1. Contents1. Introduction.........................................................................................................................................................................................3

1.1 Thesis Topic..............................................................................................................................................................................31.2 Short presentation of the thesis..................................................................................................................................................31.3 Acknowledgements...................................................................................................................................................................3

2. Current status of seepage monitoring and control...............................................................................................................................42.1 Brief history of the development of seepage control methods..................................................................................................42.2 Failure modes............................................................................................................................................................................42.3 Seepage detection and monitoring.............................................................................................................................................4

2.3.1 Interstitial pressure and piezometric levels measurements...............................................................................................42.3.2 Seepage flow measurements.............................................................................................................................................42.3.3 Geophysical methods........................................................................................................................................................42.3.4 Remotely operated vehicles (ROV)..................................................................................................................................42.3.5 Thermal detection.............................................................................................................................................................42.3.6 Rezistivity measurements.................................................................................................................................................42.3.7 Tracers for determining the seepage paths.......................................................................................................................52.3.8 Visual observations...........................................................................................................................................................5

2.4 Seepage control..........................................................................................................................................................................52.4.1 Seepage control through embankment dams....................................................................................................................52.4.2 Seepage control through dam’s foundation......................................................................................................................52.4.3 Filters................................................................................................................................................................................52.4.4 Geosynthetic materials......................................................................................................................................................5

3. Theoretical basis of water flow through porous media.......................................................................................................................63.1 Darcy’s law. General form. Limits of its validity......................................................................................................................63.2 Flow net.....................................................................................................................................................................................63.3 Characteristics of porous media................................................................................................................................................63.4 Unsaturated regime....................................................................................................................................................................63.5 Transient regime........................................................................................................................................................................6

4. Seepage analysis using mathematical modeling.................................................................................................................................64.1 Characteristics of a mathematical model...................................................................................................................................6

4.1.1 How to realize a mathematical model...............................................................................................................................64.1.2 Porous media schematization...........................................................................................................................................64.1.3 Boundary conditions schematization................................................................................................................................74.1.4 Mathematical models calibration......................................................................................................................................7

4.2 Finite differences method..........................................................................................................................................................74.2.1 Boundary conditions.........................................................................................................................................................74.2.2 Calculation algorithm.......................................................................................................................................................74.2.3 Transient regime...............................................................................................................................................................7

4.3 Finite element method...............................................................................................................................................................74.3.1 Differential formulation....................................................................................................................................................74.3.2 Variational formulation....................................................................................................................................................74.3.3 Transient analysis.............................................................................................................................................................74.3.4 How to realize the moddel................................................................................................................................................74.3.5 Steps in the Finite Element Method calculation...............................................................................................................7

4.4 Boundary integral equations method.........................................................................................................................................74.5 Specialized software for seepage analysis.................................................................................................................................7

5. Use of mathematical models for the analysis and the selection of constructive solutions - case study Ostrovul Corbului...............75.1 Object of the study.....................................................................................................................................................................75.2 Site description..........................................................................................................................................................................85.3 Site geology...............................................................................................................................................................................85.4 Cause of the piping phenomena.................................................................................................................................................8

5.4.1 Analyzed assumptions......................................................................................................................................................85.4.2 Mathematical model.........................................................................................................................................................85.4.3 Simulations for checking the assumptions.......................................................................................................................8

5.5 Solutions to stop the piping phenomena....................................................................................................................................85.5.1 Proposed solutions............................................................................................................................................................85.5.2 Changing the pumping regime effect...............................................................................................................................85.5.3 The cutoff wall effect........................................................................................................................................................9

5.6 Conclusions...............................................................................................................................................................................96. Checking the efficiency of remedial solutions on dikes characterized by poor drainage – Frunzaru study sase...............................9

6.2 Site description..........................................................................................................................................................................9

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6.2.1 Foundation soil nature....................................................................................................................................................106.3 Sealing phenomena analysis....................................................................................................................................................10

6.3.1 Current state....................................................................................................................................................................106.3.2 Analyzed hypothesis.......................................................................................................................................................106.3.3 Mathematical model.......................................................................................................................................................106.3.4 Simulations for checking the assumptions.....................................................................................................................106.3.5 Remedial solutions proposed..........................................................................................................................................11

6.4 Conclusions.............................................................................................................................................................................117. Calibration of the mathemathical models by measurements and investigations using infrared thermography – case study Ostrovul

Mic.........................................................................................................................................................................................................117.1 Object of study.........................................................................................................................................................................117.2 Site description........................................................................................................................................................................117.3 Behavior of the work during service.......................................................................................................................................117.4 Analyzed solutions for the reconstruction of the sealing........................................................................................................117.5 Sealing solutions analysis........................................................................................................................................................12

7.5.1 Fundamentals..................................................................................................................................................................127.5.2 Mathematical model.......................................................................................................................................................127.5.3 Model calibration............................................................................................................................................................127.5.4 Sealing systems analysis.................................................................................................................................................127.5.5 Sealing solutions effect on dam stability analysis..........................................................................................................137.5.6 2D horizontal plane effect analysis of the cutoff wall realized from the dike’s crest....................................................137.5.7 Rehabilitation works.......................................................................................................................................................137.5.8 Sealing solutions multi-criteria analysis.........................................................................................................................14

7.6 Conclusions.............................................................................................................................................................................148. Conclusions, original contributions and perspectives.......................................................................................................................14

8.1 General conclusions on the study of infiltration......................................................................................................................148.2 Original contributions and future research directions.............................................................................................................15

Selective bibliography...............................................................................................................................................................................16

Keywords:

seepage associated phenomena; detection and monitoring; infrared thermography; embankment dams; porous media; sealing; drainage; hydrodynamic picking up; mathematical modeling; finite element method; multi-criteria analysis

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1. Introduction

1.1 Thesis Topic

Making reservoirs is recorded since ancient times, and dams are one of the oldest man-made construction. Nowadays hydraulic structures and hydraulic development have a vital role in the management of water resources. However, in addition to their beneficial role, hydraulic structures can be the source of some accidents.

