Presented by: GM Prasad General Manager (Design & Engg.) THDC India Limited, Rishikesh
CONTENTS
Introduction General Site Site Selection Selection Criteria Criteria for for Dams General General Inve Investiga stigations tions for Dam Sites Sites Case Study of Tehri Tehri Dam Investiga Investigation tionss for Tehri Dam ( Earth Earth & Rock Fill Fill Dam) Dam) Investigations Investigations for Pipalkoti Pipalkoti Dam Dam (Concret (Concretee Gravity Gravity Dam).
INTRODUCTION
Selection of suitable site for dams is a very important aspect of planning for Hydro Electric Projects. It depends upon various parameters viz., topography, geology / seismicity of the area, foundat foundatio ion n chara charact cteri eristi stics cs,, wate waterr avail availabi abilit lityy, avail availabi abilit lityy of required required material within reasonable distance, distance, accessibility accessibility of the site site etc. etc. After the preliminary selection of site, another important aspect is thorough thorough investigations investigations of the site. Howev However er,, an experienced Engineer can predict the likely type of dam for a given site without requisite requisite investigations investigations being done. Time and Money spent in doing the investigations always leads to a wise engineeri engineering ng decisions and avoids avoids the various various kinds kinds of surprises during execution, delay in completion and making the project project expensiv expensive. e.
GENERAL SITE SELECTION CRITERIA FOR DAMS
Topography:
The River cross – section at the dam site should preferably hav have a narro narrow w gorg orge to reduc educee the the leng length th of the the dam. dam. The gorge should open out upstream to provide large basin for for a reserv eservoi oirr. In case there is a confluence of two or more rivers in the sel select ected reac each, the dam dam shou shoulld preferab erablly be locat cated downstream of the confluence to take advantage of the flow of all rivers. The dam should be preferably located where the river bed is high, high, so so that that the the heig height ht and cost of the dam is reduc reduced. ed.
GENERAL SITE SELECTION CRITERIA FOR DAMS
Foundation:
Suitable foundation should exist at the site for the particular type of dam. If suitable foundation is not available but it can be improved by adopting various measures, the site may be consider co nsidered ed for selection. selection. In gener eneral al foun founda dati tion onss shou should ld be free free from from seam seams, s, open open pock pocket etss or faul faultt plan planes es.. Formation Fo rmation of hard and soft layers layers alternates alternates are are not generally generally good, because the penetration of water may weaken the soft layer layerss and lead to move movement ment along them. Requirements of foundation are more stringent for Concrete Gravity dams as compared to the Earth & Rockfill dams. Stab Stabil ilit ityy of con oncr cret etee dam dam is con ontr trol olle led d princ princip ipal ally ly by the the adequa adequacy cy of the engine engineeri ering ng prope properti rties es of the foundat foundation ion material such as - Load bearing capacity , Shearing strength, Permeab Permeability ility and Erosion Erosion Resista Resistanc ncee etc. etc.
GENERAL SITE SELECTION CRITERIA FOR DAMS
Seismicity of Area: In highly seismic zones, first choice for the construction of the dams is for Earth & Rockfill type dams as such dams have inherent property of high shoc shock k abso absorp rpti tion on duri during ng the the even eventt of sha shaki king ng..
GENERAL SITE SELECTION CRITERIA FOR DAMS
Reservoir and Catchment Area:
The site should ensure adequate storage capacity of reservoir basi basin n at minim minimum um cost. ost. The cost of land and property submerged submerged in the water spread spread area area should should be minimu minimum. m. The geological conditions of the catchment area should be such such that that perc percol olat atio ion n loss losses es are are mini minimu mum m and and maxi maximu mum m runoff runoff is obtaine obtained. d. The site should be such that a deep reservoir is formed after the construction of the dam. A deep reservoir is preferred to a shallow shallow reservoir because because in the former former the evaporation evaporation losses are small, the cost of land acquisition is low and the weed growth is less.
GENERAL SITE SELECTION CRITERIA FOR DAMS
The site should be such that the submerged area is minimum. It should should not submerge submerge costl costlyy land and property property.. It should should not affect the ecology of the region. Monuments of historical and archit architectu ectural ral importanc importancee should should not not be submerg submerged. ed. The dam site should be such that the reservoir would not silt up quickly. The life of the reservoir depends upon the rate of silting. The site should be selected such that it avoids or excludes the water from those tributaries which carry a high perce percentag ntagee of silt silt The soil soil and rock mass at at the reserv reservoir oir site site should should not contain contain any objectionable soluble minerals which may contaminate the water. water. The stored water should be suitable for the purpose for for which hich the the water ater is requi equirred. ed.
