Presented by: GM Prasad General Manager (Design & Engg.) THDC India Limited, Rishikesh

G. M. Prasad

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Page 1: G. M. Prasad

Presented by:

GM Prasad

General Manager (Design & Engg.)

THDC India Limited, Rishikesh

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General Site Selection Criteria for Dams

General Investigations for Dam Sites

Case Study of Tehri Dam

Investigations for Tehri Dam ( Earth & Rock Fill Dam)

Investigations for Pipalkoti Dam (Concrete Gravity Dam).

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INTRODUCTION Selection of suitable site for dams is a very important aspect of

planning for Hydro Electric Projects. It depends upon variousparameters viz., topography, geology / seismicity of the area,foundation characteristics, water availability, availability ofrequired material within reasonable distance, accessibility of thesite etc.

After the preliminary selection of site, another important aspectis thorough investigations of the site. However, an experiencedEngineer can predict the likely type of dam for a given sitewithout requisite investigations being done.

Time and Money spent in doing the investigations always leadsto a wise engineering decisions and avoids the various kinds ofsurprises during execution, delay in completion and making theproject expensive.

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The River cross – section at the dam site should preferablyhave a narrow gorge to reduce the length of the dam.

The gorge should open out upstream to provide large basinfor a reservoir.

In case there is a confluence of two or more rivers in theselected reach, the dam should preferably be locateddownstream of the confluence to take advantage of the flowof all rivers.

The dam should be preferably located where the river bed ishigh, so that the height and cost of the dam is reduced.

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Suitable foundation should exist at the site for the particulartype of dam. If suitable foundation is not available but it canbe improved by adopting various measures, the site may beconsidered for selection.

In general foundations should be free from seams, openpockets or fault planes.

Formation of hard and soft layers alternates are not generallygood, because the penetration of water may weaken the softlayers and lead to movement along them.

Requirements of foundation are more stringent for ConcreteGravity dams as compared to the Earth & Rockfill dams.Stability of concrete dam is controlled principally by theadequacy of the engineering properties of the foundationmaterial such as - Load bearing capacity , Shearing strength,Permeability and Erosion Resistance etc.

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Seismicity of Area:

In highly seismic zones, first choice for theconstruction of the dams is for Earth & Rockfill typedams as such dams have inherent property of highshock absorption during the event of shaking.

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Reservoir and Catchment Area: The site should ensure adequate storage capacity of reservoir

basin at minimum cost.

The cost of land and property submerged in the water spreadarea should be minimum.

The geological conditions of the catchment area should besuch that percolation losses are minimum and maximumrunoff is obtained.

The site should be such that a deep reservoir is formed afterthe construction of the dam. A deep reservoir is preferred to ashallow reservoir because in the former the evaporation lossesare small, the cost of land acquisition is low and the weedgrowth is less.

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The site should be such that the submerged area is minimum.It should not submerge costly land and property. It shouldnot affect the ecology of the region. Monuments of historicaland architectural importance should not be submerged.

The dam site should be such that the reservoir would not siltup quickly. The life of the reservoir depends upon the rate ofsilting. The site should be selected such that it avoids orexcludes the water from those tributaries which carry a highpercentage of silt

The soil and rock mass at the reservoir site should not containany objectionable soluble minerals which may contaminatethe water. The stored water should be suitable for the purposefor which the water is required.

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Spillway site: A good site for a spillway should exist at or near the darn site. The valley

should be sufficiently wide to locate the spillway if it is an integral partof the dam. If the spillway is to be located separately, the best site ofspillway is that in which there is a saddle near the dam site which isseparated from it by a hillock. In that case, the main dam can belocated in the gorge and the spillway can be constructed in the saddle.

If the site conditions are such that the spillway must be located in themain river channel, the types of dams that can accomodate an overflowspillway should be considered.

Material: Material required for a particular type of Dam should be available

nearby, cost of construction is quite dependent on this factor. For theEarth & Rockfill dam, material for placement in all the zones should beavailable locally. In case of concrete dams, the main requirement is foravailability of aggregates for production of concrete.

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It would be preferable to select a site which is connectedby a road or rail link or can be conveniently connected sothat Man , Machinery and other equipments can easilybe transported to the site.

Low cost of real estate:

The cost of real estate for the reservoir site, dam,dwellings, roads, railways, etc. should be low.

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Minimum overall cost: The site should be such that it entails the minimum overall

cost of the project, including subsequent maintenance. Generally, two or three probable sites are selected and rough

estimates are made. The site which entails the minimumoverall cost is then selected.

