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Hydro PowerStructures

(Including Canal Structures and Small Hydro)

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.CONTENTS

Part I Water Power Development

'Power in Flowing Water 1-51 Energy in surface water; power estimates; the design

or normal rated head.

Power Honse - Layonts and Definitions 6-322 General; layout of power stations; terms relating to

hydro power tariff; definitions.

Characteristic Types of Development 33-403 Characteristic types of development; hydropower

structnres; world's largest capacity hydroelectricplants.

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::,rl

--------.----..----.-----_. --'.,---- -

Sediment Ejector & Cross Drainage Works _ 227-26710 Introduction; sediment ejector; important design_

- considerations of cross drainage works; design ofan aqueduct; design of a syphon.

ix.CONTENTS

438"476

40(i-437

General; valleys suited for arch dams; laying outdam in Vlan; types of arch dams; the appurtentantworks; design of an arch dam; trial load analysis;stability at abutments.

Dams 326-380

General; furces acting on dam; typeS of loads;stability analyses methods safety criteria; gravityanalysis; internal stress calculations; gravhicaldetermination of shear streSS; R.S. Varshney's stresscoefficients for dam design; criteria for earthquakeresistant design; instrumentation "in concrete ;'dam.

381-399

Gates and Valves 477-54616 General; control equip:nent; hydraulic gates;r"dial

gates; air vent; weight estimates; illustrative designof radial gate 'it,

Galleries and SluicesGeneral; stress concentration at holes; ovenings indams; reinforcement for circular galleriesrectangular gallery; multivle ovenings; design of agallerY"- -iVith semi circular top; design ofrectangular gallery; outtet sluices-general; hydraulicsof flow through a sluice; intake structures; Vrofilesof sluices in concrete dams; transition shapes;aeration of sluices; structural desigt\ of outlet

sluice.

15

Areh Dams

14

Buttress DamsGeneral; tYVes and selection of buttress dam; designVi-inciples; the most economical profile having notension; buttress design by unit column theory;design on the, basis of unit column theory.

13

Gravity

12

183-206

CONTENTS

'Power channel; design of earthen channels; linedchannels; drainage and presstire release arrange-ments; design of a lined section.

Pewer Channel - Earth. & Lined

Forebay and Escape Fall 207-2269 _General layout of forebay; escape structure; design

of a glacis fall; design -example of a glacis fall(flumed); vertical drop fall; design example of aSharda type vertical drop fall.

Part 3 Dams and Their AppurtenaBt WorksJi;;tb and Rocklill Dams 210-325n Introduction; foundation f?r earth- dams; causes of

failures of earth dams; dpsign criteria of dams;prevention of erosion C embankment details;seepage through dams; co;'trol of seepage throughfoundation; drainage in' -. earth dams; stability-analysis; : selection of type -- of earth dams;maintenance -,and treatment of common troublesin earth dams; rockfill dam'S.

');lil

B

IUvel' Training Works 140-1821 Necessity of river training; river regions

and river characteristics; classification ofrivers on alluvial plains; causes of meandering;meander parameters; cutoff; curvature of flow;methods for training of rivers; guide banks;design criteria for guide bunds; design of guidebunds and launching aprons; groynes or spur&;design of repelling spurs; special - types; pitchedisla:nds; bank protection;_ bridge piers.

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x CONTENTS CONTENTS xi

Part 4 Water Conducting System Part 5 Power HouseaIntak~ Strnctures

22 Introduction; hydraulic thrust; structural design.

The power house; turbines; generators; governingand protection equ'ipment; tr~nsformer; turbineefficiency determination; setting of turbines-drafthead; hydraulic design of spiral case; hydraulicdesign of draft tube; dimensioning of power house;preliminary dimensions of a power 40use;approximate dimensions and weight of peltonwheels; approximate dimension and weight ofpropeller turbine.

17

Tnnneis

18

548-616

Gelleral; location and intake type; trash racks;shape of inlet; intakes through concrete dam;aeration of inlets; design of intake; sedimentexclusion arrangement.

617-698

General; rock mechanics and tunnels; stresses inrock tunnels; geometric design; hydraulic design;structural design of concrete lining in rnck;structural design of concrete lining in soft strataand soils; tunnel supports; rock bolts; grouting;shotcrete lining; tunnelling methods; new AuS!riantunnelling method (natm); use of freezing methodin tunnelling.

Hydranllc Design of Power Stations

21

Strnctoral Design of Hydel Power Statloll

822-889

890-919

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oPenstocks & Pressure Shafts

TIdal Power Stations

PumpetlStorage Plants

19699-749

General; Types of pen'tOck; ecOnomical diameterof penstock; design of penstocks; number ofpenstocks and equivalent penstock diameter;pressure conduits in rock or concrete; anch,ors and

, supports (saddks); other details; sp.ecial types ofpenstock construction.

23

24

920-935

Introduction: types of tidal plants and theiroperation; possibilities of tidal power in India.

936-946

., Introduction; types of pumped storages; theeconomics of Pl!mped storage; pump turbines.

()()()(j'.J

()Surge Tanks

2075(1-320

General functions of a surge tank; types of surgetanks; design considerations of surge tanks;methods of surge analysis; simple surge tank;restricted orifice surge tank; differential surge tank;stability of surge tank; downstream surge tank.

Part 6 Small Hydro

Small Hydro Scenario 948-953

25 Small hydro power-development scenario; benefitsof 'Small hydro'; smal!, h)'!,,,

gS4C\lJlf

982-101S

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;~trOd'!~tiO.n; Ch'it~&iks.Of)l1e4.iUl!l and higl1 headIlli6ro/Il)ini'iiYdro "schemes; civil works of 10:-" ..head,rriicr,,/mini hy(kij sdheines; giites for smailhydro s~he;l)es. . . "

'''''y::' ,

xii

.... -,. - .,:) , . :"":';'Sniall Hy~o Stations - Design .,

26

'1} . ; .)j'Torbines.aod Generators for Small Hydro

,''.

CHAPTER IPower in Flowing Water

1.1 ENERGY IN SURFACE WATERThe hydrologic cycle is a continuous process by which water

is transported from the oceanS to the atmosphere, to the landand back to the sea. Many sub cycle exist. The evaporationof inland water and its subsequent precipitation over landbefore returning to the ocean is one example. The drivingforce for the global water transport system is provided bythe sun, which furnishes the energy required for evaporation.

While evaporating from the seas into the atmosphere, thewater masses gather poten tial energy, a portion of which isused in the process of precipitation from the clouds whilethe remainder is dissipated in the coarse offiow in streamsand rivers. The particle of water starting from a hillside andrunning toward the sea possesses more 01 less kinetic energydepending upon the changes in the velocity of the stream flow.However the amour,t of kinetic energy is insignificant ascompared to the dissipating potential energy; the changein kinetic energy are negligible. Thus the dissipation ofpotential energy of run-off waters in mountainous and hillyregions, regardless of small and negligible quantities, doesnot mean a gain in kinetic energy.

The potential energy of the run-off is dissipated to overcomeinternal friction of tur ulent water, to supply energy. to

1

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POWER ESTIMATES 3

For evaluating power and energy, it is better to work outavailable power for a 90 percent water availabiliry year todetermine firm power and for an average discharge: .year todetermine turbine units for installation. Two types of averagedischarge years are taken by different organisations. i.e. onein which the year which has 50% availability or a syntheticyear which has average flows for different time units 'chosensay 10 day flows (this type of average flow synthetic yearwilf normally be about 60 to 70 percent yearly availabilityyear). In case of peak power installations, sometimes evena lower availability, even lower than 50%, is taken intoaccount. This would give higher installation.

Operational studies are carried out to determine power andenergy available at different time intervals. A typicaloperational table will have the following columns : (for acase of a reservoir scheme)

I. SI. No.2. Month (starting from July and ending in June next year)3. Period (l0 day or weekly, for which flow data are

available)4. Inflow in the stream (mS!s)

This can be found under three conditions viz:(i) Direct if measurements are available for the

ret.ervoir site(ii) lndirect either compoted or generated on the

basis of either upstream or downstream dischargesite e.g. in case of Lakhwar reservoir data ofTajewala downstream was taken

(iii) Share of some interstate agreement5. Releases from reservoir. They can be either equal or

more or less than the discharge in col. 4 above (ms/s)6. Retention in the reservoir (col 4 - col 5) (ms/s)7. Incremental storage (mS) = col 3 x col 68 Storage initial Mms9. Progressive storage Mms (cumulative of item 8 above)

10. Total storage Mms (col 8 + col 9)11. Reservoir elevation at the beginning. 1m)

1.2 POWER ESTIMATES

(1.5)(1.6)

(1.4)and generator

POWER IN FLOWING WATER

whirls, eddies and spiral flows, to scour river bed and totransport the bed load.