Seepage, both through the body or foundation of the structures, is one of the main causes of dams failure, all retention structures being subject to these phenomena.

In order to prevent the associated phenomena of seepage, such as internal erosion, piping, excessive pressures, to appear or evolve, an early detection of atypical behaviors is necessary and also a close monitoring and an extensive analysis, followed by the determination of remedial solution of the situations.

1.2 Short presentation of the thesis

The paper is divided into 8 chapters, first of them having an introductory role. Chapter 2 presents a brief history on the evolution of seepage control methods, the phenomena associated to seepage, current methods of detection and monitoring and a synthesis of the existing constructive methods to control seepage. Chapter 3 presents the theoretical background on water flow through the porous media. Chapter 4 presents the characteristics of mathematical models and the description of three methods used to analyze: FDM, FEM and boundary integral equations. Chapter 5 contains the analysis of dams characterized by the lack of cutoff walls, with consequences of high values of the hydraulic gradients and downstream concentrations of the gradients. The structure characterized by this problem is the Ostrovul Corbului protection dike where subsidence of land adjacent to the pumping station was reported repeatedly in its downstream area. The study focuses on the use of mathematical models for analysis and selection of constructive solutions necessary to stop the phenomena. Chapter 6 contains the analysis of dikes with drainage problems. This situation is characterized by the occurrence of adverse events with specific effects – springs, wet areas, etc. The location characterized by this situation is represented by Frunzaru Dam which faced infiltration phenomena after the execution of the sealing works. In this study case the effectiveness of remedial solutions was studied, which in this cases focuses on collecting and draining the seepage water. Chapter 7 includes the analysis of the situation in which infiltration and hydrodynamic picking up appear mainly in the dam foundation, when cutoff walls are imperfect. This situation was encountered at Ostrovul Mic Dam. In

this analysis the model was calibrated using thermal investigations carried out on site. The study case presents the analysis of the possible remedial solutions using, as technical criteria, the value of hydraulic gradients, the level of seepage emergence, the value of seepage flow and slope stability. The chapter ends with the presentation of the multicriteria analyses used for a better selection of the remedial solution.

The thesis concludes with a chapter which presents general conclusions regarding seepage control, the author contributions and future research directions.

1.3 Acknowledgements

I sincerely thank and send out my gratitude thoughts to Phd. Eng. Prof. Dan Stematiu for his dedication in guiding my every step over the study years, for his trust during my doctoral studies and especially for the valuable comments and exchange of ideas that contributed to the developing of the thesis.

My gratitude goes to the staff of the Department of Hydraulic Engineering, part of the Faculty of Hydrotechnics for being a great example to follow in my future and for contribuing to my personal development as an engineer.

I express kind thanks to the Institute of Hydroelectric Studies and Design and the entire staff of the Department of Dams for the understanding, the support and the opportunity to work with real professionals.

Last, but not least, I would like to thank my family for the confidence, the patience and the constant and unconditional support they have shown me all this time.

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2. Current status of seepage monitoring and control

2.1 Brief history of the development of seepage control methods

2.2 Failure modes

The downstream flow of the water from the reservoir can be realized through the dam’s body, under dam or beside it. The following types of phenomena associated with infiltration are known: downstream excesive presures, internal erosion, piping, chemical erosion, increased interstitial pressure and saturation of the filling materials.

2.3 Seepage detection and monitoring

There are a lot of factors which, following the monitoring phase, can lead to conclusions on seepage through the dam or foundation. Among them we can mention: increasing of seepage flows, variations of the interstitial pressure, temperature anomalies, physical and chemical properties of the water, the transport of fine particles by seepage water, wet zones, wetland vegetation, slope instability, changes of known phenomena, etc.

The monitoring and the measuring of these factors can be achieved by different methods – from the simplest, which involves the direct measurement of the interstitial presure or of the seepage flow, to the most complex which involves the use of geophysical methods. But, the most common, easiest and most reliable way of monitoring is based on visual observations conducted periodically.

2.3.1 Interstitial pressure and piezometric levels measurements

2.3.2 Seepage flow measurements

2.3.3 Geophysical methodsGeophysical methods, commonly applied in the engineering geology, can be used also for internal inspection of hydraulic structures. In this way you can detect any anomalies or any particular elements such as: heterogeneity, gaps, holes, cracks, preferential flow paths, etc. These methods are based on the propagation of mechanical or electromagnetic waves and have the advantage of being nondestructive. They allow easy determination of some of the material internal characteristics [57]. Among these the following can be mentioned: seismic reflection and refraction, ground penetrating radar, ultrasonic tomography, infrared surveys, the sonar.

