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RD 2011CFRD 15

The Second International Symposium on Rockfill Dams 1

PUNTILLA DEL VIENTO DAM DESIGN

Luis San Martn M 1, Marcela Quezada C 2

1, Civil Engineer, SMI Ingenieros Ltda.2, Civil Engineer, SMI Ingenieros Ltda

ABSTRACT

In 1960 the Direction of Hydraulic Works, Ministry of Public Works of Chile began the construction of the Puntilla Del Viento Dam . Initially the construction of diversion tunnel was considered on the left bank of thevalley, which was aborted, leaving only part of the entrance and exit portal of the tunnel built. Excavations at theportal of entry showed the massive presence of fluvial material, which made the excavation work and progress very difficult.

In 2009 ran the re-study and project design. A detailed geological analysis was performed and conducted a seriesof field surveys, especially drilling, which determined the presence of a paleoflow of 60 to 70 m depth on theleft side of the valley, as in 1960 year was being crossed by the works begun in the diversion tunnel . In thissituation, the construction of diversion tunnel on the right side of the valley was studied and decided . In addition,the presence of a paleoflow forced to consider the construction of a diaphragm wall and an articulatedhorizontal plinth near it.

Another project condition is the very high rate expected sedimentation coming in the reservoir which wasdiscussed extensively on sediment management techniques available. Finally, recommended the implementationof a hydrodynamic and transport model of sedimentation, to evaluate the morphological effects expected over thelife of the reservoir. As an immediate step in the design stage is recommended to use the bottom dischargesystem exclusively for the sediment purge and meet the demand for irrigation through another tunnel dedicatedfor this purpose. This recommendation is consistent with the aims of the project, since it is desired long lifeproject, which shall not be affected in the short term effects of sediment deposition.

For the flood evacuation system had initially considered a frontal gated spillway in order to maximize thevolume of the reservoir. For technical reasons, such as risk in the operation, this view was changed by a sidechannel non gated spillway, but with the possibility to place over the ogee a Rubber Dam, giving the possibilityto increase the storage capacity because of demand growing conditions and / or lowering effects capacity due toexpected sedimentation. The design condition is that being the inflated rubber dam is to generate a flow over theweir as to maintain the original design conditions, in terms of peak water level and drainage capacity.

Key Words: Paleoflow, Articulated Plinth

1. INTRODUCTION

Puntilla Del Viento dam is located in the Aconcagua River in central Chile, at 970 masl. TheAconcagua River has a snow - rain mixed regime . The basin has an area of 2,100 km2, of which 65%corresponds to the snow component . The annual average flow of 31.0 m3/s.-The purpose of the workis the accumulation of water to irrigate about 8,000 ha located downstream of the dam and making up,together with 30,000 ha existing pole agricultural development in central Chile. Resources will be dammed principally during the months of May to September, to be delivered regularly in the summerseason.

Geological and hydrological conditions in the area forced to make some special considerations in thedesign of the works, details of which are presented below.


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2. PRINCIPAL CARACTERISTICS OF THE WORK

The main work of the reservoir is a compacted gravel fillings, which will be waterproofed through aconcrete screen disposed on the upstream face (CFGD Dam type)

The main characteristics of this dam can be summarized as follows:

Dam volume : 112.000.000 m 3 Surface area : 345haVolume of fillings : 3.517.000 m 3 Total height : 107 mCrest length : 400 m Crest width : 10 mUpstream slope (H: V) : 1,5 : 1 Downstream slope (H: V) : 1,6 : 1Concrete face volume :17.771 m 3 Concrete face thickness :0,30-0,52mDiversion tunnel length : 630 m Tunnel design flow : 330 m3/sLateral spillway length : 214 m Spillway design flow :2.273 m3/s

Figure No. 1 shows a diagram with the general layout of the works described above.

Fig. 1 Schematic arrangement of the dam

3. GEOLOGICAL-GEOTECHNICAL ASPECTS OF THE PROJECT

In the dams area appears volcanic , volcanic - clastic and sedimentary rocks with weak stratification,which are partially covered by quaternary deposits of unconsolidated soils of different origin,especially at the foot of the slopes and talus at the bottom of Aconcagua river valley area and side creaks .

In the right or north abutment the rocks are partially covered by debris and some debris cones of side canyons . This debris has a maximum thickness of 3m. The geotechnical quality analysis shows thatthe layer of poor quality has a maximum capacity of 10m thickness . After that, a layer of mediumquality of 10 to 35m thickness appears. From this area develops a horizon of high geotechnical


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quality .Left or south support exhibits a cross-slope stronger than the right support . These rocks are affectedby an incipient hydrothermal alteration, and generally have a moderate weathering. The geotechnical analysis of quality shows that the layer of poor quality has a maximum thickness of 10m . After that, alayer of medium quality of 15 to 50m thickness appears. From this area develops a horizon of highgeotechnical quality.