By erecting dams and weirs, a considerable portion ofpotential energy iu any stream or in a river basin is utilised.This energy can be utilised to generate hydroelectric power.This is usually done in two ways viz :

(i) By diverting the whole or part of the flow into an artificialby-pass usually termed power chan nel, the slope ofwhich is considerably flatter than that of the originalstream. The difference in elevation between the powerchannel and original river course is gradually increased.and a head is created for power generation at the mostsuitable site.

(ii) By short cutting the rOilte of water conveyance, thusenabling the fall available in the river to be utilised.This is possible if the river course is sinuous and thevalley is characterised by sharp or horseshoe bends.These bends can be cut by a channel (e.g. ChillaPower Station UP) or a tunnel (e.g. Chibro PowerHouse UP)

If the difference in water levels between the intake of thewater conducting system and at the end of turbine i.e. at theentry to tail race is H, and if the flow is Q, then the overallpotential power P, is :

( V12_V.2)P- v Q H + 2g (m.kg!s). (1.1)The portion of power that originates from changes in velocity

is generally negligible as against the potential power arisingfrom differences in elevations and thus we can say that power

P = v Q H (m. kg/s) (1.2)Hence power in SI units (g = 9.81 m/s2and v - 9810 N/mS)

P = 9810 Q H (Nm/s) 0.3)Since I Nm!s = I watt and 1000 Nm/s = I kW

P = 9.81 Q H (kW)If 'I)' and 'I). the are efficiencies of turbine

respectively, we can sayEoet .. 9.81 'I)' 'I). QH (kw)

~ 8 to 8.5 QH (kw)

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1.3 THE DESIGN OR NORMAL RATED HEADThe normal head or des'gn head Hd is the lowest head

under' which the entire wheel discharge capacity can beutilised; at full gate, at the required speed.

The following are the factors to be considered affecting thechoice of the normal head viz:

(a) Theoretical energy output should be maximum(b) Unit cost of hydroelectric power (Rs. per kWh) should

be ,mininium. This requirement may favour a highernormal head since any increase in Hd results in reducedturbine diameter and high speed whereby the size andcost of machinery and even the size of power housestructure are reduced. (Caution: In case of heavysediment load in the stream and when the sedimentparticles are of hard rock such as quartz, it is desirableto go for a one step lower speed of turbine. This

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365DAYS

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The approximate determination of the design(normal or rated) head-after M. Seidner

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H. =

THE DESIGN OR NORMAL RATED HEAD

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Fig, 1.1

would increase size of machine but will be a safetyfactor against erosion and cavitation)

For preliminary studies, Hd (or Hn) may be assumedwithout serious error as the arithmetic mean of availableheads i.e.

According to M. Seidner, the normal head can be found byplotting the rough power curve on the basis of the assumedaverage efficiency value The head pertaining to the averagepower output agrees fairly well with the practical value ofthe normal head. The average power line usually intersectsthe power curve at two points, of which the one indicating thelower head should be chosen (Fig. I.I).

A discussion on rated head in case of Francis turbinesin particular is given in clause 2.3.1.

POWER IN FLOWING WATER

Evaporation losses (m) : a curve between evaporationloss in m vs reservoir surfacearea is required

Reservoir elevation at the end of the period'(collI - col 12 + rise in elevation due to col. 9),Mean head (Elevation (ll + 13)/2 - tailwater level)Power output MW = 9.81(coI5) x (col 14)/1000Units generated (coIlS x col 2 in hours)

General criteria followed in reservoir operation studies are:0)' Reservoir will be at dead storage on 1st July each year

or on the date chosen on the basis of seasonalvariation or the location of the reservoir.

(ij) Reservoir will be filled (with least spilling) during themonsoon by surplus water (i.e.) after meeting drinkingwater and irrigation commitments)

(iii) Depending on irrigation requirement (if it is amultipurpose reservoir) a uniform high discharge may beassumed to pass for Rabi (October to January) andrelatively lesser discharge thereafter until June. Thispa.ttern usually matches with the winter and summe"

'peak requirements of the grid.

4

12.

13.

14.15.16.

8 'POWER HOUSE LAYOUTS AND DEFINITIONS LAYOUT OF POWER STATIONS 9

Fig.2.2 Semi outdoor-plant--Kentucky dam U.S.A.

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LAYOUT OF POWER STATIONS 7

2000 kW2001 to 15,000 kW15,000 to 50,000 kW

over 50,000 kW

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CHAPTER 2Power House

Layout:s and Definit:ions

2.1 GENERALIt is rather difficult to:trace wheD exactly water power

devel"pment made its beginning. The water wheels have beenin existence from very early periods of human civilization.Records of water wheels being in use on the Hwang Ho, theNile and the Euphrates are in evidence. They were mainlyused for husking corn. Even in the interior parts of ArunachalPradesh we see ample evidence of such wheels for huskingcorn. The adoption of the water wheels for generationof electric power is comparatively of a recent origin. Fora considerable time, oil and coal were considered the onlysources for generation of electric energy. This is especiallyso because, invariably hydro-electric development nsed to bean out-of-the-place in the higher hills of the watershed andneeded long transmission distances before making it availablefor the several load centres. But thanks to the present daytrends in the development of long distance transmission,particularly so of the high altitude and high voltagetransmission, hydro power development has gained importanceand has become a priucipal competitor to the other modes ofelectric power generation. Another important cause for thistrend has been the remarkable strides made in the developmontof the hydraulic turbines as a prime mover. it is simple,efficient, easily controlJed and longlived.

6

2.2 LAYOUT OF POWER STATIONSBased on various features, the power houses ca.n be grollped.

2.2.1 Classification On Load Characteristics1. Isolated Plant: A power hOllse which actsiridependently.2. Interconnected Plant: A power house, where it is a part

of a net work, is referred to as interconnected plant. Suchpower houses can again be further sub divided as :

I i! Base Laad StationsThe power stations operate continuously at a constant

power. These operate at relatively high load factors almostequivalen t to firm power steam stations. These cater forpower at the base of the load curve.

(it) Peak Load PlantsPower Houses supplying power to clip off the peaks of the

load curve are calJed 'p~.lk load plants". Tilese supply poweras and when needed and hence may not be operatingcontinuonsly with a reslliting low load factor.

2.2.2 Classification on the Capacity of PlantsThese are classified as below :L Midget plants-capacity upto2. Low capacity plants3. Medium capacity plants4. High capacity plants

2.2.3 Classification on Hydranlic Cbaracteristi~s :Depending upon the source of water power stations are

called as :. 1. Run-of-tbe-river type: They use water as available

in the stream.2. Storage plants : Here water is stored in periods of

high flows and utilised during dry periods.3. Tidal plants: "fiere the rise of water level in the tides

is used for generation.

10 POWER HOUSE LAYOUTS AND DEFINITIONSLAYOUT OF POWER STATIONS II

M.\X. HoW~:~l~~TAKE

Fig. 2.4 Section through uliderground Guayabostation-EI Salvador

Fig. 2.3 Outdoor power house-Lower Salmon river plant,Snake river Idaho-U.S.A.

head less than 20 m.head 20 to 60 m.head above 60 m.

1. Low head plants2. Medium head plants3. High head plants

2.2.5 Classification based on headThe classification is bas~d on the operating head on the

turbine.

6. Construction does not interfere with the over groundfeatures.

7. The underground power station is relativ~I)' more stableand foundation problems are minimised. Vibration an4fatigue stresses are consideraQly reduced.

8, With suitable location, the surge chamber may bedispensed wi:h. .

9. Underground power station, in some cases, may assist inestablishing an increased net head and thereby poS'iQleincreased potential.

The chief disadvantages are in the provision of costlyapproaches, ventilation and cable tunnels and artificiallighting. The underground station has a depressing effect onthe workers inside, unless air conditioning, adeqm'te lightingand other necessary facilities are provided~ The choicebetween an otltdoor, semi-outdoor or indoor type is dependenton the following :

(i) level of approach to the power station, .(ij) severity of climatic variations at site,

(iii) height of tail water level with respect to generatorfloor and

(iv) the seismicity of the area.Power stations without the generator hall are to be preferred

in seismic areas. The outdoor gantry is about 50 to 100%expensive as compared to the crane inside tlie power house.With high tail water conditions, the downstream wall willhave to be made for the protection of generator agaiustflooding and a semi outdoor or indoor type inay be selecteddepending upon economics.

PENSTOCK

H"'~H.W..- ,"_~- . "",.,.c,F""""lr--

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POWER. ~I~ ACCESS SHAFT::::~~~~.:J'j]' ~:i;:;::;;(w:~.

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4. The arrangement permits for providing penstocksthrough rock which need comparatively thinner liner ascompared to the thick plates needed for penstocks above theground. The saving in steel in the pipe line is a veryimportant consideration in India, since it saves import ofhigh quality steel from outside.

5. The power station is better suited to seismic areas andhas b,tter protection against bomb attacks.

'--

, ~'.>.

