Dam Axis Research Articles

Evaluation of Damsites Groutability Using Secondary Permeability Index, Rock Classification (Case Studies)

Problem statement: The dam body and foundation seepage is one of the important points in design parameters. This index is related to the permeability. There fore permeability is one of the basic parameter in design grout curtain, prevent water escape from foundation and avoid negative seepage pressure in borrow materials, so grouting is used. Grouting means injected materials like cement or chemical materials under pressure to rock or soil layers. Approach: Purpose of grouting is treatment fractures, fissures, increase strength and decrease seepage. Grout curtain is used for sealing beneath dam body and other structure to obtain above aspects. Results: The present research dealing to study the geotechnical characteristics and conditions of two damsites (Kavar and Sheshpir dam) regarding to their permeability and groutability. Permeability in rock mass is controlled by jointing and their characteristics. The Kavar dam may be constructed on Gareh Aghaj river. This site is located in 40 km southwest of Fars province, Iran. The river flow direction is parallel to the local syncline axis. The main geological features of damsite are faults, fractures and gypsum lenses that they may lead to water escape from reservoir. The rocks in the right abutment of the dam are heavily crushed with low strength. Therefore it is expected that water escape from this zone and some parts of dam axis may happen. This is approved from boreholes logs analysis and Water Pressure Tests (WPT). The other dam which called Sheshpir is located in west of Shiraz, Iran. Regarding to structural geology of this site, two large anticlines are extended in general direction of Zagros mountain range (northeast- southwest). The geology index features are karstic and jointed rock with faults. In general, regarding the grout-curtain design for each case, it is necessary to evaluate the volume of grout and injection system. Because of complexity of local structural geology especially rock foundation, it is difficult to estimate above-mentioned parameters. Conclusion/Recommendations: Therefore, through a number of WPT in both sites, the Permeability and hydro fracture tests results were used in the foundation for the design of the grouting program. using SPI rock classification systems, it is possible to identify the karstic features of the formation and the relationships between the permeability and groutability. This method that first expressed by foyo identify the special classification for grouting. In this system use results of Water Pressure Test (WPT) and Rock Quality Design (RQD) simultaneously. With the help of these two parameters rock class and grout complex is identified.

The effect of geology on the suitability of the Olynthios river dam site, Northern Greece

The paper draws attention to the adverse geological and tectonic setting for a proposed dam on the Olynthios river in Northern Greece. The preliminary investigation showed the foundation area to be at the boundary of two dissimilar units in tectonic contact: Mesozoic metamorphic strata juxtaposed with Neogene sedimentary strata. It discusses the significance of the conglomerates in the Neogene deposits and the importance of the clay lenses and clay matrix for the stability and potential settlement of the dam. Although numerous faults are present, the major faults dip steeply at right angles to the dam axis. The study has indicated that there is little loose disturbed material associated with the faults. In this particular environment, in addition to selective grouting, it is recommended that a clay blanket is placed upstream to lengthen the flow path. The feasibility study has shown that despite the theoretically unfavorable conditions the site can be considered suitable.