GENERAL SITE SELECTION CRITERIA FOR DAMS
Spillway site: A good good site for for a spillwa spillwayy should should exist at or near near the darn site. site. The valle valley y should be sufficiently wide to locate the spillway if it is an integral part of the dam. If the spillway is to be located separately, the best site of spillway is that in which there is a saddle near the dam site which is separated from it by a hillock. In that case, the main dam can be locat located ed in the gorg orge and the spill spillwa wayy can be co const nstruc ructed ted in the the saddle saddle..
If the site conditions are such that the spillway must be located in the main river channel, the types of dams that can accomodate accomodate an overflow spillway spillway should be considered. considered. Material : Material required for a particular type of Dam should be available nearby, cost of construction is quite dependent on this factor. For the Earth & Rockfill dam, material for placement in all the zones should be available locally. In case of concrete dams, the main requirement is for availability availability of aggregates aggregates for production production of concrete. concrete.
GENERAL SITE SELECTION CRITERIA FOR DAMS
Accessibility: It would be preferable to select a site which is connected by a road road or rail rail link link or can can be con onvvenie enient ntly ly con onne nect cted ed so that Man , Machinery and other equipments can easily be tran transp spor ortted to to the the site site..
Low cost of real estate:
The cost of real eal esta estatte for the the reser eservvoir sit site, dam, am, dwellings, dwellings, roads, railways, etc. should s hould be low. low.
GENERAL SITE SELECTION CRITERIA FOR DAMS
Minimum overall cost:
The site should be such that it entails the minimum overall cost co st of the project, project, including including subsequ subsequent ent mainte maintenanc nance. e. Generally, two or three probable sites are selected and rough estimates are made. The site which entails the minimum over overall all cost is then select selected. ed.
Other considerations considerations:: For the development of a particular backward area, the dam
may be constructed constructed in that region. Sometimes public opinion and economic consideration of the region may affect the site sele select ctio ion n of a dam. dam.
GENERAL INVESTIGATION FOR DAM SITES
Base Based d upon the sta stage of a projec ject, inv investi estiga gati tio ons carried out for for selection of site can be categ categorised orised into follo following wing three three stages: stages:
Preliminary Investigations Detailed Investigations and Pre Construction Investigations
Preliminary Investigations:
To avoi oid d expe expend ndit itur uree on deta detaile iled d inv investi estiga gati tion on it is always advisable to prepare preliminary feasibility report on the basis of preliminary investigations. investigations. Preliminary Investigations cover mainly the following aspects:
Water Availability Studies Studies Hydrological Studies Topographical Survey Geological and Fo Foundation undation Surveys
Detailed Detailed investig investigation ations: s:
Detailed Investigations are carried out if after preliminary investigation project is found to be feasible, so that detailed project report can be prepared in order to evalu evaluat atee cost and benefits benefits from from proje project ct with with accur accuracy acy..
Pre construction investigations investigatio ns
Pre constructi construction on investigations investigati ons are done to do detailed structural design.
GENERAL INVESTIGATION FOR DAM SITES
Based upon the type of investiga igations, it can be categorised as below:
Surface Investigations Subsurface Investigations Geotechn Geotechnical ical investiga investigation tionss on co constru nstructio ction n materials materials.. Hydro Meteorological Investigations
Surface Investigations: Preliminary Surveys Satellite Imageries. Topographical opographica l Surveys Sur veys Geological Mapping.
Sub Surface Investigations:
Drilling and logging.
To deciph decipher er the the thickn thicknes esss of over overbur burden den,, weathe eatherin ring g and distressing of rockmass, depth of sound rock and rock mass qualit qualityy value. value.
Test Pits.
An excavation made to examine foundation and to determine whether an area is suitable for dam construction; includes the taking of rock samples and the determining of the depth of overburden.
Drifts. Geological features predicted based upon drilling and logging can be physicall physicallyy verified verified through through drifts. drifts. Drifts Drifts also provide providess a facility facility for con conduct ducting ing in-situ in-situ tests tests..