12 POWER HOUSE LAYOUTS AND DEFINITIONS LAYOUT OF POWER STATIONS Il

The low' head plants have, however, a characteristics layoutwhich ',inay be used on some of the medium head plants aswell, but it differs significantly from the arrangement fer highhead. Comparative features of low, medium and high headplants are given i 11 Table 2.1.

The three types of power stations are shown in Fig. 2.5.Two typical layouts for high head plants are exhibited inFig. 2.6 and 2.7.

LOW HEAD r-----.j:)()~':ERPLANT

NorE:~The statement within brackets indicates an uncommon occurrence

TABLE 2.1Comparative features of low, medium & high head plants

rNSTOCk

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TUN~EL

Fig. 2.5 Typical layout of low, medium and highhead plants

SURGeRESE~V01~ ft ~ C~AMBER~L

~:r~~ 14 POWER HOUSE LAYOUTS AND DEFINITIONS

LAYOUT OF POWER STATIONS 15. .INiIW.e GATE

~ RESf.R\lOIR

Run~of-the-Riv~r PI~nts

The supplies in the river areavailable throug~out the year.A bridge is obtained acrossthe river without substantialadditional cost,

There is no lowering of thewater table.

NO such problems and entirewater can be used. for developmentof pow~t. The~e is scenic beautyand sco~ for sport.

Navigation channel has to bedeveloped and maintained withthe stream.Rise in ground water level due tohigher ponding in the valley.Higher evaporation losses butseepage loss may 'not be high sinceriver gets inflow from seepage.

Re~locatioD of big\:lways, fail {..>adsetc. only if these facilities arelocated close tot~e dver.Operation is convenient since allthe structure! are';8t one place.

No such possibilities.

Utilization is poss_bIe by a highweir which is expen.'-jve.

causessubsoilcreates

Plants on Diversion Canal

(4) Due to small width of wateron the canal evaporation lossesare less and seepage los~es canbe avoided by lining.

(5) Alignment of power channelinterferes with exi.

16 . POWER HOUSE LAYOUTS AND DEFINITIONS LAYOUT OF POWER STATIONS 17

',q/'vA, AhO CANAl ANI< l)E1II$TOCII unUlAnolY--)UT C>"'~~ ~~STOc.. C~lwe IN A'VE/l

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etc. being done at the upstream face of the dam. The powerstation building is separated from the dam by expansion jointbut galleries are made at the toe of the dam for housing powerstation accessories and cables etc. Often the space betweenthe dam and power house is used for locating the transformers.Such types of power stations are provided at Mettur, GandhiSagar, Panchet, Rihand and Hirakud. A section through theRihand power station is shown in Fig. 2.9.

~~ ,lfAM~HeAO~ GATESj e",.. I " I ffle-CA~.\.'

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Fig. 2.8 Layouts of diversion schemes

Power Station below Dams

Power stations are located at the toe of dam where suitablefoundations are available and the only head to be utilised isdue to the creation of the dam. The most common arrange.ment is to feed the turbine by a separate penstock which isembedded inside the dam, provision of gates and trash racks

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U.S.A. (ii) by the construction of high dams (iii), by cuttingacross watershed, when the adjacent vdley may be lower asinstanced by the Koyna development in Maharashtra or byshort circuiting long reaches of the same river having steepslopes as in the case of Yamuna Hydroelectric Scheme StageII in U.P. The power potential of high head schemes islarge and plants should always be based utilising the entiredischarge available; storage of surplus water at any time

.greatly increases the generation capacity.

~

18 POWER HOUSE LAYOUTS AND DEFINITIONS TERMS RELATING TO HYDRO POWER TARIFF 19

G = annual growth in generation (per cent)U = per capita generationc = a constant = 0.02(population growth rate)+ 1.33

(2.2)(2.3)

10gG = c- 0.J510gUG = 100/Uo'1Sor

In case of underground stations, (Fig. 2.7) the powerstation comprises three separate halls, one for the valves atthe upstream, a central one for the machines and a hall forthe transformers downstream. One hall alone will need alarge opening, which is thus avoided. The valve hall isconnected by a bye pass to the tailrace so that in case of anybreak down of the penstock or valve, water would. pass to thetailrace and the power station will be saved froinfl60ding..

Pumped storage plantsThe pumped storage plants operate on the prinCiple that

the machines are used for generation of p,.werdtiring peakload when power is given to the net work and for pumpingwater back into the reservoir during off peak periodS. Theprovision is based on economics of operation and theavailability of enough spare ca">acity in the grid to operatethe machi~es as pumps in tile low load. periods. The geoeralarrangements of pumped storage plants are discussed inChapter "Structural Design of Power Houses" (Hydro PowerStructures Part II)

2.3 TERMS RELATING TO HYDRO POWER TARIFF

2.3.1 Power and Energy Terms1. Water Power PotentialElectrical energy in kW = 8 to 8.5 Q.H (2.1)

whereQ = discharge in cumec, H = operating head in metre.There are many methods of forecasting the load demand.

They take into eonsideration (i) the class-wise consumption

E = electricity consumptionM = index of manufacture of production

t = time for which consumption is to be projectedK = adjustment facter

2. Normal Water Level It is the highest elevation of waterlevel that can be maintained in the reservoir either with a gatedor ungaled spillway, without spillway discharge.

3. H,ghest Average Level A level above and below whichequal amounts of power are developed during an average yeari.e. 50% units between N.W.L. and H.A.L. and 50% unitsbetW.eenHA.L. and min W.L.

4i Mlnimnm Water Level It is that water elevationproducing minimum net head on the units (65% of designhead).

5. Gross Head (H.) It is the difference in elevatio~between the water levels of the forebay and tail race.

6. Net Head (HD) It is the gross head less all hydrauliclosses except those chargeable to the turbine. Net head isthe head available for doing work on the turbine. Spiralcase and draft tube losses are chargeable to the turbine.

7. Maximum Head (Hmax) It.is the gross head resultingfrom the difference in elevation between the maximum forebaylevel and the tail race level without spillway discharge and withone unit operating at no load speed (Q = approximately5 percent of rated Q). Under this condition, hydrauliclosses are Degligible and may be neglected.

S! :Mlilitniun .Head (HmID) It is the Det head resultingfrom the difference in elevatioD betweeD the minimum forebaylevel and the tail race level with all turbiDes operatiDgat full gate.

9. Weighted Average Head (H) It is the net headdetermined from reservoir operation calculations, which willproduce.cthe same amouDt of power in kilowatts betweeD thatheadand maximum headas is developed between the same

-

20 POWER HOUSE LAYOUTS AND DEFINITIONS

Belgium formula.E = K MO'6 2'4661

where

2.4

TERMS RELATING TO HYDROPOWER & TARIFF 21

head and minimum head. This weighted average head willusually lie approximately two thirds of the total range ofhead above minimum.

10. Design Head (Hd) It is the net head under whichthe turbine reaches its peak efficiency ofsynchronous speed;usually difference of N.W,L. and Min. Tail W.L. = 125% ofdesign head. This is tlie head which determines the basicdimensions of the turbine and therefore of the power plant.

11. Rated Head (Hr) It is the head of which thespecifications requite that the turbine operating at full gateshall produce sufficient power to deliver the name plate poweroutput. This rated head should equal the design head of theturbine to assure maximum overall plant efficiency.

U.S.B.R. recommends following formula to determine H,

Hr = Hm.x (f:isl = 0.91 Hm (P.F} (2.5)where

P.F. = geDerator power factorNote: The factor 1.15 is for a generator having an overload

capability of 115 percent of its rated capacily.12. Critical Head (He,) It is the head at which the full

gate output of the turbine produces the permissible overload onthe generator at uDit power factor (usually 115 p.c. of thegenerator k.V.A. rating). This head will produce the maximumdischarge through the turbine.

Fig. 2.10 illustrates these water levels and head forhydroelectric power plants with Francis type turbines.

13. Firm Power It is the maximum continuous poweravailable from a plant under the most adverse hydraulicconditions.

14. Secondary Power It is the excess :,ower availableover the firm or primary power say in night or monsoonperiod.

15. Installed Capacity. Total capacity in kW or MWofall the turbine gonerators units installed in the power house iscalled installed capacity.

16. Dependable Capacity 1t is the load carrying abilityof the power station with respect to the load characteristicsduring a specific time interval and is decided by the powerfactor, capability aud the load OD a statioD.

'-

22 POWER HOUSE LAYOUTS AND DEFINITIONS TERMS RELATING TO HYDROPOWER & TARIFF 23

17. Load Curve It is a plot in between power generatedin kW or MW and time. If time is in hours, it is called dailyload curve, in days monthly load curve, in months-annual loadcurve \Fig. 2.11). The demanded load is plotted againsttimesequence, the sequence beginning at midnight. The daily loadcurve gives the operator, necessary information as to thegeneral characteristics of load, that is being enforced on theplant. With this information the operator can take suitableaction in advance to be ready to supply increased demandwhen it is needed.

ilot i;\J~r thAn "j,.M~ AI

24 POWER HOUSE LAYOUTS AND DEFINITIONS

This factor may exceed unity, which indicates that loadshave been carried in excess of the rated capacity of theequipment. Hany modern plants have this built in reservecapacity. If utilization factor approaches unity, it may signalthe need for additional capacity.