2.3.4 Remotely operated vehicles (ROV)The use of remotely controlled vehicles, both the underwater (ROV) and the flying ones (FPV), is an investigation method that is taking the lead. Using underwater ROVs, body or foundation defects of the structure can be detected without endangering divers. Using quadrocopters equipped with FPV system wet zones, slides and others can be detected and seen from another perspective.

2.3.5 Thermal detectionThe temperature measurement method for detecting seepage withinin the body of the dams is being used since 1950, the lake’s water temperature acting as a natural tracer during the seepage flow and creating temperature anomalies within the embankment. These anomalies can be detected and localized.

The interpretation of results is based on the fact that seasonal variations in water temperature cause, through seepage water, seasonal variations within the embankment. A constant value of the embankement’s temperature corresponds to a moderate seepage current but, a large seasonal variation is a sign of an important seepage flow [57].

In order to detect the temperature within an embankment, several methods detailed in the thesis can be used: temperature measurements in wells made in the embankment, continous measurement of temperature using networks of optical fiber, detection of differences in land surface temperature using infrared thermography.

2.3.6 Rezistivity measurementsThe investigation of embankment dams by resistivity measurements is based on the fact that the soil resistivity depends on soil properties and the pore water properties.[30]

The measurements are performed using electrodes mounted on the dam’s crest and their purpose is to detect a change of resistivity. For this reason, the resistivity measurement technique is recommended to represent a form of periodic tracking of dams behavior.

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2.3.7 Tracers for determining the seepage pathsThis method is used in order to determine the seepage pathways and speed. The method consists in injecting special substances into the dam’s body and observing the downstream points where these substances and seepage water spring. Knowing the starting and the ending point, possible flow paths caan be determined. After measuring the time required for the tracer to reach the downstream point, the speed of the seepage current can be estimated and aslo the zones with high permeability.

2.3.8 Visual observationsVisual observations performed periodically are essential in preventing the development of incidents into failures. Visual observations serve to indicate the occurrence of unknow phenomena, of adverse phenomena in areas that are not equipped with measuring devices or of phenomena that can not be detected by the measurement system.

2.4 Seepage control

Figure 2.16 presents schematically the constructive measures used for seepage control (sealing of the dam or of it’s foundation and dam drainage). It is not necessary to apply all these measures to a dam - each dam must be analyzed separately and only the measures that are necessary must be adopted.

Fig. 2.16 – Elements to control seepage through and under embankment dams (adapted from [13])A – impervious core, B,D – filters, C – shoulders, E – upstream blanket, F,G – cutoff wall/ grout courtain, H – drainage wells, I –

downstream berm, J – cutoff wall

2.4.1 Seepage control through embankment damsAccording to US Army Corps of Engineers [84] there are 3 methods to control seepage through embankment dams: embankment with flat slopes, embankment zoning and drainage (filters). A fourth method is to provide a sealing element made of unearthly materials. The paper details all these methods.

2.4.2 Seepage control through dam’s foundationSeepage control through dam’s foundation is necessary in order to prevent excesive pore water presures, hydrodinamic picking up of the fine parts and excessive loss of water from the lake. The main methods to control seepage through the foundation are: cutoff walls or ground courtains, drainage, upstream impervious blanket and downstream berms.

It should be noted that, although the paper presents the sealing systems separate from the drainage systems, they should be designed in a unitary manner. In this way the effects of the sealing system are complemented and enhanced by the presence of the drainage system.

2.4.3 FiltersA filter can be defined as the practical realization of the idea of reducing the hydraulic head losses in areas which are subject to hydrodinamic picking up which leads to the diminution of the interstitial pressure values. In order to achieve their objectives, filters must fulfill two functions / requirements ([47], [77]): the stability function and the permeability function.

The thesis presents: mandatory characteristics of a perfect filter in order to reduce to minimum the risks of operational failures or incidents / accidents, flow conditions of the filters and a number of calculation methods.

2.4.4 Geosynthetic materialsThe geosynthetic materials gather several products realized mainly of plastic and whose production developed especially

after 1970. They are used in geotechnical and environmental engineering. They can be used also for development of new dams or for old dams repairing. The paper describes only the geosynthetic materials used for seepage control: geotextiles, geomembranes and geocomposites.

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3. Theoretical basis of water flow through porous media

3.1 Darcy’s law. General form. Limits of its validity

The basis for all seepage analyses consists of the Darcy's law: v=kI . For the general situation - anisotropic environment

this can be written: v⃗=|k|grad (H ). The Darcy’s law has two limits of validity, detalied in the paper: lower limit for clay

and upper limit for gravel.

3.2 Flow net

The flow net can be defined as a graphical representation of two families of curves: equipotential (ζ=ct or H=ct) and flow lines (ψ= ct or Q’=ct). Its main properties are: equipotential lines and flow lines intersect in a 90o angle; equipotential lines and flow lines do not intersect each other; the flow net does not depend on the absolute value of the coefficient of permeability (k), but only on the ratio of these coefficients in different areas of the domain.

3.3 Characteristics of porous media

The permeability coefficient characterizes the physical properties of the soil in terms of seepage. It is determined by the following factors: granulometry, shape and size of the particles, mineralogical composition, structure and texture of the soil, soil physical condition, soil saturation degree and water viscosity (variable with temperature).