Geophysical Investigations. Assessment of elastic and stress strain characteristics of rocks in the the area area..
In situ Tests Tests.. In-s In-sit itu u test testss are are carr carrie ied d out out to find find actu actual al valu values es of vari variou ouss parameters (e.g. Permeability, stress etc.) at a particular site to take take into into acco accoun untt the inf inf luenc luencee of site co cond nditi ition ons. s.
Geotechnical Investigations Investigations Depending upon the type of Construction Materials to be used:
Grain Size Distribution Mineralogical composition Atterberg Limits Shear & consolidation Parameters Density Permeability Dispersivity Dispersivity & erodibility etc. Crushing value Soundness, Alkali aggregate reactivity Abrasi Abrasivity vity etc. etc.
Hydro Meteorological Investigations:
River Discharge Rainfall Silt Load Temperature Wind Velocity Snow Cover Area.
Based upon the Investigation informat information ion is gener generate ated: d:
data,
following
Geological Geologica l & Tectonic Tectonic Data. Flood Estimation at Dam site. Water Availability . Sedimentation Data Design of foundation treatment scheme Selection of fill f ill materials and Design properties Design of Diversion Structures. Fixation of Dam height. Design of Spillway System . Power Potential . Life of reservoir reser voir etc.
Case Study Tehr ehrii Dam Site
Salientt Feat Salien Feature uress of Tehri Tehri Dam
Type Height of dam Base Width Width at top Length at the top U/s Slope D/s Slope
: : :
Earth & Rockfill 260.5 m 1125 m : 25.5 m : 592.7 m : 2.5H : 1V : 2H 2 H : 1V
Tehri site was having following features which made it an ideal location for building such a huge Gigantic structure.
Main topographi topographicc feature feature which which makes makes it ideal location are:
The River cross – section at the dam site was a narrow gorge and it opens upstream. Which means reduced length of Dam and sufficient sufficient space space for for reservo reservoir ir.. Dam is situated situated at the the conf confluence luence of two rivers rivers i.e. Bhagirathi Bhagirathi and Bhilana Bhilanagana gana and havin having g advantag advantagee of flow f low of both both Riv Rivers ers..
Material For Construction: For con onst stru ruct ctio ion n of such such a giga gigant ntic ic stru struct ctur uree hug huge amount of construction material (about 279.8 279.8 lac cu.m) cu.m) was required. At Tehri site, main construction materials required required for the core core and shell zones of the dam i.e. clay and and sand sandyy-gr graavelly elly mate materi rial al were ere avail vailab able le with within in a distance of about 3 to 5 km of Dam site, which made made it a favourable site for construction of an Earth & Rockfill dam.
Investigations for 260.5 m high Tehri Dam ( Earth & Rock Fill Dam)
After selection selection of ideal topographical topographical location for the Tehri Tehri Dam, detailed Geological investigation were carried out in the dam foundat foundation ion area area and Geo-t Geo-tech echnic nical al Inv Investiga estigatio tions ns on the fill materi materials als,, for for finalisa finalisatio tion n of vario various us design design detail details. s. Geological investigations were started in 1961 by GSI and were furthe furtherr continu ontinued ed till till the constru onstruct ction ion stag stages es.. Geo-t Geo-tech echnic nical al Investigations were carried out with the involvement of various reputed agencies viz., IRI, Roorkee; CSMRS, New Delhi; IIT, Roorkee; Roorkee; CMRI, Dhanbad etc.
To determine the thickn thickness ess of river borne material, ma terial, at the Dam site following geological explorations exploration s were were made: Drilling, Drifting, Geophysical Geophysical profiling, Geological logging of the under riverbed riverbed portion of the foundation foundati on through exploratory drifts. etc.
Besides Besides above above,, follo following wing investig investigation ationss were also made:
In situ water percolation percolation tests in i n dam foundation, foundati on, Test grouting in dam core foundation, foundati on, Compression tests on rock core, In situ rock modulus tests, Hydro fracturing tests in dam da m core core foundation foundat ion etc.
Exploration by Drill Holes
In order to decipher the thickness of overbur overburden, den, weathering and distressing of rockmass and depth of sound rock on either abutment and in the riverbed section, about 76 nos. of dril drilll h hol oles es were ere dril drille led d in in the the entir entiree dam area. area. Dept Depth h of the the dril drilll hol holes es ~ 12 12 m to 150 150 m. m. Out of total 76 drill holes, 27 drill holes fall in the core seat area. The thickness of estimated river borne material in the main dam core ore area area ~ 4m to 14m. 14m. In the coffer dam core area, on the R/A, thickness of overburden ~ 25m and depth to relatively sound bedrock as 40m.