26. Peak Diversity FactorP:D.F... Max. demand of consumer group + Demand of

. consumer group at the time of system peak demandsum of individual maximum demands

=simultaneo.us maximum demand

Since there is a diversity of uSe by each consumer, thenumerator in the above expression is greater than thedenominator with the result that the. diversity factor is morethan "nity.

27. Demand FactorD.F = Maximum demand (Time to be specified)

. Connected load28. Cold Reserve This is that portion of the installed

reserve kept in operable condition but not placed in serviceto supply peak.

29. Operating Reserve This is the capacity in service inexcess of the peak load.

30. Spinning Reserve It is that reserve generating capacityconnected to the bus bar and ready to take load.

2.3.2 Tariff Terms1. Tariff: The formula determining the price of supply

of electricity.2. Maximum Demand: The highest value 0 f the power

kilovolt amperes, or other quantity such as the current,' takenwithin a demand assessment period; the appropriate quantitybeing

26

is a diversiondevelopmenttunnel or sometimes a

27DEFINITIONS

16. Erection Bay (Assembly floor) is a separated area insideor next to the power house bays, having it platform ofadequate load bearing capacity for mour ting one,exceptionally two machinc units.

17. Exposed penstock (free penstock) is a pipeline locatedover the terrain or laid in an open ditch or in a servicetunnel.

18. Fixed"blade propeller type turbine wheel is an axial flowreaction turbine with blades keyed to the hub, unlikethose of the Kaplan turbine.

19. Follower type gate valve has a prolonged sliding plateWith an opening on its lower part and of an area equalto the cross-section of the penstock. In flow-throughposition this opening faces the pipe cross-section; inclosed position the upper part of the plate blocks thecross-section.

20. Foreby/closed bay is thc upper by of a run-of-riverstation with an intake structure at the entrance.

21. Francis turbiue/wheel is a radial inflow reaction turbine.22. Fullscroll/spiralcase/casing (full spiral) has a nose angle

of 320 degree to 340.23. Gate chamber is the part of the power conduit where

the gate is accommodated.24. Gate valve has a leaf-like closing organ sliding in a

plane perpendicular to the penstock axi&.

12. Diversion tunnel developmenthaving a pressure powerfree-flow-tunnel.

13. Diversion weir or dam is a weir or dam created forthe sole purpose of diverting flow from the water courseinto a power conduit.

14. Dynamic draft head (dynamic suction head). in reactionturbines is the sum of the velocity head ,of the waterleaving the runner and the static draft head.

15. Effective rack or screen cross"section (net rack or screencross-section) is the entire free area Le;'the' sum of allclearances between the rack or screen bars"

POWER HOUSE LAYOUTS AND DEFINITIONS

2.4 DEFINITIONS

Compiled from I.S.l. Specifications (IS. : 44iO) nndMosony's book

!. Anchor block (onchor, al,chorage) is a concrete ormasonry block on a hillside for supporting and fixing thepenstock.

2. Axial flow turbine/wheel is a collective term for turbineswith axial flow along the runner blades which are eitheradjustable or fixed. Both propeller turbines and Kaplanturbines are axial-flow wheels.

3. Banded penstock (hooped penstock) is a pressure pipereinforced by external ring-like or spiral-like bandages toresist extremely high pressure.

4. Brake nozzle is a small-size nozzle built into the Peltonturbine for directing its water jet towards the back of thebucket, thus making it possible to stop the machinequickly.

5. Bnried penstock (covered penstock) is a pressure pipelaid in a ditch and subsequently covered.

6. Bntterfly valve (clock/flap valve) has a disc-shape closingbody rotating around a shaft perpendicular to thepenstock axis.

7. Control valve (couduit/penstock valve) is the valve,generally of the butterfly type, installed between thesurge tank or the head reservoir and the penstock or thepressure shaft at the top of the latter.

8. Dead storage/area/space, (dead storage area) is the lowestportion of a storage basin from where the water isusually not drawn.

9. Diameter ratio (jet ratio) in Pelton turbine is the ratioof the pitch circle diameter to the jet diameter.

10. Diversion canal development (Power canal development)is a diversion development having a free-flow powercanal only.

11. Diversion development does not create the head inthe river course itself but in a power canal and/or tunnelbypassing a stretch of the river or connecting two riversas different elevations.

(

f', ,,

28 POWER HOUSE LAYOUTS AND DEFINITIONS DEFINITIONS29

has an

25. Cross rack or screen cross-section is the entire flowcross-sectiol1al area of the hydraulic structure protectedby the rack or screen.

26. Guide vaue is G:.c streamlined movable blade of the guidewheel (wicket gate), regulating inflow to the turbine runner.

27. Guide wheel (entrance/inlet/turbine gate) is built in th'intake portion of the entrance flume of low-head andmedillm-head power-houses, or in the headpoint of ahigh-head power plant, to control inflow into the pen stock.

28. Head"ace is that p3rtion of the power canal whichextends from the intake works of the power house.

29. Headr.ace surge tauk chamber/basiu (upper/upstreamtank/chamber/basin) is located between the lower end ofthe pressure tunnel and the penstock or pressure shaft.

30. Headwater is the water upstream from the power house.In general the water upstream. from any hydraulicstructUre creating a head.

31. High capacity water power plant or station has aninstalled power out-put of 10000 kW or higher.

32. High-head-water power plaut or station has a gross headabove 50 m (This terminology can only be used forstatistical purposes).

33. High-speed Fraucis turbine/wheel is characterized by aspecific speed of 200 to 300 r.p.m.

34. Inlet valve (turbine valve) is the valve installedimmediately ahead of the turbine i.e. at the bottom of thepenstock (unit penstock) or the pressure shaft.

35. Intake (intake works, headworks) is a hydraulic structurebuilt at the upstream end of the diversional canal (ortur:nel)ror . controlling the discharge and preventing silt,debris and ice from entering the diversion.

36. Intake. tower is a pressure tUIlnel intake erected separatelyin the reservoir for housing the flow control valvesor gates.

37. Kaplan tnrbine/wheel (adjustable-blade turbine/wheel,feathering-vane turbine/whee-I) is an axial-flow reaction

turbine with adjustable runner blades, used mainly underlow heads.

38. Live storage (useful storage/area/space) is that part of areservoir which can be utilized for power production orother purpose.

39. Low-capacity water power plant or stationinstalled power output of 100 kW to 999 kW.

40. Low-head water power plant or station has a grosS headless than IS metres.

41. Low-speed Francis tnrbine/wheel is characterized by aspecific speed of 50 to 125 r.p.m.

42. Manifold (header) is the lowest portion of the penstockwhere from the unit penstocks bifurcate.

43. Maximnm surge is the greatest amplitude occuring duringsurge oscillation (usually on load rejection) I.e. themaximum among the subsequent surges.

44. Multistage water power development 'scheme 'projectconsists of subsequent water power plants of which thedownstream station uses the spent water of theupstream one.

45. Needle valve has a streamlined regulating body movinglike a piston in the enlarged housing of the valve.

46. Normal-speed Francis turbine/wheel is cnaracteriz,d hy aspecific speed of 12 to 200 r.p;m.

47. Nose angle is the characteristic angle of the spiral case,enclosed by the radius perpendicular to inflow and theradius pertaining to the nose.

48. Open bay/forebay is the, upper bay of a run-of-the-riverstation without any intake structure at the entrance.

49. Partial scroll case/casing \partial spiral case/casing,partial spiral. semi-spiral) has a nose angle less than 3200

50. Pelton turbine/wheel is the main type of reaction turbinesused under high heads.

50. Penstock (pressure pipe) is a pressure pipe line conveyingthe water in high-head developments from the headpond orthe surge tank to the power house.

'f--....-'"-

32 POWER HOUSE LAYOUTS AND DEFINITIONS

surface of the scroll case. The stay vane serves mainlystructural purposes, though it may have a hydraulicfunction too, i.e., to increase or de.crease the momellt ofmomen tum of inflow.

71. Snrge tank/chamber/basin is a hydraulic structure erectedin the power conduit of high head developments betweenthe pressure tunnel and the penstock (or pressureshaft to protect the pressure tll nnel from water hammereffects, to diminish over-pressure due to water hammerin the penstock itself, a"d to store water for suddeD loaddemand. A surge tank can also be located between thedraft-tube and the tailwater tunnel, if any.

72. Surge-tank-system consists of two or more surge tanks,hydraulically cooperating, either on one side or on bothsides of the turbine.

73. Tailrace is that portion the power canal which extendsfrom the power house to the recipient water course.

74. Tailrace surge tank/chamber/basin (downstream/lowersllrge-tank/chamber-basin) is located between the draft-tllbeof the reaction turbine & the tailwater tunnel.