Distribution patterns of fishes in a canyon‐shaped reservoir

In August 2004 and 2005, an extensive study of the fish community was carried out in the largest water supply reservoir in the Czech Republic and Central Europe, the canyon‐shaped Želivka Reservoir, using a fleet of Nordic multimesh gillnets. Fishes were sampled at eight locations along the longitudinal profile of the reservoir and at five benthic depth layers covering depths from the surface down to 18 m (benthic gillnet 1·5 m high), and at three pelagic depth layers down to the depth of 5 m above the bottom (pelagic gillnets 4·5 m high). Catches of both juvenile (age 0+ year) and adult (fishes >1 year) fishes were highest in the upper layers of the water column (i.e. in the epilimnion down to 5 m, and down to 10 m in the benthic habitats). Along the tributary–dam axis in the pelagic habitats, both juvenile and adult fishes preferred the upper part of the reservoir, where the maximum number of species and also the greatest abundance of zooplankton were found. In the benthic habitats, fishes selected location according to factors other than trophic status. More juvenile fishes were recorded in the benthic habitats than in the pelagic habitats. Depth had the largest explanatory power for predicting fish community composition, followed by the affiliation with benthic and pelagic habitats, and location on the longitudinal axis of the reservoir. The fish community was represented mainly by cyprinids and consisted of two distinct groups of species, with bleak Alburnus alburnus, rudd Scardinius erythrophthalmus and asp Aspius aspius dominating the offshore group while perch Perca fluviatilis and ruffe Gymnocephalus cernuus were affiliated with the inshore group of the adult fish community. Roach Rutilus rutilus, bream Abramis brama and pikeperch Sander lucioperca occurred in important proportions in both the inshore and the offshore zones. All species, with the exceptions of adult perch (1+ year and older), 0+ year perch and 0+ year roach, preferred the most eutrophic tributary part of the reservoir. The fish community was relatively stable between the 2 years sampled.

Influence of watershed activities on the water quality and fish assemblages of a tropical African reservoir

Agricultural and fisheries activities around the watershed of an African tropical reservoir (Oyun reservoir, Offa, Nigeria) were found to contribute significantly to water quality deterioration of the dam axis of the reservoir, leading to eutrophication of that part of the reservoir. This is evident from the high amount of nitrate (6.4 mg/l), phosphate (2.2 mg/l) and sulphate (16.9 mg/l) in the water body which was higher than most other reservoirs in Nigeria. These nutrients originate in fertilizer run-offs from nearby farmlands and were found in higher concentrations in the rainy season which is usually the peak of agricultural activities in the locality. The eutrophication was more pronounced on the dam axis because it is the point of greatest human contact where pressure and run-off of sediments were high. The eutrophication altered the food web cycle which consequently affected the fish species composition and abundance with the dominance of cichlids (planktivorous group) and decline of some species in the fish population. Best management practices (BMP) to control and reduce the eutrophication and improve water quality and fish assemblages should be adopted and adapted to suit the situation in the reservoir.

Investigation of leakage at Ataturk dam and hydroelectric power plant by means of hydrometric measurements

Ataturk dam, with its 84.5 million m 3 fill volume and 169 m height above the foundation, is one of the largest dams in the world. Its 1.2 million m 2 grout curtain surface area is the largest constructed in the world to date. The Bozova fault crosses the dam axis in the left bank and extends towards the downstream valley. The grout curtain constructed in karstic limestone is 180 m deep in the thalweg and partially extends to 250–300 m depths in some local zones on both the right and left sides of the valley. In this study, we have investigated leakage at the Ataturk dam and hydroelectric power plant by means of hydrometric measurements. The hanging grout curtain indicates that leakage at Ataturk dam is of vital importance in defining the longevity of the dam. Hydrometric measurements, including discharge, temperature and electrical conductivity of springs and leakages, were carried out for 9 years periodically, in addition to monitoring performed before, during and after construction. The leakage increased as the reservoir water level rose seasonally but it was also discovered that this regular increase was not observed in the last three measurements. The decrease in leakage volume in the last three measurements proved the integrity of the grout curtain and the efficiency of the microsol grout works. Recent seepage levels are within design limits and therefore have no serious effect on the stability of the dam structures.

Earthquake Response Characteristics of a Fill Dam with a Semicircular Dam Axis

Earthquake Response Characteristics of a Fill Dam with a Semicircular Dam Axis

A dye-tracing test as an aid to studying karst development at an artesian limestone sub-aquifer: Zagros Zone, Iran