Exploration Through Drifts
About 31 nos. of exploratory drifts of size 2m X 2m involving about about 1470 1470 m of drifting drifting were were excav excavated ated.. 10 drifts (4 on the L/A and 6 on R/A) involving about 400m length were excavated in the dam core portion to assess the rock mass conditions in the abutments and demarcate the limits of overburden mass, weathered or distressed rockmass, slump zone and also to decipher sound rock profile. It helped in determining the extent of stripping in the core foundation area. The inferred depth of stripping on the left abutment ~ 15m to 30m 30 m and and ~ 15m 15m to 21m 21m on the the rig right abut abutme ment nt.. Plate load tests were also conducted inside the exploratory drif drifts ts for for det determi ermini ning ng the the modu modulu luss of elas elasti tici city ty and and the the modulus modulus of deformatio deformation n of rock rock mass. mass.
Geophysic Geophysical al Investig Investigation ations: s: Objectiv Objectivee of geoph geophysic ysical al inve investiga stigations tions was:
Identification of the thickness and state of the zones of surface weathering weathering and distressing; Determin Determination ation of surface surface of relativ relatively ely sound bedrock; bedrock; Identifica Identification tion and aerial aerial tracing tracing of tectonic tectonic fault zones; zones; Assessment of elastic and stress strain characteristics of rocks rocks in in the area area in thei theirr beddi bedding; ng;
To fulfil above mentioned objectives ob jectives following works were were carrie carried d out at the Dam Dam site: site: Dam Dam core core abutment abutment area area was cove covere red d with with 2774 2774 m of seismi seismicc profile profiles. s. ~ 2024 2024 m of seismi seismicc shooti shooting ng in the the L/B L/B.. ~ 750 m in the R/B. The main work was done with 5-6 m spread of geophones, shot-points spaced at 25-115 m and maximu maximum m hodogr hodograph aph length length of 230 m. m. The total number of profiles covering the left & right bank slopes was 14 & 6 respectively.
Main findings of the Geophysical Investigations:
The average thickness of the zone of surface weathering and distressing on the left bank slope ~ 10-20 m and that on the rig right bank bank ~ 20-2 20-255 m. The bedr bedroc ock k withi ithin n the zone of weathering and distressing was characterized characterized by Longitudinal wave wave velocity velocity (Vp) values varying from 1.0 to 3.9 km/s. Value of Vp rang ranged ed from 3.2 to 6.3 km/s in relati relative vely ly sound sound bedrock of the left bank rock mass and from 3.2 to 5.2 km/s in the right bank rock mass. The modulus of deformation D varied from 2000 to 16000 Mpa. The bedrock within the tectonic fault zones was characterised by Vp values varying from 1.6 to 2.6 km/s. The modulus of deformation in the zones of tectonic tectonic faults varied from 500 to 2000 Mpa.
In situ water percolation percolation tests: test s:
Tests Before Stripping of dam core foundation area : Water percolation tests were carried out in about 58 nos. drill holes, out of whic which h 27 nos. nos. of of holes holes were ere in the the co core re seat seat area area.. Tests ests Aft After Stri Stripp ppin ing g of dam dam core ore found oundat atio ion n area area : 5 nos. of additional in-situ permeability tests were conducted to get addit addition ional al inform informat ation ion about about the perme permeabi abilit lityy of found foundat ation ion rocks rocks..
Tests ests during during Curta Curtain in Grout Grouting ing Work: ork: Variati V ariation on of in-situ permeability permeability of rockmass rockmass along the grout curtain, curtain, was also observed by conducting water loss tests in u/s row of grout curtai curtain n durin during g the co cours ursee of actual actual grout groutin ing g work. ork. It was seen that the permeability in the riverbed zone was very low. Further, permeability of the right abutment was slightly more as compar compared ed to the left abutment abutment..
Test grout grouting: ing:
To ascertain the groutability groutabil ity of rock mass mas s and to develop suitable grouting programme, programme, Test grouting was carried out on three platforms in the dam area in three grades of phyl phyllit lites es expose exposed d on the right right bank. bank. A triangular pattern of trial grouting grouting was adopted for all the platfo platform rms. s. Effic ficiency of grouting was measured in terms of percentage of reduction of pre grout permeability of inspection holes.