The central area corresponds to the alluvial plains and terraces of the valley of Aconcagua , which present fluvial deposits in the dam zone corresponding to loose materials and permeable on surface and a little more compact in depth. The profile of contact between soil and rock has a paleoflow that reaches about 60m in depth, as shown in the profile of the figure No. 2 and No. 3 .- This situationforced to make a detailed analysis of leakage below the dam and the consideration of an articulated horizontal plinth .-

Figura N 2 : rock basement analysis

A static-dynamic analysis of the dam through the application of finite elements was performed. It wasdetermined that the maximum vertical settlements are on the order of 0.50 cm during the constructionand 2.0 m for design earthquake. The situations considered by the model correspond to the scene of empty dam, which is the most unfavorable from the viewpoint of stability and vertical settlements .Probabilistic analysis of seismic risk for the site of the dam gave maximum horizontal accelerations of0.32 g with 10% probability of being exceeded in 100 years. From deterministic seismic hazard analysis, was selected as design earthquake occurred on March 3, 1985, which hit central Chile and was a magnitude 7.8 Richter .


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Fig. 3 Estimated rock profile in the dam area

4. PLINTH DESIGN

4.1. Left Right Support

Both , left and right abutments, considered the traditional design of a plinth. It was not considered inthe solution to build an internal plinth to reduce excavations because the rock has some degree of

fracturing, and a traditional injections solution was considered.

This also

led to the decision toconsider , in the upper part, a plinth of a minimum length of 3.5 m in length that would give room for the execution of injections designed.

The layout of the plinth was a way of reducing the number of breaks, allowing defining a plinth for the left side 6 vertices and 8 vertices for the right one . The plinth is anchored to the rock with 3 linesof bolts ( 25 mm , 4.2 m), distributed symmetrically along the plinth. Figure No. 4 shows a schematic layout of the concrete slab and the layout of the plinth and the table No. 1 the main features of it.

RMR Gradient H mean L plinth ThicknessFrom To estimated (m) (m) (m)

1 965 984 60 14 93 7.0 0.65

2 984 1030 63 14 60 5.0 0.603 1030 1067.5 47 10 19 3.5 0.50

Plinth Level (masl)

Table 1 Characteristics of the plinth, piecewise


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diaphragm wall and the horizontal plinth will take place only when the fillings are placed or at leastone important part of them, so they have already produced the settlements and deformations expectedfor construction.

It is proposed for this stretch a plinth full width of 7m , the same as that defined for the lateral plinths at the bottom side, but sectorized in two tranches of 3.5 m each.

4.2. Injections

To ensure better consolidation and above all a better waterproofing under the plinth, it was considered to do the injections following the GIN method, using a maximum pressure of 5 to 10 kg/cm2 up to 15m depth and 30 kg / cm2 under 15 m depth . Initially considered a GIN value of 1,500 bar l / m andthe maximum volume of grout injected 300 l / m.

Consolidation Injections: 2 lines of consolidation Injections were designed to fill the rock surfacenear the plinth 15 m depth . The holes are perpendicular to the plane of the body of the plinth, with variable spacing according to the width of the plinth.

Waterproofing Injections: 3 types of waterproofing injections were designed according to thefollowing criteria:

Primary injection of 45 m long spaced 12 m each. Secondary injections of 30 m in length, placed between two primary injections , thus leaving a

6m separation between them . Tertiary injections of 15 m length, placed between a primary and a secondary , thus leaving 3

m apart from each other.

5. TUNNELS

It is usual in this type of project consider that in the diversion tunnel correspondent works areimplemented for the delivery of the regulated flow from the reservoir. However, in the case of thisproject, this situation is addressed separately, because the considerations made in respect of theexpected high rates of sedimentation. Indeed, during the development of the project a lot of analysismade on the amount of material deposition expected for the reservoir, with an estimated rate of about820,000 m3/year. This means that within 50 years the dam would reduce its capacity in about 1 / 3.

There was extensive discussion on sediment management techniques available and applied in variousparts of the world, according to the characteristics of each work, taking into account the following:

Fig. 6 Detail articulated horizontal plinth

Small Diaphragm Wall Flexible Joint

El 965.0

7.0 m

33.69


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Reducing the incoming load of sediment into the reservoir (management and sediment control in theupper basin and / or construction of sediment retention dams upstream, build a bypass channel ormake side storage)

Management of sediments within the reservoir (sediment distribution to certain rules of operation ofthe dam, dredging selective barriers of obstruction)

Evacuation of sediments from the reservoir (flushing , sluicing , density currents, dredging, drydredging, hydro suction)

Finally, the implementation of a hydrodynamic and sediment transport model was recommended toevaluate the morphological effects expected over the life of the work. As an immediate step in thedesign stage, it was recommended to use the bottom discharge system exclusively for the evacuationof sediment and meet the demand for irrigation through other tunnel dedicated exclusively for thispurpose, as shown in Figure No. 7. In addition was required a third tunnel to provide access and ventilation to the bottom discharge system .