The paper presents a set of practical formulas for determining the permeability and a range of permeability values for different types of soils.

3.4 Unsaturated regime

Water flow in unsaturated regime is governed by the same principles as in the case of the saturated one. Darcy’s law is still aplicable. The most important difference is that the permeability coefficient is not a constant, but varies with the humidity which varies itself with the pore water pressure. In this paper are presented the three elements that characterize the characteristic curve of a material - Air Entry Value (AEV) , the slope of the function (mv) and residual water content.

3.5 Transient regime

For the analysis of the transient regime, the same general equations are used as in steady state regime, with the difference that they are integrated using time-varying boundary conditions and special conditions on the free surface. If we analyze the motion parameters at a certain moment of time, the problem can be studied as a steady state one, and because the motion varies from one moment of time to another – the transient flow can be studied as a sequence of steady state stages.

4. Seepage analysis using mathematical modeling

4.1 Characteristics of a mathematical model

According to Pietraru, numerical simulation involves three concepts: the nature, the scheme and the mathematical model.

Nature is known by its scheme and the mathematical model represents the mathematical description of the scheme.

Acording to Krahn [32] there are 4 main reasons for using mathematical modeling and these are detailed in the thesis: A. making quantitative predictions; B. comparing alternatives – simulation; C.identification of major parameters; D. discovery and understanding of physical processes.

4.1.1 How to realize a mathematical modelTo achieve a correct model, careful planning is required and within this subchapter a series of recommendations for achieving a more accurate mathematical model are presented and illustrated. The conclusion of this chapter is that the mathematical modelling is made by the user, not by the software – the user decides which are the most important elements for the problem and the software realizes only the mathematical calculations.

4.1.2 Porous media schematizationThe natural soil where infiltration takes place is always inhomogeneous and anisotropic. Still, in the analysis, it is considered: Homogeneous, Zoned, Isotropic or Orthotropic

This assumption, plus those regarding the geometric shape of the domain and those regarding the causes that induce water motion (bonduary coditions), represents the schematization process.

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The schematization process is very important in seepage analysis because if the scheme is inadequate, the results will be inaccurate.

4.1.3 Boundary conditions schematizationIn general, seepage problems involve 5 types of boundary conditions: a. impermeable boundary (k=0); b. inputs and outputs (zones with given potential); c. spring zone; d. seepage line; e. imposed specific flow.

4.1.4 Mathematical models calibrationSoftwares are tools that receive as input a set of data obtained from nature and that provide values of the paramatrs found in nature as results. This way, the entire process, which is based on data from nature gives back the data from nature and allows the confrontation of the data. A mathematical model is correct only if its results are consistent with data from nature and for this reason the analysis on mathematical model should never be dissociated from reality, requiring a careful calibration.

As data from nature that can be used to calibrate mathematical models we can specify: values for seepage flow, values for interstitial pressure, levels of depression curve in body filler, etc.

4.2 Finite differences method

This chapter is detailed in the thesis and contains a description of the finite differences method and also presents the calculation relations, how the bondary conditions are treated, stages of the calculation algorithm and how the transient regime is modelled.

4.2.1 Boundary conditions

4.2.2 Calculation algorithm

4.2.3 Transient regime

4.3 Finite element method

The finite element method, which is detailed in this thesis, is based on two mathematical formulations that are equivalent: differential and variational [55]. The paper presents the mathematical relations for the two equivalent formulations, computing steps, how the transient regime is modelled and a series of guidelines for meshing.

4.3.1 Differential formulation

4.3.2 Variational formulation

4.3.3 Transient analysis

4.3.4 How to realize the moddel

4.3.5 Steps in the Finite Element Method calculation

4.4 Boundary integral equations method

4.5 Specialized software for seepage analysis

There are currently a large number of software packages for the water flow through porous media analysis. In the thesis were chosen two well-known models - one based on finite difference method (MODFLOW) and the other one based on the finite element method (SEEP / W).

5. Use of mathematical models for the analysis and the selection of constructive solutions - case study Ostrovul Corbului

5.1 Object of the study

Iron Gates II hydropower plant includes, for the protection of Ostrovul Corbului area, an embankment dam founded on highly permeable and easily trained alluvial soils. The initial protection dike project did not supply a cutoff wall and the solution adopted to maintain a certain water level within the enclosure area was to pump the excessive water over the dike and out in the lake.

Groundwater movement between the retention arrangement and lower levels created by pumped water lead to damages brought to the soil structure. Such a phenomenon is reported near the pumping station, which is emphasized by changes in water levels after the drainage process.

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This study aims to identify the potential causes of the piping phenomena which have appeared at Ostrovul Corbului dike and the possible remedial solutions using the available data and mathematical modeling.

5.2 Site description

5.3 Site geology

5.4 Cause of the piping phenomena

5.4.1 Analyzed assumptionsIt is estimated that the piping phenomena of the sand from the pumping station area have the following causes:

dam foundation soil which is composed of irregular sand and gravel, unstable in terms of hydrodynamic picking up

Pumping station’s caisson which represents an obstacle in the flow net of the groundwater flow from the dam to the drainage channel

Water pumping from the drainage channel into the storage lake takes place as a transient flow with 2 high capacity pumps.