Other considerations: For the development of a particular backward area, the dam

may be constructed in that region. Sometimes public opinionand economic consideration of the region may affect the siteselection of a dam.

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Based upon the stage of a project, investigationscarried out for selection of site can be categorised intofollowing three stages:

Preliminary Investigations

Detailed Investigations and

Pre Construction Investigations

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Preliminary Investigations:

To avoid expenditure on detailed investigation it isalways advisable to prepare preliminary feasibility reporton the basis of preliminary investigations.

Preliminary Investigations cover mainly the followingaspects:

Water Availability Studies

Hydrological Studies

Topographical Survey

Geological and Foundation Surveys

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Detailed investigations:

Detailed Investigations are carried out if afterpreliminary investigation project is found to be feasible,so that detailed project report can be prepared in orderto evaluate cost and benefits from project with accuracy.

Pre construction investigations

Pre construction investigations are done to do detailed structural design.

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Based upon the type of investigations, it can becategorised as below:

Surface Investigations

Subsurface Investigations

Geotechnical investigations on construction materials.

Hydro Meteorological Investigations

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Surface Investigations: Preliminary Surveys Satellite Imageries. Topographical Surveys Geological Mapping.

Sub Surface Investigations:

Drilling and logging. To decipher the thickness of overburden, weathering and

distressing of rockmass, depth of sound rock and rock massquality value.

Test Pits. An excavation made to examine foundation and to determine

whether an area is suitable for dam construction; includes thetaking of rock samples and the determining of the depth ofoverburden.

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Geological features predicted based upon drilling and loggingcan be physically verified through drifts. Drifts also provides afacility for conducting in-situ tests.

Geophysical Investigations.

Assessment of elastic and stress strain characteristics of rocks inthe area.

In situ Tests.

In-situ tests are carried out to find actual values of variousparameters (e.g. Permeability, stress etc.) at a particular site totake into account the influence of site conditions.

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Geotechnical Investigations Depending upon the type of Construction Materials to be used: Grain Size Distribution Mineralogical composition Atterberg Limits Shear & consolidation Parameters Density Permeability Dispersivity & erodibility etc. Crushing value Soundness, Alkali aggregate reactivity Abrasivity etc.

Hydro Meteorological Investigations: River Discharge Rainfall Silt Load Temperature Wind Velocity Snow Cover Area.

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Based upon the Investigation data, followinginformation is generated: Geological & Tectonic Data.

Flood Estimation at Dam site.

Water Availability .

Sedimentation Data

Design of foundation treatment scheme

Selection of fill materials and Design properties

Design of Diversion Structures.

Fixation of Dam height.

Design of Spillway System .

Power Potential .

Life of reservoir etc.

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Case Study

Tehri Dam Site

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Salient Features of Tehri Dam

Type : Earth & Rockfill

Height of dam : 260.5 m

Base Width : 1125 m

Width at top : 25.5 m

Length at the top : 592.7 m

U/s Slope : 2.5H : 1V

D/s Slope : 2H : 1V

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Tehri site was having following features which made itan ideal location for building such a huge Giganticstructure.

Main topographic feature which makes it ideal locationare:

The River cross – section at the dam site was a narrow gorgeand it opens upstream. Which means reduced length of Damand sufficient space for reservoir.

Dam is situated at the confluence of two rivers i.e. Bhagirathiand Bhilanagana and having advantage of flow of both Rivers.

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Material For Construction:

For construction of such a gigantic structure hugeamount of construction material (about 279.8 lac cu.m)was required. At Tehri site, main construction materialsrequired for the core and shell zones of the dam i.e. clayand sandy-gravelly material were available within adistance of about 3 to 5 km of Dam site, which made it afavourable site for construction of an Earth & Rockfilldam.

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Investigations for 260.5 m high TehriDam ( Earth & Rock Fill Dam)

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After selection of ideal topographical location for the Tehri Dam,detailed Geological investigation were carried out in the damfoundation area and Geo-technical Investigations on the fillmaterials, for finalisation of various design details.

Geological investigations were started in 1961 by GSI and werefurther continued till the construction stages. Geo-technicalInvestigations were carried out with the involvement of variousreputed agencies viz., IRI, Roorkee; CSMRS, New Delhi; IIT,Roorkee; CMRI, Dhanbad etc.

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To determine the thickness of river borne material, at the Dam site following geological explorations were made:



Geophysical profiling,

Geological logging of the under riverbed portion of the foundation through exploratory drifts. etc.

