into the bedrock, the other one considering a 0.5 m gap between the cut-off wall deep end and and the bedrock. The values are listed in table 1.

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Table 1. Permeability coefficients

kx (m/s) ky (m/s) Dike body 2 x 10-3 2 x 10-2 Alluvium foundation 4 x 10-3 4 x 10-3

Bedrock 1 x 10-6 1 x 10-6 Deteriorated concrete face 2 x 10-4 2 x 10-4

Cut-off wall 1 x 10-6 1 x 10-6 Rehabilitated concrete face 1 x 10-5 1 x 10-5

New cut-off wall 1 x 10-6 1 x 10-6 Geomembrane 1 x 10-7 1 x 10-7

The emergence level on the downstream face shown by the flow net in figure 3 corresponds to the measured one. Simmilar values were obtained in both cases – closed or flowting cut-off – thus revealing that the main cause of the recorded seepage phenomena is not the deficiency of the performed foundation sealing. The simulations pointed out that the most significant contribution to seepage and to the high level of seepage line is done by the deteriorated slabs on the lower part of the concrete face and especially by the damaged joint between the concrete face and the cut-off wall. The excavations in front of the upsream toe, required to implement the cut-off wall in an open trench, have exposed the most pervious layer above the bedrock to direct infiltration.

Three rehabilitation alternatives were proposed – jet grouting diaphragm, additional cut-off wall and PVC geomembrane on upsream face. In order to support the final decision three comparison criteria were selected: the seepage flow, the condition of the downstream face and the maximum seepage gradient.

INVESTIGATION OF PALTINU DAM AGING BASED ON IN SITU DYNAMIC MESUREMENTS

The Problem

The challenge of managing dams aging became a principal focus of dam engineering throughout the world. In many practical situations damage can be reasonably interpreted and quantified as a stiffness reduction, with the significant advantage of treating a state of damage in a linear elastic

context. The concrete strength reduction and the extended fissures lead to Young modulus reduction that is regarded as a damage indicator. A variation in the elastic modulus induces a change in the dynamic response of the structure.

The main tool available to reveal the changes in the dam dynamic response is the recording of the free and forced vibrations of the dam and the processing of recordings in order to identify the vibration periods and shapes and the critical damping eventually.

The dynamic tests consist in recording the vibration field in the dam induced by ambient noise excitation (free vibrations) or by the hydromechanical equipment vibrations (forced vibrations). The spectral power density and Fourier spectrum of the recordings can render evident the first vibration modes, mainly the natural frequencies.

When evaluating the dynamic behaviour of concrete dams, it is reasonable in most cases to assume that the response is linear for low or moderate-intensity dynamic loading. Such an assumption of linear response simplifies both the formulation of the mathematical model used to represent the dam, reservoir water and foundation rock system and also the procedures used to calculate the response.

The present paper section deals with the field testing of dynamic response of Paltinu arch dam and the corresponding calibrated model used to asses the dam safety or its deterioration due to aging phenomena.

Paltinudam

Located on Doftana River, Paltinu arch dam was constructed between 1960 and 1971. The

57



dam height is of 108 m and the crest length is of 330 m. Storage has been purposed mainly to provide the discharge for drinking and industrial water supply.

The cross profile of the location is characterised by a pronounced asymmetry caused by a terrace located on the left bank having a width of about 100 m along the valley. The dam foundation consists of Carpathian flish and includes detrial sedimentary rocks: sandstone, micro conglomerates and shistous clays.

The foundation rock presents a marked bedded arrangement. The rock mass is affected by a dense cracking system, more evident in sandstone. The dam area is crossed by several faults generally located transverse on the valley. The permeability of the foundation rock was quite large (2…8 lugeons) and an extensive grout curtain was provided.

Dam structure is made up of central symmetrical double curvature arch body that rests on a pulvino by means of a peripherical joint and a parabolic wing extending over the left abutment terrace (figure 1).

During the first 3 years after commissioning the reservoir was operated at lower levels. At the new cycle of reservoir filing in 1974 when the water levels have exceeded the previous recorded ones, an abnormal behaviour of the dam was noticed: dam displacements larger then predicted, movements at the foundation level, joint openings, cracks at rock surface and significant increase of seepage (from 10 l/s to 150 l/s).

Remedial works had in view the causes of the unusual loading scheme. They consisted mainly of additional rock watertightening upstream the fault, drainage downstream and additional mass on the left abutment. To reduce the seepage forces within the rock mass the upstream face of the rock was covered with concrete and a new grout curtain was performed along the left abutment. To prevent possible water pressure effects a new drainage system was provided by means of two drainage galleries located 10 m downstream the grout curtain. To increase the abutment stability a concrete cover was performed on the left bank downstream area that allowed also rock mass consolidation by grouting and controlled drainage (fig. 4, 5).

Figure 4. Paltinu arch dam

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Figure 5. Downstream view of Paltinu dam

Field Dynamic Measurements

The dynamic response of Paltinu dam was achieved by means of highly sensitive KINEMETRICS equipment. Consequently, a chain of SS-1 RANGER seismometers in simultaneous configurations, were used in order to record the response (figure 6).

The SS-1 Ranger is a structural dynamics instrument, used in the determination of multi-modes of vibration under low-level excitation. The positioning scheme for the transducers on the dams was adopted in accordance with the expected response.

Figure 6. Positioning of equipment on dam crest

The layouts of the sensors were performed in order to point out the maximum displacements at the crest level, the characteristics of the fundamental eigenmode of vibration of the dam, together with the frequency content of

the recorded signals. The location of seismometers used in the 2006 campaign for the dam response recording is shown in figure 7.