5.4.2 Mathematical modelTo confirm the assumptions on the causes of the phenomenon, a transient 2D horizontal model was realized using the finite element method. The model was realized with Seep/w and the mesh had 2707 elements.

5.4.3 Simulations for checking the assumptionsTo highlight the increase in hydraulic gradients induced by the pumping, the situation before the pumps were starting was modeled first – high water level in drainage channel. The initial head boundary conditions adopted were: H = 42.00 maSL in the storage lake and H = 37.00 maSL in the drainage channel.

Simulation results are presented in the paper as the lines of constant head, the hydraulic gradients and the flow vectors. As a conclusion of the analysis it has been discovered that the geometrical features of the drainage channel cause concentrations of large gradients when seepage water exits within the channel. These values (0.18) are qual to the critical ones.

To highlight the effect of the current pumping regime it was modelled as a transient one: lowering the water level in the channel from 37 maSL to 35 maSL in 3 hours. The time step of the model was 5 minutes.

As a result of the analysis we observe the increase of the gradients over time as the water level decreses. At the end of the pumping process the value of the gradients is 0.26. These values are greater then the values of the critical hydraulic gradients and in this way we validated the causes of the hydrodinamic picking up.

5.5 Solutions to stop the piping phenomena

5.5.1 Proposed solutionsTwo types of interventions have been proposed as corrective measures:

Changes to the pumps in order to create the possibility of adjusting the flow in order to limit the water level offset into the channel and to lower the water speed.

Create a sheet pile cutoff wall on an alignment parallel to the channel bank and then rotated to the limit of the drainage channel, behind the pumping station, in order to create a hydraulic barrier to reduce the flow gradients below the critical values.

5.5.2 Changing the pumping regime effectThe effect of changing the pumping regime was highlighted by analyzing a transient model – controlled pumping, which leads to lowering water levels from the drainage channel from 37.00 maSL to 36.00 maSL in 3 hours.

After analyzing the maximum gradient it was noted that the measure does not bring significant changes in seepage regime. Compared with the currently practiced pumping regime maximum gradient reductions are obtained from 0.26 to 0.22.

After analyzing the results, it was found that this intervention by itself does not change the seepage regime. The difference between the actual pumping regime and the proposed one is insignificant – the value of the hydraulic gradient decreases from 0.26 to 0.22, but the risk of hydrodynamic picking up still remains.

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5.5.3 The cutoff wall effectThe cutoff wall leads to radical changes within the infiltration flow net. We analyzed 2 assumptions - maintaining current pumping regime and change it according to the previous chapter.

Figure 2.19 presents, for example, the effect of the cutoff wall for the first assumption examined.

SP

36.

5

36.5

37

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37.5

37.5

38

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38.5

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39.5

SP 0.0

5 0.05

0 .1

0 .1

0.1

0 .15

Fig. 2.19 – Equipotential and equal gradientlines – Transient analysis : Slow pumping (37 – 36 - 3h)Step 3 – after 180 minutes. Effect of the cutoff wall

The 2 analyzed assumptions showed that presence of the cutoff wall has a beneficial effect – diminution of output gradients below the critical value. For the second assumption we obtain a safety factor of about 1.8 for the hydrodynamic picking up.

5.6 Conclusions

The analysis on the mathematical model showed that hydrodynamic picking up of the sand from the dam foundation has 4 main causes:

the presence of easily trained alluvial soils;

the configuration of the terminal area of the drainage channel;

the presence of the caisson which represents a barrier for the groundwater flow;

the pumping regime characterized by an important oscillation of the channel water level.

To control the seepage phenomena, which may jeopardize the integrity of the pumping station, the following are proposed :

the development of a sheet pile cutoff wall behind the pumping station, in order to create a hydraulic barrier to reduce the flow gradients below the critical values

changes to the pumps which create the possibility of adjusting the flow in order to limit the water level offset into the channel and to lower the speed of the water.

A numerical simulation on a 2D horizontal model showed that the cutoff wall has the maximum effect in stopping hydrodynamic picking up of the sand from the foundation. The combined solution – installation of the cutoff wall and change of pumping regime – is recommended.

6. Checking the efficiency of remedial solutions on dikes characterized by poor drainage – Frunzaru study sase

6.1 Object of study

After completion of the works, following the raising of the water level of the lake Frunzaru from 69.00 to 69.50 at the NRL (71.00 ASL) an increase of seepage through the dikes was detected, which resulted in additional sealing works that required emptying the lake. Once filling the lake at NRL after completion of the new sealing works, the seepage phenomena returned. This situation has led to the present study which aims to analyze the causes of the infiltration phenomena occurring after completion of the sealing works and to establish remedial measures that do not involve emptying the lake.

6.2 Site description

Frunzaru hydropower plant, located on the Lower Olt River is the third hydropower plant on the Slatina – Danube sector. The dam is composed of the hydroelectric power plant, the spillway, the embankment dam and the navigation lock.

Longitudinal dikes delimiting the lake on both sides are homogeneous dikes made of heterogeneous filling

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protected on the upstream slope with concrete mask.

The "E" section, analyzed in the study, is characterized by an upstream slope of 1:3 and the dam body is composed of a 1.00 m thick layer of normal drainage material on the upstream slope, and also an horizontal drain in foundation.

The filling of the rest of the dike consists of heterogeneous material (ballast mixed with clay in proportions of 30% to 60%).