Besides above, following investigations were also made:

In situ water percolation tests in dam foundation, Test grouting in dam core foundation, Compression tests on rock core, In situ rock modulus tests, Hydro fracturing tests in dam core foundation etc.

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Exploration by Drill Holes

In order to decipher the thickness of overburden, weatheringand distressing of rockmass and depth of sound rock oneither abutment and in the riverbed section, about 76 nos. ofdrill holes were drilled in the entire dam area.

Depth of the drill holes ~ 12 m to 150 m.

Out of total 76 drill holes, 27 drill holes fall in the core seatarea.

The thickness of estimated river borne material in the maindam core area ~ 4m to 14m.

In the coffer dam core area, on the R/A, thickness ofoverburden ~ 25m and depth to relatively sound bedrock as40m.

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Exploration Through Drifts

About 31 nos. of exploratory drifts of size 2m X 2m involvingabout 1470 m of drifting were excavated.

10 drifts (4 on the L/A and 6 on R/A) involving about 400mlength were excavated in the dam core portion to assess therock mass conditions in the abutments and demarcate thelimits of overburden mass, weathered or distressed rockmass,slump zone and also to decipher sound rock profile. It helpedin determining the extent of stripping in the core foundationarea.

The inferred depth of stripping on the left abutment ~ 15m to30m and ~ 15m to 21m on the right abutment.

Plate load tests were also conducted inside the exploratorydrifts for determining the modulus of elasticity and themodulus of deformation of rock mass.

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Geophysical Investigations:

Objective of geophysical investigations was:

Identification of the thickness and state of the zones ofsurface weathering and distressing;

Determination of surface of relatively sound bedrock;

Identification and aerial tracing of tectonic fault zones;

Assessment of elastic and stress strain characteristics ofrocks in the area in their bedding;

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To fulfil above mentioned objectives following works werecarried out at the Dam site:

Dam core abutment area was covered with 2774 m ofseismic profiles.

~ 2024 m of seismic shooting in the L/B.

~ 750 m in the R/B.

The main work was done with 5-6 m spread ofgeophones, shot-points spaced at 25-115 m andmaximum hodograph length of 230 m.

The total number of profiles covering the left & rightbank slopes was 14 & 6 respectively.

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Main findings of the Geophysical Investigations:

The average thickness of the zone of surface weathering anddistressing on the left bank slope ~ 10-20 m and that on theright bank ~ 20-25 m. The bedrock within the zone ofweathering and distressing was characterized by Longitudinalwave velocity (Vp) values varying from 1.0 to 3.9 km/s.

Value of Vp ranged from 3.2 to 6.3 km/s in relatively soundbedrock of the left bank rock mass and from 3.2 to 5.2 km/s inthe right bank rock mass. The modulus of deformation D-varied from 2000 to 16000 Mpa.

The bedrock within the tectonic fault zones wascharacterised by Vp values varying from 1.6 to 2.6 km/s. Themodulus of deformation in the zones of tectonic faults variedfrom 500 to 2000 Mpa.

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In situ water percolation tests:

Tests Before Stripping of dam core foundation area : Waterpercolation tests were carried out in about 58 nos. drill holes, out ofwhich 27 nos. of holes were in the core seat area.

Tests After Stripping of dam core foundation area :5 nos. of additional in-situ permeability tests were conducted to getadditional information about the permeability of foundation rocks.

Tests during Curtain Grouting Work:Variation of in-situ permeability of rockmass along the grout curtain,was also observed by conducting water loss tests in u/s row of groutcurtain during the course of actual grouting work.It was seen that the permeability in the riverbed zone was very low.Further, permeability of the right abutment was slightly more ascompared to the left abutment.

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Test grouting:

To ascertain the groutability of rock mass and to developsuitable grouting programme, Test grouting was carriedout on three platforms in the dam area in three grades ofphyllites exposed on the right bank.

A triangular pattern of trial grouting was adopted for allthe platforms.

Efficiency of grouting was measured in terms ofpercentage of reduction of pre grout permeability ofinspection holes.

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In- situ Rock Modulus Tests:

In order to determine the modulus of elasticity and themodulus of deformation, cyclic plate load tests werecarried out in different grades of rocks in 14 driftslocated on the right bank of Tehri Dam prior to Dec.1973. The tests were carried out across and along thefoliation planes.

Further (after Dec. 1973) in-situ plate bearing anduniaxial jacking tests have also been performed inexploratory drifts for determination of modulus ofelasticity and deformation modulus in different gradesof phyllites.