75. Tidal power pI ant or station utilizes the potentialhydraulic power originating from the tidal cycles of thesea.

76. Turbine efficiency is the entire efficiency of the turbine,Le. the product of hydraulic, mechanical and volumetric

.efficiencies.77. Undergronnd power station is a development where atleast

the machine hall is located in an excavated cavern.78. Water Power development/project/selteme is used in two

_s'enses; . '(il water power plant.

(ii) two or more water power plants exploiting thepotential power of a river or a river systems accordingto a common power concept.

CHAPTER 3Characteristic Types of

Development

3.1 CHARACTERlSTIC TYPES OF DEVELOPMENTThere are usually three types of development viz :1. Diversion Canal Type and Ruu-of-tlte-Rher plants witlt

head water levels more or less constantIn such plants, there is continuous operation and there is

slight variation in generated power In the case of diversioncanal types, fluctuations, as compared to the head, are relativelysmaller; consequently also variations in power output aresmaller. .

The classic example of an old development of this type isthat of Ganga Canal in UP, where a series of power houseswere constructed before Independence and are still in use.They are Pathri. (replaced Bahadrabad power house in fiftees),Mohammadpur, Sumera, Palra, Bhola, Salava, Chitaura andNirgajni. .

Example of the utilisation of long river stretches is Up 'erRhine river forming boundary of France and Germany(Fig. 3.0. There are four power stations with shiplocks in the navigable Grand Canal d'Alsace : Kembs,Ottmarsheim,Fessenheim and Vogelgrun. There are fourdiversion type projects with short by-pass navigable) canals(power stations and locks in the passes) : Marckolsheim,

33

\

(

34 CHARACTERISTICS TYPE OF DEVELOPMENT HYDROPOWER STRUCTURES

,,'I.,

t~'}

:~

/,35

('~

.~

I

1/~

~ .A.lS8OURG 0 \0 20 $~OVRG "'''';' Keitt

or. 'it Pol!. STll" . :

rJ 0 : Mll"HI~ ._ ~:n ElM "S'~lll".~ r.e~TII "~9 tf-C>IlSHE'M llll,/, MI. 10., :::~ ,",,"I"U' tt':oi llH,>,AU 1':' ./ ..i;. .. ''-- "S"tE

'; ~.: SWITlElllA>lD.:- .. ~"': ,

Fig. 3.1

Rhinau, Gerstheim and Strassbourg. There are two riverbarrages (weir, power plant and loci's) in the Rhine bed:Gambsheim and Iffezheim.

2. Run-of-the-River PlantIn case of the more common version of run-of-the-river

plants, the head water level is raised slightly above 6.ood waterlevels, sometimes remaining even below it. Power generation,therefore, can be continuous or interrupted. Examples ofsuch generation are:On river Bhagirathi - Pala Maneri (P); Maner! Bhali Stage

I (C); Maneri Bhali Stage II (UC)On river Alakhnanda - Vishnu Prayag (UC); Vishnugad

Pipalkoti (P); Bowala Nand Prayag(P); Shri Nagar (UC)

(p = planned; UC = under construction; C = constructed)

These projects can tither be (j) power house on a diversionchannel (e.g. Shri Nagar) or a tunnel development (e.g. Chibrop.h. utilising waters of Tons river or Mane-fi Bhali Stage IUttar Kashi power house); Meneri Bhali Stage II (Dharasupower house).

3. Storage Scheme

In such schemes, water is stored behind a dam and the powerhouse can be either at the toe of the -dam (e.g. Rihand in UP;Bhakra in Punjab) or at the end of a tunnel (e.g; Tehri in UP).

A typical storage scheme is presented by Upper IndravatiHydroelectric Project in Orissa India, where four damsconstructed on four different rivers, form a common reservoirto fed a single power house, having a tunnel head race waterconductor, followed by open penstocks and an open tail racechannel. (Fig. 3.2)

3.2 HYDROPOWER STRUCTURESHydroelectric schemes plants may be arranged according to

the layo"t shown for Yamuna Hydroelectric Scheme Stage II,U.P India (Fig. 3.3) and Koyna Hydroelectric DevelopmentMaharashtra Iodia (Fig. 3.4). The main structures, in ahydroelectric scheme, are :

(0 the weir (jf it is a diversion canal type of development)or a diversion/storage dam, with appurtenant structures.

(i0 the intake(iii) the head race and appurtenant structures.. For high head

development tunnels with or without surge tanks andpressure shafts are necessary.

(iv) the control equipment like gates and valves.(v) the penstocks.(vi) the power house.

(vii) the tail race with or without downstream surge tank.Arrangements and designs of the different types of storage

dams and other structures are given in pages to follow.Detailed discussion on foundation problems and treatmentprecede the design details 0 f such structures.

.0"1l

Oll/Ell n ...1UII '.'.rUNllnPUISTOC~ ;''''!L ""'CE C""'IIIft:-..

, ..... Pu~

LEGEND;- , ii~g:1~~T,llA!-!NA~NmV"R~lLLJr_{ --tUliA"

Fi: {::~:t

IK1Allt "'T ICKAIU

_'C_Sl:IlflTNlIlR!$T~lCnOOftl!l~t

uPt

Fig. 3.3 YAMUNA-HYDRO-ELECTRIC SCHEME STAGE II [V.P. (INDIA)]SCHEMATIC PROFILE

r38 CHARACTERISTIC TYPE.OE DEVELOPME'f WORLD'S LARGEST HYDROELECTRIC PLANTS 39

o.a.

n.a.

1996

o.a.

na.

1975

1973199619951968

n.a.

n.a.

o.a.

n.ae

19771955

1994

D.a.19611977

o.a.

1983194219861984

1%819951979

Year ofinitial

operation

20,00013,40012,00010,83010,3007,2606,4006.0006,0005,3285,0204,5004,3204,2004,1504,0003,6003,6002,400-3,6003,4893,4783,4D93,3003,3003,2003,0002,8002,730

Rated capacity MWNow Planned

Country

TABLE 3.1World's largest capacity hydroelectric plants (constructed andunder constrnction) - after Water Power &: DliinConstruction.

May 1990

3.3 WORLD'S LARGEST CAPACITY HYDROELECTRICPLANTS

Table 3.1 lists the world's largest capacity hydroelectric plants.

1. Turukhanok C.l.S.2. Three Gorges China3 ltaipu Brazil 10.5004. Grand Coulee U.S.A. 9,7805. Guri (Raul Leoni) Venezuela 10,3006. Tueurui (Raul G Lbano) Brazil 3,9607. Sayano-Sbu,bensk C.I.S.8. Krasnovarsk C.I.S. 6,0009. Corpus ArgentinaParaguay -

10. La Grande 2 Caoada 5,32811. Xingo Brazil12. Bratsk C.I.S. 4,50013. USt-llim c.r.s. 3,84014. Xiaowang China15. cabora Bassa Mozambique 2.42516. Bogucbany C.I.S.17. Rogun C.I.S.18. Kanev c.r.s.19. Kalabagb Pakistan20. Barakshetra Nepal21.Tarbela Pakistan 1,75022. Paulo Afonso r Brazil 1,52423. Ertan China24. Pati Argentina25. Ilha Solteira Brazil 3,20026. Chapeton Argentina27. Ronacador Brazil~Argentina28. Bennett W.A.C. Canada 2,730

Rank Name of theorder plant/project

.,;,ii:

...

'"

~~Clsa!(

~

Iag:

~b~~~~~

~~a~~w~ ~co 0.w w>-o .. ~Zo"0.

tj '"' ....z ~

. OF- t-JM -

'.>11 Io:."' II:: 0'11) 0";0''' 4~ ... 0

V..

J~ 0z Jz 0'" tJ.

40 CHARACTERISTIC TYPES OF DEVELOPMENT

TABLE 3.1'.

Rock Mechanics 6-Foundation Treatment

CHAPTE,R .4

4.1 FOUNDATION ROCK PROBLEMSFor centuries, the hydroelectric design engineer has been

designing on the basis of past successes and, m~re important,past failures. It would be seen that this was not an easy task,judging from the endless list of failures and the very slowdevelopment of technical skills.

Leaving aside overtopping by unexpected floods, 1he maincause of structure failure has always been failure of thefoundation.

The foundation is one of the natural data relative to anydam site and, as such, it is extremely difficult to grasp it in allits complexity. it is a' pity that there have had to be terribledisasters to prove that rock foundations are not always strongenough.

The important thing is to make a correct assessment of thesituation. It would be pointless to develop scholarly behaviourpatterns for the rock matrix while overlooking the fact thatthere is a potential failure mode by sliding of large volumesalong existing geological Surfaces of separation.

Classification of Foundation Problems(I) Foundation investigation methods.

(ii) Foundation design methods.(iii) Foundation treatment methods. '

41

I.

"

19671985199319801955195419802005

1995

n.a.

1980

n.a.

n.a.

19821955

1996

198219691976198419921979198119761958199319911988197919962000

19961972/77

Contd.