A dye-tracing test is employed to study the karst development and flow regime at an artesian limestone sub-aquifer, the Khersan3 Dam site, Zagros Zone, Iran. Tracer breakthrough curves showed an early dominant peak followed by a pronounced tailing effect. The peak concentration was a response to induced pressure during dye injection. The results suggest that the dye was pushed into the small pores and fissures around the injection point during dye injection. Hence, the dye moved out as a result of matrix and fissure diffusive processes and created a long pronounced tailing. The maximum flow velocity in the upper artesian sub-aquifer ranged from 0.97 to 2.9 m/h. However, the mean tracer velocity ranged from 0.19 to 0.51 m/h based on the mean residence tracer time; consequently, the flow regime in the artesian sub-aquifer was determined to be mainly diffusive. The results reveal (1) a low hydraulic gradient from upstream of the dam axis to downstream; consequently, there is no considerable flow; (2) poor karst development and diffuse flow at the tracing test area; (3) a discharge zone at a location downstream of the dam axis which is the main terminal of general flow direction at the dam site.

Engineering geological assessment of the Obruk dam site (Corum, Turkey)

Engineering geological properties of rock masses, such as discontinuities, degree of weathering, strength and hydraulic conductivities along the dam axis, and the excavatibility of diversion tunnels, power tunnels and spillways, were investigated in order to determine probable problems and necessary precautions to be taken prior to construction. In this study, the Obruk dam site, constructed on the Kızılırmak River to the north of Çorum, central Anatolia, was investigated from the standpoint of the aforementioned engineering geological aspects. Quaternary alluvium at the dam site has a thickness of about 65 m and overlies the Eocene basalt basement. Diversion and power tunnels were constructed in the basalts. In order to determine the distribution of the basalt, both horizontally and vertically, core samples were collected from preliminary geotechnical boreholes drilled by the General Directorate of the State Hydraulic Works (DSI), and the depth to groundwater was measured in these boreholes. The geological and geomechanical properties of the basalt are controlled by tectonism and weathering, and these properties vary over short distances. The mineralogical, petrographical, and geomechanical properties of the basalt core samples were determined. Based on these test results, the rock mass cropping out at the dam site has been classified for tunneling, and possible support systems suggested.

Earthquake safety assessment of concrete arch and gravity dams

Based on research studies currently being carried out at Dalian University of Technology, some important aspects for the earthquake safety assessment of concrete dams are reviewed and discussed. First, the rate-dependent behavior of concrete subjected to earthquake loading is examined, emphasizing the properties of concrete under cyclic and biaxial loading conditions. Second, a modified four-parameter Hsieh-Ting-Chen viscoplastic consistency model is developed to simulate the rate-dependent behavior of concrete. The earthquake response of a 278m high arch dam is analyzed, and the results show that the strain-rate effects become noticeable in the inelastic range. Third, a more accurate non-smooth Newton algorithm for the solution of three-dimensional frictional contact problems is developed to study the joint opening effects of arch dams during strong earthquakes. Such effects on two nearly 300m high arch dams have been studied. It was found that the canyon shape has great influence on the magnitude and distribution of the joint opening along the dam axis. Fourth, the scaled boundary finite element method presented by Song and Wolf is employed to study the dam-reservoir-foundation interaction effects of concrete dams. Particular emphases were placed on the variation of foundation stiffness and the anisotropic behavior of the foundation material on the dynamic response of concrete dams. Finally, nonlinear modeling of concrete to study the damage evolution of concrete dams during strong earthquakes is discussed. An elastic-damage mechanics approach for damage prediction of concrete gravity dams is described as an example. These findings are helpful in understanding the dynamic behavior of concrete dams and promoting the improvement of seismic safety assessment methods.