This recommendation is consistent with the aims of the project, since it is desired a work to deliver long life, which will not be affected in the short term effects of sediment deposition . Then the project considered:

Projecting the diversion tunnel having in mind that in it be located the bottom discharges system, soduring the operation stage of the project can flushing finer material deposited in vicinity . Forreasons of access, operation and venting it meant to consider building a third tunnel for this purpose.

Projecting a tunnel to the work of delivery, separate from the above that happens to be 24 m higherin elevation than the work of diversion. A modular grid was considered for the work . That is, if after50 years of operation the sediment in the reservoir reaches levels close to the grid, it is possible to seal with tiles the lower 5 m below the grid , there is still a 5 m stretch of bars that have the capacity necessary to operate the system . This work provides a period of 5 years to decide to increase theheight of the tower or building a new one, probably connected the delivery tunnel with a shaft .

In the operational phase of the reservoir is necessary an accurate control of the sediments level intothe reservoir in order to know the effective deposition levels and define the necessary measures tobe taken (e.g. mechanical dredging).

In times of floods, operate the work of bottom discharge located in the diversion tunnel in order tomobilize the material in suspension that comes with the flood.

Diversion tunnel

Operation tunnelAccess tunnel

Drainage

El 976.0

El 1000.0

El 1010.0

El |971.0

Fig. 7 Schematic layout of tunnels


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6. FLOOD EVACUATOR (SPILLWAY)

With regard to the work of flood evacuation , was initially considered frontal gated spillway 3 spans of16 m height and 8 m width with the purpose to maximize the volume of the reservoir , since it isrestricted in its capacity due to the presence of other infrastructure in the floodplain area . Particularimportance is the existence of a feeder canal for hydroelectric Chacabuquito that develops from the left bank of the valley that surrounds the area of inundation of the dam, whose level cannot be exceeded. This means that the maximum water level that can reach the reservoir is the elevation 1069 meters above sea level .

For technical reasons and risk in the operation, this point of view was changed by a side channelspillway, 214 m length . In order to maintain the possibility to increase the reservoir volume in 12 Hm3 approximately , was conceived the idea of leaving the threshold conditions for later in a 2nd stage , ifrequired , to place over the ogee a rubber dam system which will increase the reservoir capacity , asshown in Figures No. 8 and No. 9 .-

On the initial stage , the design of the work is addressed in the traditional way, by defining a profile of evacuation as shown in Figure No. 10. - Subsequently, the design considers the presence of the rubber

dams and in this situation the condition design being imposed with non inflated rubber dam togenerate a flow over the weir maintaining the original design conditions, in terms of maximum waterlevel and discharge capacity .

Future piers to anchor rubber dam

Instrumentation stall

214 m

Fig. 8 Spillway disposal to implement rubber dam


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The main considerations taken into account are:

For normal conditions of operation, when the water level is over the original spillway level, therubber dam should be maintained inflated to ensure the required regulatory capacity in thereservoir.

Upon the occurrence of a flood that exceeds the upper level of the rubber dam, it must be deflated to allow increased hydraulic head and with it the ability to evacuate.

For safety , the control mechanism of the rubber dam will have a warning system. If the level of the rubber dam is exceeded (e.g. 20 cm) and has not begun the process of deflating the mechanism should aim to make the deflated automatically, keeping alert.

The implementation of the rubber dam at a later time should not affect the operation of the collectorchannel and the chute .

The rubber dam should be installed over a horizontal concrete base wide enough to place anchoringsystems, leaving a bridge inspection structure . Figure No. 11 shows the threshold of the evacuatormodified to have the rubber dam.

It is considered small flood evacuation without deflating the rubber dam because this structure allows to be overrun with a water head up to 40% of its nominal diameter, operating without anyproblems . (This condition is verified at a later stage, once defined and agreed upon basic designconditions of this work).

The design considers the presence of 2 or 3 intermediate piers , which are anchored to the rubber, why should be considered a decrease in the effective length of the original spillway .

The design of the piers will consider conical shapes, with the aim of reducing losses caused by thepresence of these elements.

70 m70 m66 m214 m

Anchoring piers for rubber dam (Stage2) Instrumentation Stall

Fig. 9 Longitudinal spillway profile with piers available for rubber dam


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Fig. 11 Overflow crest modified

Fig. 10 Overflow crest original situation

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