In- situ Rock Modulus Modulus Tests:
In order to determine the modulus of elasticity and the modulus of deformation, cyclic plate load tests were carri arried ed out in diff ifferen erentt grad grades es of rock ckss in 14 drift riftss located on the right bank of Tehri Dam prior to Dec. 1973. The tests were carried out across and along the folia foliatio tion n planes planes.. Furth rther (af (after Dec Dec. 1973 973) in-s in-sit itu u plat platee bear bearin ing g and and uniax niaxia iall jac jacking ing tests ests have also also been been perf perfo ormed rmed in expl explor orat ator oryy drif drifts ts for for det determi ermina nati tion on of modu modulu luss of elasticity and deformation modulus in different grades of phyllites.
Hydrofracturing Hydrofracturing Tests in Core Core Foundatio Foundation n rocks:
To find out the reopening pressure of existing joints in the core foundation rocks of Tehri dam, Hydrofracturing Hydrofracturing tests using Minifrac system were conducted in about 10 nos. of drill holes. Test ested dept depth h was upt upto 15m from from the the stri stripp pped ed core ore surface. Re-ope e-openi ning ng pres pressu sure re as obta obtain ined ed from from the the tests tests had had ranged from 17.25 to 41.4 kg/cm2. Based on these test results, maximum limit of grouting pressure in the dam foundation was kept in between 10 to 12 kg/cm2.
EXPLORAT EXPLORA TORY DRIFT UNDER THE RIVERBED RIVERBED
During During the period fro from m 1963 1963 to 1971, a numbe numberr of vertica verticall and inclined drill holes were put down in the river bed sectio section n and from from both both the the abutme abutments nts.. The excessive loss of core recovery in the inclined holes coupl oupled ed with with othe otherr featu featurres indi indica cate ted d exis existe tenc ncee of a major shear zone on downstr stream. However, the complete absence of its exposure on any of the abutments, particularly in the reaches where river flows through the bends had created doubts about its existenc existencee and dispositio disposition. n.
For maki making ng a reass easses essm smen entt , addi additio tiona nall verti ertica call and and inclined holes were put down in the river bed. The drilling data did not indicate any prominent fault zone in the riv river bed. bed. 3 drill drill holes were ere electric electricall allyy logged logged observations made from which provided support to the interpretation that the drill holes have not intersected any any maj major or shear shear zone zone.. Subsequently, in early 1987, a 63.9 m long exploratory drift was was driven driven under the riverbed riverbed from the the R/B to the L/B. This drift was located at about 55m d/s of the main dam axis. 3D geological logging of this drift convincingly disproved the existence of a riverbed shear zone at the dam dam sit site.
GEOTECHNICAL INVESTIGATIONS INVESTIGATIONS FOR DAM FILL MATERIALS: Construction Constructio n of Tehri Tehri dam required about abou t 27.98 M Cu.m of fill materials for the core, transition, shell & riprap zon ones es.. In order to eval evalua uatte the the suit suitab abil ilit ityy of thes thesee materials, preliminary investigations at 14 different sites for core material (within a distance of 5 to 35 km from dam site), 2 sites for transition and shell materials and various various sites sites for riprap riprap material material were co conduc nducted ted..
After preliminary investigation for construction material few sites were selected to carry out the detailed investigations. For clay Koti borrow area and for shell materi material al Dobata Dobata area area was was select selected. ed.
Inv Investi estiga gati tion onss at Koti oti borr borro ow area area (for (for cla clay): y):
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 as deep deep as 20 20 m. To determine the Grain Size Distribution 82 samples were collected from the 22 pits at various depths. From these, the average gradation curves for the pits were worked worked out. Atterberg’s out. Atterberg’s Limits for all the 82 samples were determined and samples were classified as per BIS classification. ,
For determini For determining ng Mineralog Mineralogical ical Composit Composition, ion, three samples samples were were subjec subjected ted to X-ray X-ray diffr diffract action ion analy analysis sis.. In order to check the Dispersivity of soil, 76 samples taken from different pits at various depths were subjected to various tests viz, Crumb test, S.C.S. Laboratory dispersion tests, Percentage Percentage of sodium salts salts in in pore pore water water and Pin Pin hol holee test. test.