AUGMENTATION OF WATER RESOURCES AT WELLINGTON, TAMIL NADU-INTEGRATED GEOPHYSICAL INVESTIGATIONS FOR ITS PROPOSED DAM

ABSTRACT A 12 m high and 112 m long earthen dam was proposed to be constructed for augmenting surface water resources at Wellington, Coonoor district, Tamilnadu. For designine the foundation, the quality of sub surface formation was to be assessed and the geologically weak structures were to be delineated. As the topography of the area is not plane and also as thick lateritic formations are capping the deep-seated charnockite (bed rock), use of two different geophysical methods viz 1. Seismic Refraction and 2. Electrical Resistivity, was necessitated. The results of these two geophysical surveys have been integrated while characterizing the foundation. Two continuous seismic profiles along the proposed dam axis and four vertical electrical soundings in its reservoir bed were taken. The results revealed a four-layered stratum. The compressional wave velocities vary from 330 m/sec to 3345 m/sec and the resistivities from 22 ohm-m to 907 ohm-m. The low velocity and resistivity values (330 m/sec and 22 ohm-m respectively) are attributed to a loose and soft formation and their higher values (3345 m/sec and 907 ohm-m respectively) are attributed to a compact and hard formation. The depths of different geological layers obtained from these results were also co-related well with the borehole litho-logs. No geologically weak structures have been identified in the area by these investigations. The presence of compact formation and the absence of geologically weak structures at the site are favourable for founding the proposed structure.

Seepage problems in the karstic limestone foundation of the Kalecik Dam (south Turkey)

Seepage phenomena through the karstic limestone foundation of Kalecik Dam are investigated. The dam, designed as a rock-fill dam with a height of 77 m, is already used for irrigation. The foundation consists of Mesozoic ophiolite, Paleocene allochthonous units composed of different lithologies, and Miocene conglomerate. The conglomerate lies unconformably on the other units. Quaternary basaltic lava patchily covers the others. Seepage from upstream to downstream occurs through the allochthonous karstic limestone. This limestone, which is overlain by conglomerates, has a thickness increasing towards the right abutment. To prevent seepage at the right abutment, a 200-m long and 60-m deep grout curtain along the dam axis was constructed. After the impoundment, some springs occurred downstream, and for this reason, extra grouting was performed. However, the seepage problem could not be solved. In order to determine the seepage direction and karstification pattern, hydrological studies have been done. Additional investigation boreholes have been drilled to observe fluctuations of the groundwater level and to analyze hydrochemistry. Also, dye tracer tests have been carried out. As a result of these hydrogeological studies, seepage paths were observed in the karstic limestone located between the dam and the spillway. As those carbonate rocks continue beneath the spillway, the seepage problems are enjected to continue.

Geologic hazards of an embankment dam constructed across a major, active plate boundary fault

Abstract The geological structures associated with the site of the 55 million m 3 Karameh embankment dam constructed in the Jordan Valley and the tectonic effects on dam foundation and reservoir margins were reviewed. The dam crosses the strike-slip fault of the Jordan Valley Rift Zone. Trace evidence of the fault indicates a displacement of 8 to 15 m over a rupture length of some 130 km, which probably took place several centuries ago. Earthquakes with Richter magnitudes as great as 7.8 have occurred along the Jordan Valley Fault. Deterministic studies by Tapponnier (1992) indicated that the dam design should incorporate the possibility of a 7.8 event, a maximum horizontal rupture displacement on the fault of 10 m and a design peak ground acceleration (PGA) of 0.74 g at the site of the dam. These values are consistent with those which would be used in the USA for a similar case. However, the dam was actually designed by a consultant and constructed for a PGA of about a quarter of this value, based on seismic hazard analysis following guidelines of the International Committee on Large Dams (ICOLD) (1989). Moreover, the dam was designed for displacements of 6 m horizontal and 2 m vertically. Liquefiable sand layers were found in the dam foundation. A PGA of 0.50 g will trigger liquefaction of the sand layers in the dam foundation which would be expected to result in a crest settlement of 4.4 m. Slope stability analysis indicated deep failure planes in the foundation zone. The excavation of loose materials from under the dam foundation has not precluded the possibility of liquefaction occurring under the expected earthquake. Field mapping of geological features during the dam foundation excavation and construction revealed that: a) the most likely location of the Jordan Valley fault is in the area where the Wadi Mallaha stream crosses the dam axis, b) zones of en echelon type open fissures have been defined in the laminates sub-parallel to the Jordan Valley Fault Zone, c) at the Wadi Mallaha stream bed a parallel zone of faulting and warping of the Lisan Formation was identified, and d) the alignment is clearly confirmed by the exposure immediately upstream of the core at Ch 1375. The main wrench fault zone crosses the embankment footprint (upstream to downstream approximately) and reaches the surface around Ch 1375. The critical safety elements of the embankment are the core, the downstream fine filter, the chimney drain and the drainage blanket. To resist large earthquake events safely, the following safety measures should be implemented: 1. A freeboard of 7.0 m instead of the 5.0 m constructed. 2. The foundation of the dam should be stabilized against liquefaction. 3. The embankment internal zoning should be designed to accommodate damage resulting from earthquake events with a magnitude of 7.8. 4. The foundation needs relief measures downstream to lower the pore pressure. This paper describes the measures taken during construction as overall defense against future fault movements through a wide plastic core, an extensive upstream blanket, a 5.0-m thick downstream chimney filter and drain zones, a 5-m freeboard and an upstream crack stopper zone which may be critical for normal faults with a lateral extension component. The geological determination of the main wrench fault alignment resulted in the addition of an extra 2-m width to each of the already wide chimney filter and drain zones. In order to reduce potential seepage, local cut-off trenches or slush grouting were used for treatment of any open fissures at the upstream edge of the external blanket and the right bank ridge. The scale and scope of this dam and inherent engineering geological hazards are unprecedented. The design is considered deficient. This paper documents serious safety issues with the dam. The constructed dam presents serious safety risks and represents a case history of a disaster waiting to happen.