2,200

2,4002,400 2,400

2,340 2,7152,160 2,700

900 2,7001,800 2,700

2,7001,613 2,6802,650 2,6501,736 2,6102,563 2,563

2,5602,520

1,500 2,5002,460 2,460

2,4242,424

2,416 2,416

2,4002,360

2,304 2,3042,300 2,300

l,gl5 2,tOO2,100 2,1001,424 2,1002,080 2,0802,069 2,069

9g0 2,0302,000 2,000

2,000

2,2682,136-432 2,136-432Yugoslavia

RomaniaArgentinaBrazil

c.I.S.

TurkeyU.S.A.Canada

BrazilMexico

C.I,s.

EgyptU,S.A.ArgentinaBrazilU.S.A.U.S.A.BrazilIndia

53. Garabi

29. Gezhonba China30 John Day U.s.A.31. Nurek C.I.S.32. Revelskoke Canada33. Ya~yreta Argt'otina34. Sa9 Simao Brazil35. La. Grande 4 Canada36. Mica Canada37. Volograd 22nd Congress C.I.S.38. Macagua II Venezuela39. Segredo Brazil40. Itaparica Brazil41. l'au10 Afonso IV Brazil42. Caruachi Venezuela43. Tocoma Venezuela44. Shrum G.M. Canada

(Portage Mt.)45. IIha Grande46, Chicoasen' Manuel

Moreno Torres)47. At.tu'k48. Gregory County49. La Grande 3SO. Volgo VI Lenin

(Kuihy,hev)5I. Dniester52. Djerdup ([ron Gates)

54. A.swan High55. Bath County56, Piedra del Aguila57. Itl.lrJ;lbiara58. Chief Joseph59 Mc'Jary60. Saito Santiago61. Tehri

~

:r "j' I,; .'

, /~

42 ROCK MECHANICS & FOUNDATION TREATMENT FOUNDATION INVESTIGATION METHODS 43

4.2 FOUNDATION INVESTIGATION METHODSThe mechanical appraisal of a rock mass includes:- a qualitative estimate of the response to the loads applied

by the structure; this includes an assessment of possible failuremodes.

- quantitative measurement of parameters used in thenumerical . analysis of the behavionr of structure andfoundation.

This must be done in several different ways:(a) a study of geology and hydrogeology of the site,(b) detailed description of the structure (geometry of

discontinuities infilling, etc) and determination of engineeringidentification indices;

(c) direct measurement of mechanical parameters for use inthe analysis;

(d) monitoring the foundation after completion of thestructure.

1. Geology and Hydrogeology:The .site investigation begins with the conventional

geological survey. The geologist sometimes bas to answer avery difficult questiolJ, whether a given fault at the site canbecome .active or whether it can be reactivated by theimpounded' water. There are hundreds of dams built onfoundations with sometimes dozens of shear faults that can beof large sizes in some cases. The fact that none has ever beenreactivated to any measurable extent is because the probabilityofa given fault moving in the short span of geological timerepresented by the life of the structure in infinitely small.

Studies have been done by Japanese Engineers (1967) andR. S. Varshney (1971) on the effect of fault location andorientation on dam behaviour.. It has been found that solong the fault remains at heel and upto middle point withorientation towards heel, the situation is not disheartening.But when the fault is located on the downstream half of thefoundation and is inclined downstream, the problem becomesalanning.

2. In-Sitn Measurements(a) Graphical Presentation of the Geological Structure

The major structural features must be drawn on maps withsome means of indicating their orientation in space, forinstance by means of rectangles showing a perspective viewof the unit square (Muller, 1963). Polar diagrams by projectionof a unit hemisphere are widely used. The "equal area

. ' . .

projection" is often preferred by' struCtural geologists as italIows for easy plotting of the distribution frequency in space.

b GeophrsicsGeophysical investigations are tabulated in Table 4.1.

TABLE 4.1Ontline summary of geophysical methods Jor subsurface

exploration

I. Seismic methods ntilizing artificially induced earth tremors,usually created by explosives or weight impacts.(A) Refraction methods.(B) Reflection methods.

2. Gravity methods measuring variations in the earth'sgravitational field as related to subsurface geologicalstructure.

3. Magnetic methods measuring contrasts or variations inmagnetic susceptibility of rocks.

4. Electrical methods.(A) Methods measuring natural potentials.(B) Methods measuring potential dwp in current

transmitted between electrodes.(C) Methods measuring distortions in natural or induced

electrical and magnetic fields.5. Methods based on measurement of radioactivity of rocks.

Seismic methodsThe speed of longitudinal waves, VL depends on the elastic

constants of rocks according to the relationship in eq. 4.1.Velocities of seismic longitudinal waves in natural materialsare given in Table 4.2.

44 ROCK MECHANICS & FOUNDATION TREATMENT FOUNDATION INVESTIGATION METHODS 45

\.-

\~ .

."

A = energy travels parallel to earth's surface;B = energy is refracted at critical angle, travels in medium of

velocity VI and provides impnls.s which return to surface;C = energy is partly refracted into medium of velocity Vl and

is partly refracted back to the surface

Fig. 4.1 Paths followed by energy from an artificiallygenerated seismic impulse

and critical distances, Xl and Xs, which are identified byintersections of the time-distance curves.

SEISMIC 'D1STUR;ANCEA

1.7-4.22.8-645.7-6.43.5-5.53.5-4.43.5-7.5

AlluviumClayLoessSandGlacial. tillGranite; quartz, monzonite; granod ioriteGabbro, diabase, basaltSandstone, shaleLimestone

softhardcrystalline

Anhydrite, gypsum saltSlateSchist and gneiss

fE--j..=-v--...VL = ,,-'p' (I - 2 v) (1 + v) (4.1)

In this equation E is Young's modulus (madulus of elasticity),p is density, and v is Poisson's ratio, usuallywith a numericalvalue near 0.25 but not in excess of 0.50. It is apparentthat the velocity depends to a great extent on the value ofYoung's modulus, E.

TABLE 4.2Some velocities of seismic longitudinal (compessionaI)

waves in natural materials (from F. Press, 1966)Velocity(km/s)0.5-2.1I 1-2.50.3-0.60.2-1.20.4-1.74.6-6.05.0-6.71.4-4.8

,.. , II x.

- I.-J-o --rs -37>0

-- Xf ----to!~ tn/~~;'- - - 1/$. 2500 1rY~c.. -~------,

, -- -2' .,1-=.,-;- - \

~+. /' lis.l. P"'/"CL.~T"'::t", ,

""o

81508

~IOOI~

'>'z;:50

~;:

DiSTANCE FORM SHOT POINT IN MTf\EFig. 4.2 Paths travelled by seismic impulse which provide'first

arrival at geophones in a seismometer and time-distancecurves obtained from measurements of time of arrivaland distances of geophones ftom shot point.

Fig. 4.1 shows some paths followed by artificially generatedseismic impulses. Fig. 4.2 illustrates the basis for determiningdepths of layers)n a simple situation, and suggests the kinds ofconsiderations that are usefnl in determining the geometry ofsurface geologicstructures.

III fiitfe, 4-.2 a seismic impulse at a shot point is transmittedalong, various paths to geophones, and a time-distance graphis plotted from the times of first arrival of impulses at eachgeoPllOne. The velocities of each of the layers is obtainedby taking' the reciprocal of the slopes of each segment of thetime-distance curve. Inspection of the time-distance curvesyields iotercept time Tl and Ts, obtained by projecting tothe time coordinate of the segments of the time-distance lines,

46 ROCK MECHANICS & FOUNDATION TRATMENT FOUNDATION INVESTIGATION METHODS 47

Gravity methods

Gravity exploration of subsdrface geologic features detectsand measures lateral variations in the earth's gravitativeinstrument called a gravimeter is used to identify gravityanomalies above bodies of rock With. smaller or greaterdensities than adjacent rocks. A possible use of gravitymeasurements in engineering investigations is in theidentification of extensively altered zones which may havelower densities than the original rocks.

Magnetic methods

Magnetic methods of subs,urface exploration depends on thefact that earth materials in the earth's magnetic field displaydifferent magnetic susceptibilities. Magnetic susceptibility isthe ratio of the degree of magnetization to the intensity ofthe magnetizing force. Lines of magnetic force in the earth'sfield tend to concentrate in "ferromagnetic" and"paramagnetic" substances and are dispersed in ,"diamagnetic"substances such as rock salt and anhydrite. Most rocks areweak paramagnetic but show appreciable and measurabledifferences in susceptibility.

Magnetometers lend themselves to observations on theground or from the air, and air-borne magneto-meters havewidespread use in regional reconnaissance studies.

(4.7)

(4.6)

p=2"a(';)

p=I (.l._.l. __1 _.l.)