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Hydrofracturing Tests in Core Foundation rocks:

To find out the reopening pressure of existing joints inthe core foundation rocks of Tehri dam, Hydrofracturingtests using Minifrac system were conducted in about 10nos. of drill holes.

Tested depth was upto 15m from the stripped coresurface.

Re-opening pressure as obtained from the tests hadranged from 17.25 to 41.4 kg/cm2.

Based on these test results, maximum limit of groutingpressure in the dam foundation was kept in between 10to 12 kg/cm2.

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During the period from 1963 to 1971, a number of verticaland inclined drill holes were put down in the river bedsection and from both the abutments.

The excessive loss of core recovery in the inclined holescoupled with other features indicated existence of amajor shear zone on downstream. However, thecomplete absence of its exposure on any of theabutments, particularly in the reaches where river flowsthrough the bends had created doubts about itsexistence and disposition.

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For making a reassessment , additional vertical andinclined holes were put down in the river bed. Thedrilling data did not indicate any prominent fault zonein the river bed. 3 drill holes were electrically loggedobservations made from which provided support to theinterpretation that the drill holes have not intersectedany major shear zone.

Subsequently, in early 1987, a 63.9 m long exploratorydrift was driven under the riverbed from the R/B to theL/B. This drift was located at about 55m d/s of the maindam axis. 3D geological logging of this drift convincinglydisproved the existence of a riverbed shear zone at thedam site.

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Construction of Tehri dam required about 27.98 M Cu.mof fill materials for the core, transition, shell & riprapzones. In order to evaluate the suitability of thesematerials, preliminary investigations at 14 different sitesfor core material (within a distance of 5 to 35 km fromdam site), 2 sites for transition and shell materials andvarious sites for riprap material were conducted.

After preliminary investigation for construction materialfew sites were selected to carry out the detailedinvestigations. For clay Koti borrow area and for shellmaterial Dobata area was selected.

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Investigations at Koti borrow area (for clay): The area was first investigated with 40 auger holes of shallow depth

and samples collected and analysed for grain size classification.Subsequently 22 pits were dug in the whole area, some pits were asdeep as 20 m.

To determine the Grain Size Distribution, 82 samples were collectedfrom the 22 pits at various depths. From these, the average gradationcurves for the pits were worked out. Atterberg’s Limits for all the 82samples were determined and samples were classified as per BISclassification.

For determining Mineralogical Composition, three samples weresubjected to X-ray diffraction analysis.

In order to check the Dispersivity of soil, 76 samples taken fromdifferent pits at various depths were subjected to various tests viz,Crumb test, S.C.S. Laboratory dispersion tests, Percentage of sodiumsalts in pore water and Pin hole test.

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Permeability tests were conducted in laboratory on 10 samplescollected from various pits. Average permeability was worked out as0.63 X 10-7 cms/sec .

Specific gravity tests were conducted on 49 disturbed samples. Theaverage specific gravity of the material was 2.68.

Insitu wet density, Insitu moisture content and Insitu dry density offour undisturbed samples were also determined.

Average Optimum moisture content and corresponding optimum drydensities at standard proctor’s compaction effort for 45 samples takenfrom various pits at different depths, were worked out as 14-15% and1.827 gm/cc respectively.

The consolidation characteristics of Koti Borrow area material havebeen determined by conducting one dimensional tests on 21 samples

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Triaxial shear tests:

Unconsolidated undrained tests

Consolidated drained tests

Consolidated undrained tests

Hydraulic fracturing tests.

Dynamic Shear Modulus:

Block Vibration tests

Wave Propagation tests

Vertical Dynamic Plate Load Test

Dynamic shear parameters .

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Investigations at Dobata borrow area (for shellmaterial):

Shell material has been borrowed from Dobata borrow area.For exploring the Dobata terrace, 63 & 42 nos. of pits were dugin old and new Dobata areas respectively. Samples taken fromthe pits were subjected to various tests as below:

Grain size classification .

Direct Shear tests.

Triaxial shear tests.

Field and laboratory Permeability tests .

Oedometer tests (one dimensional compression tests) .

Long term deformability tests.

Dynamic Shear Modulus test.

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Investigations / Testing of filters:

Transition zone between core and shell zones has beendesigned as filter. Taking into consideration of the risk ofcrack formation in the core, the material used for these zonesranged from sand to gravel size. Following investigations andtests were conducted for selection of filter material.

Permeability tests.

Internal erosion of filters tests.

Crushing ratio (particle breakage).

Triaxial shear tests .