The foundation for which section "E" was adopted is marked by the existence of a first layer composed of fine sand, silty sand and clay silt characterized by low permeability and a thickness of less than 3 m. Following this layer there is a permeable layer of variable thickness - sandy gravel and cobble. The last layer in the dike’s foundation consists of the bedrock, which is considered impermeable.

6.2.1 Foundation soil nature

6.3 Sealing phenomena analysis

6.3.1 Current stateSince the beginning of 2011, when the lake was filled and the its upstream level reached NRL (71,00 mdM), various in situ visits where made that revealed a series of atypical phenomena – wet zones on the surface of the berm, griffons in the downstream channel and exfiltrations on the berm, on the slope, on the downstream channels’ slope and in the downstream channel.

6.3.2 Analyzed hypothesisBased on the existing data it was considered that the exfiltration present on the Frunzaru lake dikes following the remedial works has the following causes: settlement of the berm and the diminution of its permeability caused by intense traffic of small or heavy vehicles and the clogging of the weep holes in the concrete mask of the downstream channel. They are obstacles in the seepage path with consequences such as the diminution of drainage in the downstream channel and also the presence of springs on the berm and slope.

6.3.3 Mathematical modelTo confirm the hypotheses on the causes of seepage phenomena, a steady state seepage analysis was performed on a 2D vertical plane model using finite elementh metod.

6.3.4 Simulations for checking the assumptionsIn order to analyze the effects of berm compaction and clogging of the weepholes situated on the downstream channel concrete mask we studied 4 variants and in fig. 6.8 we presented the flow net for the first hypothesis examined:

"complete execution" - ideal situation characterized by lack of clogging; "operation 1" - a situation characterized by clogging of the weepholes situated on the downstream channel

concrete mask; "operation 2" - a situation characterized by compaction of the berm "operation 3" - a situation characterized by the combined effect of the first two operations.

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68

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NNR : 71.00 mdM

Distante orizontale [m]

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175

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te [m

dM

]

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Fig. 6.8 – Flow net and flow vectors – operation 3

As a result of the analysis we noticed that the weepholes clogging effect is reduced, the drawdown curve rising in berm’s body, but not reaching the surface. Instead, the effect of the excessive compaction of the berm is more important, the spring level beeing above the berm level.

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6.3.5 Remedial solutions proposed As a solution to remedy the current situation, the realization of horizontal drains was proposed, which should drive the seepage water into the downstream channel. The drains can be realized without emptying the lake and they serve to remove the "cap" created by excessive compaction of the berm.

After analyzing the mathematical model it was found and confirmed the fact that the draining straps have an important effect - the drawdown curve descending in the dike’s body. However, their implementation without removing the causes that led to the excessive compaction of the berm, only leads to a delay of the current situation. That is why restricting / prohibiting the auto traffic on the berm is recommended.

6.4 Conclusions

The mathematical model analysis showed that the main cause for the appearance of springs on the downstream slope and wet zones on the berm is the excessive compaction of the berm and the clogging of the weepholes in the downstream concrete mask, transforming them into obstacles to the flow. Excessive compaction of the berm due to intense traffic with large and small vehicles has a more important effect than clogging of weepholes.

As a solution to remedy the situation the realization of horizontal drains was proposed. They serve to remove the "cap" created by excessive compaction of the berm, to lower the drawdown curve and to drive the seepage water into the channel. After modeling, we have concluded that this remedial solution is effective only if there will be taken measures for restricting the traffic.

Even if the study is focused on Frunzaru dikes, this problem - berm compaction caused by the traffic - is also found in other facilities. The recommended remedial solution is removing the causes that led to the settlement and draining the seepage water to downstream channel.

7. Calibration of the mathemathical models by measurements and investigations using infrared thermography – case study Ostrovul Mic

7.1 Object of study

The hydro power station Ostrovul Mic faced infiltration problems since its commissioning, that is why the accumulation worked with level restrictions most of the time.

This study was generated by the stringent need of constructive interventions to the dikes and has as main objective the comparative efficiency analysis of the reconstruction solutions for the sealing system in terms of seepage and slip stability so that the best recovery option is chosen.

7.2 Site description

Ostrovul Mic hydropower is composed of a weir dam, a hydroelectric power plant and perimeter dams. The artificial lake dikes are made of ballast with trapezoidal cross-section and a sealing system consisting of a reinforced concrete mask and a sealing cutoff wall made as a trapezoidal ditch filled with concrete gel. They often have the following shortcomings: they are not embedded in the bedrock, there is no joint between the cutoff wall and the cutoff support beam, there are windows in the cutoff wall in the lower area of large blocks.

7.3 Behavior of the work during service

In this study the important events produced at the lake’s dikes are chronologically presented and as a conclusion we can affirm that the water seepage through the dike is a continuous and increasing in intensity process, as the water is infiltrating through preferential paths at the same time as the picking up of fine particles is taking place.

7.4 Analyzed solutions for the reconstruction of the sealing

As remedial alternatives analyzed there were considered the solutions proposed by SC ISPH SA and SC Hidroelectrica SA and also the solution proposed during the analysis made on the mathematical model:

I. the execution from the dam’s crest of a sealing cutoff wall 1 m embedded in the bedrock,II. the execution of a new cutoff wall, alongside the existing one, and the rehabilitation of the concrete mask,

III. the reconstruction of the upper sealing and the cutoff wall -concrete mask joint by laying down a geomembrane

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Emergence of depression curve

7.5 Sealing solutions analysis

7.5.1 FundamentalsAs a basis for data analysis, geological conditions of the site and also the UCC information provided both by the client - SC Hidroelectrica SA - SH Hațeg and by its designer - SC ISPH SA were used.