Development of water quality management strategies for the proposed isikli reservoir

A two-dimensional laterally averaged hydrodynamic and water quality model (CE-QUAL-W2) is used to simulate the water quality behavior in the proposed Isikli Reservoir of the Ankara Water Supply System to determine appropriate strategies for the management of water quality. Isikli application of the model is used to demonstrate the potential water quality degradation patterns in the reservoir under different pollution loads. The results of the modeling study are compared with the drinking water quality standards currently effective in Turkey to formulate various water quality management strategies for the reservoir and its catchment area. In addition, various treatment options are evaluated considering the existing treatment system of Ankara City. Based on the findings of this study, treatment or interception of wastewaters originating from the existing pollution sources around the reservoir area are introduced as the major infrastructural control techniques. Further, establishment of protection zones around the reservoir and relocating the possible pollution sources to a point downstream of the dam axis are found to be the most applicable administrative control methods for water quality preservation in the proposed reservoir.

Development of water quality management strategies for the proposed isikli reservoir

A two-dimensional laterally averaged hydrodynamic and water quality model (CE-QUAL-W2) is used to simulate the water quality behavior in the proposed Isikli Reservoir of the Ankara Water Supply System to determine appropriate strategies for the management of water quality. Isikli application of the model is used to demonstrate the potential water quality degradation patterns in the reservoir under different pollution loads. The results of the modeling study are compared with the drinking water quality standards currently effective in Turkey to formulate various water quality management strategies for the reservoir and its catchment area. In addition, various treatment options are evaluated considering the existing treatment system of Ankara City. Based on the findings of this study, treatment or interception of wastewaters originating from the existing pollution sources around the reservoir area are introduced as the major infrastructural control techniques. Further, establishment of protection zones around the reservoir and relocating the possible pollution sources to a point downstream of the dam axis are found to be the most applicable administrative control methods for water quality preservation in the proposed reservoir.