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Investigations for Riprap material: For the protection of the dam slopes and increasing stability of

slopes during seismic activity, a well graded hard blasted rock i.e.riprap (5 mm-1200 mm) has been provided on both the slopes of thedam. Suitability of prospective quarry sites for this material wasevaluated in terms of its rock quality, quantity and distance from thedam site.

Tests and studies conducted in borrow areas for checking thesuitability of Riprap material were as below: Compressive Strength. Bulk Density. Softening Coefficient. Water Absorption. Triaxial Shear Tests Experimental Study for estimation of Seismic Pore Water

Pressures . Ground Penetrating Radar Survey for estimation of overburden in

quarry areas.

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Investigations for Pipalkoti Dam (Concrete Gravity Dam)

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Salient Features of Pipalkoti Dam

Type : Concrete Gravity Dam

Height of dam : 65 m

Length at the top : 89.3 m

River : Alaknanda

Location : District Chamoli(Uttarakhand)

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For the selection of Dam site 5 different sites wereexplored during the Preliminary Stage of the Project.

Upper Barrage Site (B-1)

Site was suitable for construction point of view but advantageof discharge of two tributaries (Karmansa and Kalpa Gangariver) of Alaknanda river meeting just downstream of thissite is not there.

Lower Barrage/Dam Site 1

This site is not taking full advantage of water head available.

Dam Site 2

This site have strong possibility of Rock Fall on the rightabutment.

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Dam Site 3

This site does not appear suitable as about 20 m thick riverborne material terrace exists above water level on both thebanks of river.

Dam Site 4

Dam Site 4 was selected to carry out the DetailedInvestigation as this site was free from the Danger of Rock Fallas compared to other sites. Also on preliminary investigation,anticipated depth of overburden was less as compared toother explored sites.

Finally proposed dam site is located 50 m d/s of the D4 site.

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Geotechnical Explorations and FieldInvestigations:

For obtaining the information with regard to geological

characteristics of rockmass and depth of overburden etc.,detailed geotechnical explorations through exploratory driftsand drilling were carried out at Pipalkoti Dam site.

Drifting: In order to explore the rock condition, two exploratory drifts of

size 2 m x 1.8 m on the left and right bank of the river Alaknandawere excavated. From these two Drifts, cross cuts have beenexcavated.

In total 150 m of drifting was done at Pipalkoti Dam site.

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In order to decipher the depth of overburden and nature offormation below the Dam site, ten nos. of boreholes weredone during the DPR stage. In total 500 m of Drilling iscarried out at the Dam site. Based on the investigations, depthof overburden in the riverbed has been estimated about 20m.

Subsequently, in order to have an idea about the depth ofcolluvium material on the left bank of the river (in the vicinityof dam), three nos of boreholes were also done in the area atabout 100m u/s of the dam site. Depth of alluvium in that areahas been estimated around 40 to 50m.

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Seismic Refraction Survey:

To determine the depth of overburden, level of rock at damsite and to infer rock quality, seismic refraction survey both onland and underwater along five traverses was carried out.

The lengths of two land profiles for the survey was between100 m to 115 m while the same for underwater profiles rangedfrom 50 m to 120 m.

The subsurface strata information inferred from this surveywas used in designing the foundations of various structures.

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On Land: Top layer having compression wave velocity between 300 m/sec and

600 m/sec was inferred to be loose boulder bed (thickness 3.5 to 11m).

Land having wave velocity between 800 m/sec and 1200 m/sec wasinferred to be compact / partially saturated boulder bed.

The quartzite rock velocity along land profiles ranged between 3000m/sec and 4000 m/sec.

Underwater: Velocity below 2000 m/sec was interpreted to be saturated bouldary


Layer having velocity between 2000 m/sec and 3000 m/sec mayrepresent compact bouldary bed / weathered quartzite rock

Rock velocity varied from 4000 m/sec to 6000 m/sec.

The depth to rock varied from 10m to 22m.

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Estimation of Geo-mechanical Properties of Rock Cores:

From the dam site, test samples were collected from three drill holes.

Based upon the laboratory testing following properties of the rock were determined:

- Unit Weight (25.7 to 28.4 KN/m3)- Void Index (3.8 to 6.1)-Specific Gravity (2.8)-Brazilian Tensile Strength (9.13 to 17.95 Mpa)- Uniaxial Compressive Strength (43.2 to 102.13 Mpa)- Tangent Modulus (2.99 to 14.12 Gpa)- Poisson’s ratio (0.21 to 0.26)- Shear Strength Parameters (C = 20.71 to 37.19 Mpa, Ф = 45.8 to 47.90).

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