7.5.2 Mathematical modelTo analyze the effect of the proposed sealing solutions, a steady state mathematical model was realized by using the finite element method and the program SEEP / W 2004.

The analyzed field - left dike embankment cross section (km 1 320) - was represented as a mesh consisting of 12580 triangular and quadrilateral finite elements.

As boundary conditions there were considered: Upstream: the lake water level is equal to the restricted level for the calibration phase and to the normal retention level for the analysis phase and Downstream: the downstream channel water level is equal to 1 m above the level of the deck of the downstream channel and the potential spring area on the downstream slope.

In the first phase - calibration - the model included the body of the dike with the existing sealing systems and foundation ground, and in the second phase - analysis - the model was modified according to each analyzed alternative.

7.5.3 Model calibrationThe emergence rate of the downstream parameter depression curve was chosen as element of calibration for the model. To determine its value in situ, infrared photography was used and the investigations are summarized below in figure 7.5.

Fig. 7.5 – The position of depression curve – second version (october 2011)

Comparing digital photography with infrared photography, with the silhouette of the witness as the connecting element for the two pictures, we can affirm that the blue/red demarcation (Figure 7.5) corresponds to the depression curve’s emergence zone. As a result of the measurements the emergence rate of the depression curve was determined.

During the phase of the model calibration, the main cause of the large seepage flows and of the area below the downstream parameter wetting is the degradation of the concrete mask in the inferior area of the dam and the lack of junction between the cut off wall and the beam. The other imperfections, such as the lack of embedding in the bedrock for the groin and the presence of spaces in the areas inferior to large blocks, are secondary causes.

7.5.4 Sealing systems analysisDuring the second stage of analysis, the calibrated model was modified geometrically in order to simulate the effect of the sealing works. The effect of the works was simulated both when the works were perfectly executed and when defects appeared during execution(lack of embedding and spaces in the cut off wall).

The results for each analyzed situation are presented in the thesis grafically (as flow nets, as gradients) and as tables (seepage flows, hight above the berm for the emergence of the depression curve, emergence level, wet zones on the downstream parameter and the comparison between the output gradient and the critical gradient determined on the basis of the granulometry). As examples there are presented in the figures 7.7, 7.12, 7.17 și 7.19 the flow nets for the perfect execution hypothesis for the 3 analyzed alternatives:

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Cota lac: 465.00 mdM

Varianta : Ecran de la coronament, pereu degradatCota lac 465.00 mdM

Cota coronament : 466.80 mdM

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Cota lac: 465.00 mdM

Cota izvorare : 455.20 mdM

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Varianta : Ecran paralel cu pintenul, pereu reparatCota lac : 465.00 mdM

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Fig. 7.7 – Flow net – alternative I. Fig. 7.12 – Flow net – alternative II.

Cota lac: 465.00 mdM

Cota coronament : 466.80 mdM

Varianta : Pinten incastrat, membrana pana la cota 463 mdM (NME - 1m)Cota lac : 465.00 mdM

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Cota lac: 465.00 mdM

Cota izvorare : 452,10 mdM

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Varianta : Pinten neincastrat, membrana pana la cota 463 mdM (NME - 1m)Cota lac : 465.00 mdM

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Fig. 7.17 – Flow net – alternative III. Embedded cutoff Fig 7.19 – Flow net – alternative III. Non emebedded cutoff

Following the simulation phase, it was determined that the first remedial alternative gives a total control over the infiltration phenomenon – a diminution of flows with more than 90% and the presence of springs in the downstream channel even if defects appear during remedial works. The second alternative has benefic effects only if the works have no defects of execution, but the existing phenomena are reduced with about 30%. The third alternative leads to a diminution of the seepage flows with 50%-80% (based on the current cut off – embedded or not embedded) and represents a feasible alternative for the remaking of the sealing.

7.5.5 Sealing solutions effect on dam stability analysisTo evaluate the effect of the works on the slope stability, stability calculation were made using the limit equilibrium method for all the analyzed remedial work alternatives. The position of the depression curves was taken from the seepage calculation for each alternative.

The result of this analysis showed that the stability factors, both in static and pseudo static conditions, are superior to the values recommended by the normatives only in the remedial alternatives using cutoff wall from the crest and respectively with geomembrane. The result can be explained by the significant weight of the position of the depression curve over the stability factor.

7.5.6 2D horizontal plane effect analysis of the cutoff wall realized from the dike’s crest In order to analyse the horizontal effect of the cutoff wall realized from the dike’s crest a steady state horizontal 2D model was developed using the finite element method. Transmissivities for the modeled areas were determined based on the permeability values used in the vertical 2D model. Boundary conditions adopted are the same as for the vertical 2D analysis.

As a result of the analysis it was observed that the side effect of the cutoff wall is felt upstream and downstream on about 50 m long. In this situation the zone characterized by sealing faults is protected from seepage. The effect of a supposed defect is low, the flow spectrum not suffering major changes.