Perme ermeab abil ilit ityy test testss were ere con ondu duct cted ed in labo laborrator atoryy on 10 samp sample less collected from various pits. Average permeability was worked out as 0.63 X 10-7 cms/sec . Specific gravity tests were conducted on 49 disturbed samples. The averag averagee specific specific gravity gravity of the material material was 2.68. Insitu wet density, Insitu moisture content and Insitu dry density of four undisturb undisturbed ed samples samples were were also determined determined.. Average Optimum moisture content and corresponding optimum dry densities at standard proctor’s compaction effort for 45 samples taken from various pits at different depths, were worked out as 14-15% and 1.827 gm/cc respectively respectively.. The consolidation characteristics of Koti Borrow area material have been determined determined by by co conduct nducting ing one one dimensional dimensional tests tests on 21 sample sampless
Triaxial shear shear tests: tests: Unconsolidat Unconsolidated ed undrained undrained tests Consolidated Consolidated drained drained tests tests Consolidated Consolidated undrained undrained tests
Hydraulic fracturing tests. Dynamic Shear Modulus: Block Block Vibra Vibratio tion n tests tests Wave ave Propag Propagatio ation n tests tests Vertical Dynamic Plate Load Test Test
Dynami Dynamicc shear shear parame paramete ters rs .
Investigations at Dobata borrow area (for shell material):
Shell material has been borrowed from Dobata borrow area. For exploring the Dobata terrac terrace, e, 63 & 42 nos. of pits were were dug dug in old and new Dobata areas respectively. Samples taken from the the pits pits were ere subj subjec ecte ted d to vari variou ouss test testss as belo below: Grain Grain size size classi classifica ficatio tion n. Dire Direct ct Shear Shear tests tests.. Triaxial shear tests. Field Field and labora laboratory tory Permeabil ermeability ity tests tests . Oedomet Oedometer er tests tests (one (one dimensio dimensional nal compre compression ssion tests) tests) . Long term term deformab deformabilit ilityy tests. tests. Dyna Dynamic mic Shear Shear Modul Modulus us test. test.
Investigations / Testing of filters: Transi ansiti tion on zone zone betw betwee een n core ore and and shel shelll zone zoness has has been been designed as filter. Taking into consideration of the risk of crack formation in the core, the material used for these zones ranged ranged from sand to gra gravel vel size. Fo Following llowing investigations investigations and tests tests were ere co condu nduct cted ed for for select selection ion of filter filter materi material. al.
Permeability Permeability tests. Inte Intern rnal al erosi erosion on of filte filters rs tests tests.. Crushing ratio ratio (particle breakage). breakage). Triaxial shear tests .
Investigations for Riprap material:
For the protection of the dam slopes and increasing stability of slopes slopes during seismic activity activity,, a well graded graded hard blasted blasted rock i.e. riprap (5 mm-1200 mm) has been provided on both the slopes of the dam. Suitability of prospective quarry sites for this material was evaluated in terms of its rock quality, quality, quantity and distance from the dam dam site site.. Tests ests and and stud studie iess co cond nduc ucte ted d in borr borro ow area areass for for ch chec ecki king ng the suitability suitability of Riprap Riprap material material were were as below: below: Compressive Strength. Bulk Density De nsity.. Softening Coefficient. Water Absorpti Absorption. on. Triaxial Shear Tests Expe Experi rime ment ntal al Stud Studyy for for esti estima mati tion on of Seis Seismi micc Pore ore Water ater Pressures . Ground Penetrating Radar Survey for estimation of overburden overburden in quarry quarry areas. areas.
Inv In vestig estigations ations for Pipa Pipalko lkoti ti Dam (Concrete Gravity Dam)
Salient Salie nt Fe Feat atur ures es of Pipalk Pipalkoti oti Dam
Type Height of dam Length at the top River Location
: : : : :
Concre te Gravity Dam Concrete 65 m 89.3 m Alaknanda District Chamoli (Uttarakhand)
For the selection of Dam site 5 different sites were explored explored during the the Preliminary Preliminary Stage Stage of the Project.
Upper Barrage Site (B-1)
Lower Barrage/Dam Site 1
Site was suitable for construction construction point of view but advantage of discharge of two tributaries (Karmansa and Kalpa Ganga river) river) of of Alakna Alaknanda nda river river meetin meeting g just just downs downstr tream eam of this sit site is no nott ther there. e. This site is not taking full advantage of water head available.