Geotechnical characterisation of Obudu damsite, Obudu, south-eastern Nigeria

The Obudu dam is being built across Abeb river in Obudu area of the Cross River State (Nigeria). The earthfill dam will be approximately 18 m high with a crest length of 385 m. The dam site is located within part of the Obudu crystalline basement plateau which is a region of low seismicity. The terrain is smoothly undulating and low lying and was known to be composed of unclassified basement and decomposed bedrock (overburden). The present study was carried out to assess the suitability of the chosen dam axis based on the determination of the nature and geotechnical characteristics of the overburden and bedrock. The investigation included geophysical surveys, bearing capacity tests (cone penetrometer and standard penetration), classification and grain size distribution as well as tests for compaction, consolidation and compressive strength. The results show that the bedrock is heterogeneous, including gneisses sillimanite, biotite and granite types), dolerite, quartzite and pegmatite with an overall moderate strength (about 76.04 MN m −2 average) and fair rock mass rating (RMR). The bedrock along the dam axis is apparently lacking in major fractures which could lead to short circuiting of the future impoundment. The overburden (soil) comprises silt (MH, ML), silty clay (CL) and silty sand (SM) with a combined thickness of about 2–20 m, increasing away from the valley floor towards the shoulders. Generally the overburden is of suitable compressive strength (150–300 KN m −2), low to medium plasticity and swelling potential and low permeability (up to 1.41 × 10 −7 ms −1) which would ensure a tight reservoir. The material settlement is expected to be small and slow. Accordingly, excavation of up to 5 m, decreasing towards the valley shoulders has been suggested to expose the recommended bearing medium: fresh/competent bedrock and overburden in the valley floor and flanks, respectively. While a detailed investigation of burrow areas was not part of the present study, a few potential sites in the dam axis and reservoir areas have been suggested based on the evaluation of material properties.

Fish species composition before and after construction of a main stem reservoir on the White River, Colorado

The completion in the fall of 1984 of Taylor Draw Dam on the White River, Colorado, formed Kenney Reservoir — thus impounding the last significant free-flowing tributary in the Upper Colorado River Basin. Fishes were sampled above and below the dam axis prior to closure of the dam and in the reservoir and river downstream following impoundment. While immediate effects of the dam to the ichthyofauna included blockage of upstream migration to 80 km of documented range for endangered Colorado squawfish, the reservoir also proved to have profound delayed effects on the river's species composition. Pre-impoundment investigations in 1983–1984 showed strong domination by native species above, within, and below the reservoir basin. By 1989–1990, non-native species comprised roughly 90% of the fishes collected in the reservoir and 80% of the fishes collected in the river below the dam. Initially, fathead minnow, whose numbers quickly increased in the new reservoir, dominated all post-impoundment collections, but red shiner became the most abundant fish collected in the river below the dam by 1989–1990. While agency stocking programs for the reservoir sought to emphasize a sport fishery for salmonids, primarily rainbow trout, local enthusiasm for warmwater sport fishes resulted in illicit transfers of these species from nearby impoundments. Several species, formerly rare or unreported in the White River in Colorado, including white sucker, northern pike, green sunfish, bluegill, largemouth bass and black crappie, were present in the river following impoundment. Our investigation indicates smaller-scale, main-stem impoundments that do not radically alter hydrologic or thermal regimes can still have a profound influence on native ichthyofauna by facilitating establishment and proliferation of nonnative species.

Test Embankment Dam of Fracture Grouting

A fracture-grouting test section is a 7.1-m-high homogeneous dam of compacted silty clay on a foundation of the same material. Four test holes with different depths and lengths of casing are drilled at the dam crest along the dam axis. The grouting pressure variation curves are measured while the holes were grouted, and the shape and orientation of grout vein are recorded while the test section is excavated. The fracturing pressure is found to be directly related to the stress of the dam where fracturing started. The fracturing in an uncased hole starts near the middle height of the dam and expanded laterally and vertically. If long casing restricts fracturing within the lower part of the dam, the grout vein will have uniform thickness along the depth and be able to extend into foundation soil. All the grout veins are parallel to the dam axis and vertical.