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Figure 7. Location of the sensors

Seven sensors were simultaneously positioned at the crest level, on a horizontal radial direction, on a horizontal tangential direction and on a vertical direction. The records have been carried out taking into account vibration sources as micro tremors, together with vibrations produced by hydromechanics equipment operating in the body of the dam.

The processing of the data was carried out digitally, which allowed the determination of mean values, peak-peak values, Fourier spectra and response power spectral densities, relationships between input and output, auto correlation functions, etc. Following the process and interpretation of the instrumental data, the natural frequencies/ periods of vibration of the Paltinu dam have been established. The first – fundamental period of vibration identified by means of the measurement was T1 = 0.44 sec (f1 = 2.26 Hz).

Data Interpretation Based On The Mathematical Model

A 3 D finite element model of the dam – foundation system was developed using the ANSYS computer program (see figure 8).

The dam-foundation interaction effects were represented by a "standard" massless foundation model, in which only flexibility of the foundation rock is considered but its inertia and damping are ignored. The rock foundation was modelled with 2780 solid elements connected in 3510 nodes. The arch dam was modelled as a monolithic structure represented by 574 of finite elements of appropriate types.

The dam-water interaction effect was represented by the added hydrodynamic mass models according to Westergaard. The added water mass is realized by means of 173 lumped mass elements at the upstream face of the dam.

The sensitivity analysis leads to a concrete

corresponding to the measured fundamental frequency

The computed eigenfrequency of the system with the impounded reservoir and the added mass approach for structure reservoir interaction are presented in table 2. From the frequency response analysis it can be seen, that the first 5 eigenmodes are sufficient to represent the structural response.

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Figure 8. The finite element model

Table 2. Vibration periods

Eigenmode Frequency Period [Hz] [sec]

1 2.25 0.44 2 2.45 0.41 3 2.86 0.35 4 3.23 0.31 5 3.69 0.27

In order to acquire advance knowledge of the dynamic behaviour of the dam and also to

examine the accuracy of the results, the computed natural modes of vibration were presented in the form of deflected shapes. The first two modes are presented in figure 9.

1st Mode 2nd Mode

Figure 9. The first two vibration modes of the FE model

Performing several modal analyses on the basis of a finite element model of the dam, each of them for a different reservoir water level one can establish a graphical correlation

between the fundamental period and reservoir condition. The analysis results are presented in table 3. In figure 10 the actual variation of

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the fundamental vibration period in terms of the water level is presented. Table 2. Modal frequencies in terms of the water level

Mod 1 Mod 2 Mod 3 Mod 4 Mod 5f1 [Hz] f2 [Hz] f3 [Hz] f4 [Hz] f5 [Hz]

640 2.258 2.480 3.542 3.854 4.545635 2.283 2.540 3.583 3.865 4.573630 2.313 2.618 3.628 3.886 4.610625 2.322 2.647 3.643 3.897 4.621620 2.340 2.704 3.670 3.919 4.643610 2.354 2.757 3.694 3.939 4.658600 2.361 2.787 3.709 3.950 4.664590 2.364 2.802 3.717 3.956 4.667

Lac gol 2.365 2.813 3.725 3.963 4.670

Water level

T1

540550560570580590600610620630640650

0.42 0.425 0.43 0.435 0.44 0.445

T [s]

Niv

el in

lac

[mdM

]

T1

W

Res

ervo

ir w

ater

leve

l (m

ASL

) (m

ASL

0

Figure 10. First vibration period in terms of water level

CONCLUSION New acquired field measurements, in special annual campaigns, are used for assessing the aging effects. If the new values does not differ significantly from the previous value one can conclude that the dam has preserved its structural properties. If this is not the case, further investigation are required in order to evaluate the causes and significance of the changes which can be due to either ageing or fatigue or structural local degradations.

REFERENCES [1] Abdulamit, A., Stematiu, D., Toma, I. (1995) Identification of Dynamic Elastic Material Properties Using Hybrid Models, Proceedings New Advances in Modal Synthesis of Large Structures. Non-linear, Damped and Non-deterministic Cases, Lyon, France, October.

[2] Allen, R. (2011) Safety Evaluations of Hidden Dam – Seepage Models Meet Reality. 2011 AEG Annual Meeting.

[3] CRWMS M&O (2001). Seepage Calibration Model and Seepage Testing Data.

153045MDL-NBS-HS-000004 REV 01. Las Vegas, Nevada.

[4] FLIR SYSTEMS.(2006). ThermaCAM TM B4. User manual.

[5] Guardo, M., Rohrer, K, P. (2000) Calibration of a steady-state seepage model from transient recovery of field data† JAWRA Journal of the American Water Resources Association. Volume 36, Issue 1, pages 87–94.

[6] Moel, M. (2010). Seepage Detection within Embankment Dams Using Infrared Thermal Imaging. Proceedings of the 2010 ANCOLD Conference.

[7] Sarghiuta, R., Abdulamit, A., Bugnariu, T. (2007) Safety Assessment of Arch Dams using Global Elastic Modulus Concept. Proceedings of International Symposium Thirty Years from the Romania Earthquake of March 4, 1977 Bucharest, Romania

[8] Shutko I and col. (2009). New Technologies in Monitoring and Emergency Mapping of Water Seepage and Dangerously High Groundwater. Research report.

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[9] Stematiu, D., Bugnariu, T., Toma, I. (1995) Global Elastic Modula for Arch Dams, Proceedings of the Conference on Research and Development in the Field of Dams, Crans–Montana, Switzerland September.

[10] Stematiu, D. (2008). Use of infrared images to revealed infiltrations from tailings dams (in Romanian). Hidrotehnica, Vol. 53, Nr. 12.

[11] *** Dam Safety Law No 466 (2001) M.Of. Nr. 428/31 iul. 2001 (in Romanian).

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