Dam Site 2
This site have strong possibility of Rock Fall on the right abutment.
Dam Site 3
This site does not appear suitable as about 20 m thick river borne material terrace exists above water level on both the banks of river river..
Dam Site 4
Dam Site 4 was selected to carry out the Detailed Investigation as this site was free from the Danger of Rock Fall as compared to other sites. Also on preliminary investigation, anti anticip cipat ated ed dept depth h of overbu erburrden den was less less as compa omparred to other other explore explored d sites. sites.
Finally Finally propose proposed d dam site site is locat located ed 50 m d/s of the D4 site. site.
Geotechnical Investigations:
Explorations
and
Field
For obtai btaini ning ng the info nformat rmatio ion n with regar egard d to geolo eolog gic ical al char ch arac actteris eristi tics cs of roc ockm kmas asss and and dept depth h of overbu erburrden den etc. etc.,, detail detailed ed geot geotech echnic nical al explo explora ratio tions ns throug through h explor explorat atory ory drifts drifts and drill drilling ing wer weree carrie carried d out at Pipalk Pipalkoti oti Dam site. site.
Drifting:
In order to explore the rock condition, two exploratory drifts of size 2 m x 1.8 m on on the left and right right bank bank of the rive riverr Alakn Alaknand andaa w wer eree exca excava vate ted. d. From rom these these two two Drift Drifts, s, cross cross cuts hav have been been excavated. In tota totall 150 150 m of drift driftin ing g was was done done at Pipalk Pipalkoti oti Dam Dam site. site.
Drilling:
In order to decipher the depth of overburden and nature of formation below the Dam site, ten nos. of boreholes were done during the DPR stage. In total 500 m of Drilling is carried out at the Dam site. Based on the investigations, depth of overb overburd urden en in the riverb riverbed ed has been estimate estimated d about 20m. Subsequently, in order to have an idea about the depth of colluvium material material on the left bank of the river river (in the vicinity of dam), three three nos of boreholes boreholes were were also done in the area at about 100m u/s of the dam site. Depth of alluvium in i n that area has been estimated around 40 to to 50m.
Seismic Refraction Survey:
To determine the depth of overburden, level of rock at dam site and to infer rock quality, quality, seismic refraction survey survey both on land land and under underwa wate terr along along five five trav travers erses es was was carrie carried d out. out. The lengths of two land profiles for the survey was between 100 m to 115 m while the same for underwater profiles ranged from 50 m to 120 m. The subsurface strata information inferred from this survey was was used used in design designing ing the found foundati ations ons of variou variouss struct structur ures es..
On Land 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 11 m). Land having wave velocity between 800 m/sec and 1200 m/sec was inferr inferred ed to be compact compact / partia partially lly satura saturated ted boulde boulderr bed. The quartzite quartzite rock velocity velocity along land profiles profiles ranged ranged between between 3000 m/sec m/sec and 4000 4000 m/sec. m/sec.
Underwater:
Velocity below 2000 m/sec was interpreted to be saturated bouldary bed. Layer Layer having having veloci velocity ty between between 2000 m/sec and 3000 m/sec may may represent represent compact bouldary bed bed / weathered weathered quartzite quartzite rock Rock Ro ck veloc velocity ity vari varied ed from from 4000 4000 m/sec m/sec to 6000 6000 m/sec. m/sec. The depth depth to rock rock varie varied d fro from m 10m to 22m. 22m.
Estimation of Geo-mechanical Properties Properties of Rock Cores:
From the dam site, test samples were collected collected from three drill holes. Based upon the laboratory testing following properties of the rock were were determined: - Unit Weight Weight (25.7 to 28.4 KN/m 3) - Void Void Index (3.8 (3 .8 to 6.1) -Specific Gravity (2.8) -Brazilian Tensile Tensile Strength (9.13 to 17.95 Mpa) - Uniaxial Compressive Compressive Strength (43.2 to 102.13 Mpa) - Tangent Tangent Modulus (2.99 to 14.12 Gpa) - Poisson’s ratio (0.21 (0. 21 to 0.26) - Shear Strength Parameters (C = 20.71 to 37.19 Mpa, Ф = 45.8 to 47.90).