The situation of a small cutoff wall (150 m) realized only in the zone characterised by predominant seepage was analyzed. For this situation the effect of the sealing works is greatly reduced – the studied zone presenting flow vectors.

7.5.7 Rehabilitation worksThe alternatives studied in the previous chapters in terms of seepage control and slope stability were also economically analyzed. As a consequence, 4 types of interventions corresponding to the analyzed hypotheses on the mathematical model resulted.

a cutoff wall realized from the dike’s crest – alternative I a cutoff wall realized alongside the existing one and the rehabilitation of the concrete mask - alternative II geomembrane on the upstream slope without the rehabilitation of the concrete mask - alternative III.1

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a cutoff wall realized alongside the existing one and a geomembrane on the upstream slope - alternative III.2

The length that was proposed for sealing restoration is equal to 500 m and is found on the left side dike between km 1 000 and 1 500.

7.5.8 Sealing solutions multi-criteria analysisIn order to choose the best alternative for the remedial sealing work, a multi-criteria analysis was realized. This analysis has taken into account the results of the eficiency analysis of the seepage control and the slope stability and also the following: the costs, the duration, the way electric power production is affected during works, the possibility of a perfect execution of the works and the return of investment period based on the surplus energy.

The multi-criteria analysis showed that alternatives I and III (score difference less than 10%) are preferred.

7.6 Conclusions

As a result of the study we can conclude the following:

The mathematical model calibration was realized based on the emergence rate of the depression curve for the downstream parameter. Its value was determined in situ using the infrared thermography technique.

As a result of the mathematical model analysis of the sealing solutions we can say that the solution “Sealing with a cutoff wall from the dike’s crest” gives a total control over the infiltration phenomenon. The alternative ”Sealing with a cutoff wall parallel with the old cutoff wall and remaking of the concrete mask” has a partial benefic effect in the situation in which the cutoff wall is perfectly embedded in the impermeable horizon. The altenative “Geomembrane on the upstream slope” is a feasible one. If the existing slurry trench is embedded in the impermeable horizon then we have a total control of the exfiltrations. If the existing slurry trench is supernatant, the emergence of the depression curve is a little above the berm level, and the gradients have value comparable to the critical gradient.

Slope stability factors are above the minimal values recommended by the normative only in the alternative with remedial cutoff wall from the crest and respectively with geomembrane.

As a result of the multi criteria analysis we can affirm that the remedial alternatives with cutoff wall from the crest and respectively the one with geomembrane on the face are preferred, between the two of them there is not enough score difference to choose only one of them.

8. Conclusions, original contributions and perspectives

8.1 General conclusions on the study of infiltration

The present thesis can be concluded based on the following ideas regarding control seepage through sealing works:

At this stage, monitor seepage can be done in various ways, most notably visually and investigations that can provide data for calibration - piezometers, flow measurement, thermal detection.

Based on the law of water flow through porous media, seepage analysis can be done using mathematical modeling, for example FDM and FEM.

Mathematical modeling is a powerful analysis tool: it can determine the dominant parameters, it can investigate the causes, it can be used to compare and validate a wide range of remedial solutions and it can validate assumptions made based on field measurements.

In similar situations as the one at Ostrovul Corbului, where depth sealing is not present, gradient concentration occurs downstream with the possibility of the foundation’s fine party peaking up. In these cases, constructive intervention mainly aims the reduction of gradients and less the reduction of infiltration flows.

Currently, sealing works structure must have a zoning characterized by increased permeability towards downstream in order to safely direct the infiltration flows. Due to project infringement or local conditions, there are situations when deficient drainage appears which leads to adverse phenomena with specific effects – concentrated springs, wetting, berm saturation, etc. In such situations the constructive remedial solutions should aim the possibility of collecting and drainage of infiltrated flows.

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There are situations when infiltration and driving phenomena mainly appear in the foundation soil of the

damming works when sealing work in depth is absent or poorly made. To identify the causes and propose remedial solutions the calibrated model from phenomena/ data recorded in the field is needed. As an alternative for the areas that do not have measurements, the required calibration data can be obtained by infrared thermography. Such a technique was used to calibrate the model in Ostrovul Mic by determining the emergence rate of the depression curve for the downstream parameter.

Sealing solutions for the foundation soil are analyzed using technical criteria as: gradients, the emerging position of the downstream parameter, seepage flow and slope stability.

8.2 Original contributions and future research directions

The main original contribution in the thesis is to investigate the infiltration of atypical phenomena using mathematical modeling and the calibration of mathematical models based on field measurements.

Following the order of the chapters, below are presented the most important original contributions:

the synthesis of the current state regarding the methods for monitoring, control and analysis of the phenomena of infiltration;

the critical analysis of the software specialized in the analysis of flow through porous media; examples for the schematic illustrating techniques and the calibration of mathematical models; the establishment of criteria for the validation of constructive solutions based on gradient control compared to the

critical one investigating the causes of excessive seepage through assumptions analysis using mathematical modeling; the introduction of new investigation techniques based on infrared thermography and their use regarding the

calibration of mathematical models; establishing rehabilitation solutions for the sealing system using multi criteria analysis.

Future research directions are fully focused on using investigative techniques and field measurements in order to calibrate mathematical models. Another future direction is to predict the time evolution of the phenomena of infiltration from database field measurements.

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