Curved ray seismic tomography: application to the Grand Etang Dam (Reunion Island)

Curved ray seismic tomography is used regularly by Compagnie de Prospection Géophysique Française (CPGF) for dam and nuclear site investigation. Measurements are made between tunnels, from hole to surface or, as here from hole to hole. The hydroelectric power development proposed for the Marsouins river (Réunion Island) involves a large capacity dam. Thc only suitable site is Grand Etang, a temporary lake situated in a deep valley, fillcd with heterogeneous materials, several hundred metres thick. During field investigations, a recent basaltic flow 5-3O m thick has been detected at a dcpth of 20-40 m, interbedded with silt and scoria. The following are successively examined in this article: -the data processing programme which takes into account curved ray paths and, after several iterations ends up with an optimum solution; - the hole-to-hole tornography surveys made between 13 drill holes on a 600 m profile; - the overall results, with thc limits and the discontinuities of the basaltic flow, showing that there is no link between the recent flow and older basalts of the valley borders. The hydroelectric development project on the Marsouins river, Reunion Island in the Indian Ocean is situated in a volcanic environment. It includes thc existing Takamaka dam on the river itself; it also involves a large capacity dam for flow regulation, for which the only suitable site is in an adjacent valley at Grand Etang (Fig. 1). This small temporary lake is situated in a deep valley, filled with several hundred metres of heterogeneous material including a recent basaltic flow. The Grand Etang water level varies notably according to rainfall; it does not have any free outlet and empties only by leakage through various pervious levels. Before undertaking the design of the dam and if necessary its impervious cut-offs, it seemed neccssary to secure better knowledge of lithology, structure and permeability of different horizons. In 1981-1982, preliminary surveys included surface resistivity soundings, seismic refraction and drilling. At the projected dam site, a recent basalt flow, 5 to 30 m thick and lying on silt, was discovered under 20 to 40 m of silt and scoriaceous overburden. Figure 2 shows a geological section along the valley. In order to complete information on this flow, a line of 13 drill holes, 40 to 60 m deep, were drilled in 1982-83 along the dam axis at an average spacing of 50 m. Gamma-ray logs were run in these holes. In order to determine the continuity of the flow between the holes and to appreciate the extent of weathering, a hole-to-hole seismic tomography survey was carried out. It should be noted that a previous surface seismic refraction survey had been unable to 'see' through the high velocity basalt flow.

Construction du barrage OA-11 sur la rivière Eastmain

The OA-11 dam is the most important earth structure of the Eastmain – Opinaca – La Grande (EOL) project within the "La Grande" hydroelectric complex at James Bay, Quebec. Three rivers were diverted to the La Grande River to increase the hydroelectrical potential of LG-2 power plant. Dam OA-11, 33.2 m high, has a zoned section with an impervious central core. The soil conditions of the existing overburden along the 3214 m dam axis were very variable.The presence of pervious overburden at considerable depth along the dam axis makes it necessary to use an impervious cutoff under most of the length of the dam. On the right shore, a slurry trench was therefore built. It is 1.5 m wide, 2180 m long, and varying in depth from 3 to 22.7 m. In the river bed, the dam was built on an embankment pad of granular material pushed into the water and compacted by vibroflotation. A cast-in-place concrete wall was constructed to insure a cutoff through the pervious soils of the pad and foundation. This diaphragm wall is 595 m long, 0.6 m thick, and varying in depth from 2.6 to 35.7 m.This paper describes the different phases in dam construction with emphasis on techniques used for river closure. Key words: dam, compaction, construction, diversion, closure, cast-in-place concrete wall, river, slurry trench, vibroflotation.

A Geophysical Method of Evaluating Existing Earth Embankments

It has long been recognized that earth embankments need periodic inspection and monitoring, especially if settlement or damage is suspected. Currently, earth embankment condition is evaluated through visual inspection and analysis of data from instrumentation such as piezometers and inclinometers. A program to evaluate the condition of existing earth embankments using geophysical methods was developed. This technique is currently being used as part of a safety evaluation of existing earth embankments in the western United States. The geophysical method utilizes the “common offset” seismic refraction technique which emphasizes areas of change. Seismic lines are run along the dam axis and toe. Anomalous areas, if any, are targeted for drilling. Examples of this technique, applied to various dams, are illustrated and the results presented. It is anticipated that a base line data file will be established for all the major earth dams in the United States. New seismic lines could then be run after a dam has undergone an event such as an earthquake and the two sets of data compared. It is also anticipated that more detailed information will be gained from the seismic data as the program continues and drilling data become available for direct comparison.