~l'1 1'2 Rl R.where V is the potential difference across electrodes Band C,I is the current introduced at electrodes A and D, l'1 isthe distance from A to B, 1'2 is the distance from B to D,Rl is the distance from A to C and R2 is the distance fromC to D. In the so-called Wenner configuration the potentialelectrodes are spaced so that AB = Be = CD and Eq. 4.6reduces to :

where a = 1/3 ofthe current electrode spacing (Fig. 1.3 B).Fig,4.3 B diagrammatically shows the flow of electrical

current between two current electrodes in alluvium which issaturated with water below the water table. It is assumedthat tlte layer below the water table has a much lowerresistance (a much higher conductiVity) than the materialabove tlte water table. The confignration. of the currentand potential electrodes is the Wenner configuration' in whichthe interval between the current electrodes is divided'into equalsegments of length 'a' by the potential electrOdes. Fig. 4.3 B

Fig. 4.3 A, shows an electrical configuration commonly usedin apparent-resistivity measurements in the field. Anelectrical current is introduced into the ground through twocurrent electrodes, and the potential drop (voltage) ismeasured between two potential electrodes. The apparentresistivity between the potential electrodes is :

2" V

Electrical methods

Resistivity method frequently yield data assisting greatly ininterpretation of subsurface geology, especially in coveredareas. Resistivity techniques in general are based on themeasurement of apparent resistivity, that is resistivity measuredin homogeneous ground, commonly layered, between twopotential electrodes in an electrical field provided by twocurrent electrodes generating either a direct current or alow-frequency alternating current. The units of resistivitj inthe metric system are the ohm-centimetre or the ohm-metre.

(4.5)

(4.2)

(4.3)

(4.4)

or

or

,The depths to the layers Zo and Zl can be calculated bythe following formulas, which are most expeditiously solvedby programming into a computer:

ZO=Xl IVl Vo2 'V Vl+VO

Zo = T!_VoVl_2,fvla_Vo'

Zl =~. Tv. Vl+2 '\J V.+VlZoe Va(Vla-Vo)l/LVl (V"~Vo')lI2J

' Vo (V.'--Vl')lI', [T.- (2 Zo/Vo) [(VaL Vo')N.']ll' ]

Zl = Vl -'--,2 [(V.'-Vl.)!V.2Jl72--

and

/'

/'.'

rJ

48 ROCK MECHANICS & FOUNDATION TREATMENTFOUNDATION INVESTIGATION METHODS 49

Fig. 4.4

III II.JO\ -1'-/ r- "a-i"- R.

'IIz--.r--'" -::i c D-""

2 I ZA

IS

Fig. 4.4 indicates the kind of apparent-resistivity profilethat mi ght be obtained with fixed electrode spacing over awater-saturated fault zone buried by alluvium, and suggestsapd important application of the resistivity method. It isassumed that the water in the fault zone contains electrolytesand has considerably smaller resistivity than the adjacentrock or the overburden.

~1[-.------,I-Z

i I~ l RICH,NeE_

;~'~'~~Idealized apparent resistivity curve obtained bysurface measurements with a fixed electrodespacing over a water-saturated fat,lt buried byoverburden

Fig. 4.5 A indicates the elements of another widely usedtechnique for studying variatinns in the apparent resistivityof materials below the surface. The method, sometimescalled the "long-wire method" "tilizes a source of alternatingcurrent and a long wire extending for hundreds to thousandsof metre from one current electrode to the other. A receivingarray measures voltages along surveyed lines parallel to thewire, and results are usually plotted as voltages versusdistance. Fig. 4.5 B schematically shows a typical voltageprofile in material uf uniform resistivity. Notable departuresfrom this kind of profile indicate variations in the resistivityof subsurface materials.c. Lugeon Test

This well-known test, originally proposed by MauriceLugeon as a criterion for groutability, has since taken on new

rI--

3. Laboratory Tests

The important laboratory tests are:(i) Compression.

(ij) Radial permeability and(iii) Shear strength of joints.

which may already exist within the rock mass. They are notoften measured for structure foundations but there are caseswhere high stresses can develop neal' the surface, e.g. at thetoe of high cliffs.

One method of measuring these residual stresses uses a flatjack which has the great advantage of g'ving a direct stressmeasurement, although only to a limited depth from the ..freesurface. " .

An alternative method to use strain gauges or photo-elasticmaterials bonded into the borehole and stress relieved by"over-coring". In this way, the stresses at depth within therock mass can be measured, but the deformation modulus(which may moreover be anisotropic) must be measuredseparately.

4. InstrumentationCivil engineers have recently realizea that providing

instruments for monit0ring the rock foundations of majorstructures is a vital part of the design at least as important asmonitOring the structure itself.

(j) Geodetic MeasurementsTwo diffetent types of geodetic measurement are in

widespread use. One consists of measuring movements in theX, Y and Z axes by triangulation, and the other of measuringmovements along the Z axis only by precision levelling.

(ii) Inverted PendulumsPendulums are usually of the inverted type for rock

foundations. They consist of wires wth one end fixed at thebottom of a shaft and the top attached to ring-shapedftoatsto keep them vertical (Figure 4.6). Inverted pendulums givevery early warning of any deviation in the fuundation fromits normal behaviour.

51FOUNDATION INVESTIGATION METHODS

,_._--.,.----.,

r,ENl:l'!Aro~!---_-..JL..._----I...~!!.!!S-"''''L-~__-''--i"KK8.ut:s ANU SEDIMENT EXCLUDERSvv

0). R.$.,Varshney's design of stilling basinOn the basis of extensive model tests, R.S. Varshney has

evolved a stilling basin design, which is exhibited in figures 6.4(a) and (bJ. This design has been found to work extremelywell as compared to other ones.

A typical example of design of stilling basin on the basis ofFig. 6.4 is given in Example 6.J.

6.2.4 Downstream Floor Length and Design of DownstreamEnergy Dissipators

The main disturbance of a hydraulic jump extends upto adistance of 5(dz -dI), where dz and dl are the conjugatedepths of the hydraulic jump. In orde.. that the filter areaand stone protection be safe from the main turbulence of thejnmp, th.e desired length ofpucca floor is 6,dz~dI).

The Villue of incoming Froude number for barrages andcanal re'gulators usually lies in between 2.5 and 45, zone

. of weak.~Ydraulic jump. The energy diSSipaters and stillingbasin fo+, such conditions need cardul design. The mostsuitable.designs for low range of Froude numbers are thoseof (i) R.S. Varshnty; (ii) St. Anthony Falls; (iii) IndianStandard;StiIling Basin; and (vi) USBR. These are discussedbelow: .,

'--"C

90 BARRAGES AND SEDIMENT EXCLUDERSDESIGN OF WEIR OF BARRAGE 91

OL I !. J.5d2 /dc it c

hEIGHT OF FLOOR BLOCk.IN RELA710N TO dZDEPTH

0.21

(}6j I

&(/,

:~-O;7 O-B

HEIGHT W/~H NORMAL1I~He;"",,,'.lARIAT/ON OF SILLRIVER DEPTH

I 1 r

, I / I120r- I)

L. 20;-r ENGTH OF STILLING BASINai, IN TERMS OF FROUDE

I NUMBER AND d,

o L '---r--.\f 2 -, ... 5 ..;: ~OU.oF tJUMSJ:"R

I 1""..! -, I I I I. I I 1

t

5V

'r' -V lL

l'ITT- I'T

.-

2 I't- r I 0'4n.'

'i>'-

'0'

Q!~~~o

~I;:-

0,2Nl1fdif

VARIATION OF SILL HEIGHTWITH FROUO NUMBER

Q'rTIJ=L}~ .,,[T--n I _I-LH''d, ! ,',' I.ttl- ' I ~0. r L_LI j1 ~o UJ 4-0 fiO

1': (FROI/DE NUMSitA')ReLATION BeTWEEN FROUDE NUMBER AND til

--:r,-H1. 7".w.t.

SKETCH

4 /V~ /'

,,-..._-----I ,,/~ IJ4 1"..,o.

~~,0'~~;;:t; 0-0 -'''2 3 4" 5',

,. (PNOU.!Uf" NUM8r.'l')HEIGHT OF CHUTE BLOCKS INTFRMS OF FROUDE NUMBER

Fig. 6.4 (a)Fig. 6.4 (b) Stilling basin design for Froude number below 4.5

l:

92 BARRAGES AND SEDIMENT EXCLUDERS DESIGN OF WEIR OF BARRAGE 93

From Land 'LB relation, for L = 26 m, LB =0 6mProvide floor blocks' at a distance of 6.5 metre from toeof glacis.

(ii) ,Height: The height of floor blocks can be determinedfrom either of the two inset sketches in figures 6.4 (a) and (b).

From d2/d3 and hB/d3 relation for ds/d3 equal to 0.833,hB/d3= 0.19.

hB = 0.19 x 7.32 = 1.4 m.Fromds/o. and bId. relation curve (Pig. 6.4 b)

OS 6.1 hBfor . de = 43 = 1.415, d = 0.3

, ,hB ... 0.3 X 4.3 = 1.3 mAdopt a mean value of 1.35 m

(iii) Width: The width of floor block can be foundfrom 'the bottom sketch in figure 6.4 (a). For a Froudenumber of 2.03, WB/hB = 0.85 i.e. width of floor blocks =0.85 x, 1.35 = 1.15 m.(d) ,End Sill: The height and shape of end sill are foundfrom figure 6.4 b. For a Froude number of 2.03, H3/d3 ... 0.16:. Height of sill = 0.16 x 7.32 = 1.17 m.

Also :from the top inset sketch in figure 6.4 b, for dal de =7.32/4.3= 1.7; H./d .. 0.32

H~ = 0.32 x 4.3 = 1.37 m.The height of end sill may be taken

1.37+1.17 = I 27 I 252 . say. m.The slope of the end sill is given in top sketch of fig. 6.4 b.

(ij) Indian Standard Stilling Basin IDefinition sketch for basin I is given in figure 6.5 and

dimension sketch in figure 6.6. The length of the basin fordifferent values of Froude number is : '

455.0

44.75

34.3

23.15

Fig. 6.6 Dimension sketch for basin I

w(I) r:.V1(~!.- .0 ~2.

"" , ; SAS'.; r "C'. 1t: 'i

~",

94 BARRAGES AND SEDIMENT EXCLUDERS

CHUrl! BLOC" h':.!NO ."SIU. ~.::::..F' ":-:.-:"

t. r"- "."""",, ",., ..,,"~,= , .__.~.". ,. If:,,-" ..... ".~ ___Spa, .....,~~.,",:.~... _ r. 2'" O :

....', _._~op ..../." .. ,''''~ 0 O . 1'; ~..",. '.' .' 2d, a 'w opt"'"

..... :'.o~, .. c.'_":':"o"~::-=:::' It 0 0 0

"

DESIGN OF WEIR OF BARRAGE

l"-\.. 6--:~""'.+'- r-- ':;>.r,-~O' j... U:f1

oM~, ~

I ~I ,,' -\" ~ 'og'I (D n:J ., 1"1 0' ....~U;tO ~- ~ _~.~"t _ ..J .,. 0:.+~ ,3'& 10-. " " _ b.

- Q - ~,. - ,;-. ~ 0-~"'\. R'O!_ IC' "' 'f $....'} ~~ ID _.l if;' J...,... / .~"'f ..1 0F:- 1 -----0" ci ~

.'''''l $~

(vii) 'The height of the side wall above the maximum tail water'depth is given by Z = ds/3.

6.3 WEIR SECTION - DESIGN FOR SUB-SURFACEFLOW

The weir sectbn, glacis and downstream stilling basin arerequired to wi thstand the upl ift pressures and this objectivecan be 'l~hieved in two ways.I. Gravity Design - By making the section at all points

of sufficient thickness and weight equal to the maximumuplift pressure at the point under consideration. Thisdesign is ,known as a gravity section. The design is done~y use of Khosla's theory.

2. ,Raft Design - By designing the barrage slab as areinforced-conctete raft and utilizing the weight of thepiers and groins to assist the glacis and hearth slabs in

'resisting uplift. This design is known as the Raft design;, where adopted, care should be taken to provide some,positive pressure or weight in excess of the IOtal uplift':pressure,Where the Raft design is adopted, the reinforcement'should be designed in accordance with reinforced-concretepractice. Where the gravity design is used, the slab isoften reinforced to prevent temperature and shrinkagecracks.'rhe selection of one type or the other is a matter ofjudgement, taking into consideratio'n the comparative'cost, facilities for construction, and available materials,and skills.

96 BARRAGES AND SEDIMENT EXCLUDERS

d2'= 0.85 d2 for Fl = 5.5 to 11'( F 2= 1.0 - 8tO )ds for Fl = 11 to I7

(6.5)

(6.6)

WEIR SECTION-DESIGN

..

97

6.3.1 Khosla's Theory of Independent Variables - InfiniteDepth of Permeable Fonndation

The usual barrage and weir sections do not cor,form to aSimple elementary form and a direct solution of the Laplaceequation is tno feasible. To apply the analytic solution to

I ..... , ~J1\i ~IV..,C Vi t.lle IlVUl,

equation is tno feasible. To apply the analytic solution to

;;Jl13C -,\. - b a.~b/d

(IV)

=-'1H

J1 .__ qCJ'i

,II) 0

~01~ J01=- II

H

JE1\S--b ....l Ot (I)

WEIR SECTION-DESIGN 97any practical composite profile of a weir or a barrage,Khosla and his associates evolved the method of independentvariables. In this method a composite barrage or weirsection is s~lit up into a number of simple standard formsof known analytical solutions. The most useful standardforms among these are:(a) A straight horizontal floor of negligible thick"ess witha sheet pile at either end [Fig. 6.10 (;) and 6 10 (ii)).

(b) A straight horizontal floor depressed below the bed 'butwith no vertical cut off [Fig. 6.10 (iii,).

(c) A straight horizontal floor of negligible thicknesswith a sheet pile !ine at some intermediate position

[Fig. 610 (iv)].

(1\\)Fig. 6.1 0 St"nd"r~ f",ms _. Khosla method of

inclellendent variablesIn general, the us'!al weir section cC'nsists of a combination of

all tbe three forms mentioned above; the entire length of thefloor with any of the pile 1ines, etc. making up one such form.Each elementary form is then treated as independint of theothers. -rhe pressure at the key points are then read off fromthe curves of Figure 6.11. -rhese key points are the junctionpoi\1ts of the floor and the pile lir.e "f that ;uarticularelementary form, the bottom point of that pile line and thebottom corners in the case of 'epressed floor.

The percentage pressure observed from the curves for thesimple form into which the profile has been broken up, is validfor the profile as a whole if corrected for:

(a) mutual interference; (b) the floor thickness; and(c) the slope of the floor. .

BARRAGES AND SEDIMENT EXCUJnFRS96

is with the bottom of the floor. The pressures at the actualpoints E and C are interpolated by assuming straight linevariation from the hypothetical point E to D and also frum Dto C [Fig. 6.12]. .

98 . BARRAGES \ND SEDIMENT EXCLUDIES

Correction for ",,,t,,al interfere"ce of pilesLetC, be the correction to be applied as percentage of head,

b', tlie distance between the tW;) pile lines (Fig. 6.12),

)\

WEIR SECTION - DESIGN 99

Correction for !loor thicknessIn the standard forms with vertical cutoffs the thickness of

the !loor is assumed to be negligible. Thus as observed fromthe curves, the pressures at the junction point E and C pertainto theleve! at tb.e top of the floor whereas the actual junction

D, the depth of the pileline, the influence of which hasto be determined on the neighbouring pile of depth d. D is tobe measured below the level at whiob. interference is desired.

d, the depth of pile on which the effect is to be determined.Then,. }l

;.)......,.,,:1..\

) .\.>-- -, T- 6 9 ID II It ~ 14 ~ ..Sj~

FIG. 617 ( j, .

v:' I. I

\0 .

....

o

15 ....-----,-rr-'I~I TTInT 'II I lill I I _I;

14 t---I+IJ' i\IIIi: i': [ i\..1 E~ERGY OF HOW CURVE}'

13 l----rl+ill-i--II-++-I~ -, ; ,;'\\ I" .

u I, i :-\\~~-H++-~ ~_~_\ ----C,+-_i+-".,--'---t--+--'+---+.J.. :I

II t-1t\ttH-ttt--+H--+t\-+-\\-I'rIH\,:----+--++---'--'-+_---1-/-1-1---+----'-1...',\ \ \ r. ,l,;,, \1\ \ ' l~vl~ /~\\ l \ v? /' ~;+--+7.Ly;n7i9':fL-;4---

II \ \ . [\ ',,- f'---. .I'll -.-0-/

\\\\! \1''''~l>/Y__~ '$%'o 6 I \ \, .~" ~, 7 / j~~ 1\1 h\\\\j--- ;~e-f-r--+----I-

5 I \ \-} i

104of midpoint pressures decrease with reduction in depthof permeable stratum.maximum variation 5.5 p.c.

Case 2 - Floor with a row of piles at the centre(j) Desirable '

106. BARRAGES AND SEDIMENT EXCLUDERS VEIR SECTION - DESIGN 107

fI'

increase upstream of step and d"""ease downstream ofstep~maximum variation about 10 p.c.

To resist the indicated uplift pressures, the floor isusually designed as a pure gravity section. The requirementof floor thickness at key points may be worked out bydividing uplift pressures by the submerged density of thefloor material. 1f H is the net heat at a point and G is thedensity of the floor material, the thickness required = :- 1.

Density of concrete may be taken as 2.4. The floormay also be designed as a reinforce'.! concrete raft helddown by the weight of piers. A raft may in some cases becheaper and more desirable as the thin section of raftreduces deep excava""ion and dewatering problems. Recentexamples with reinforced concrete floor are the Farraka,barrage, New Narora barrage and Durgapl..r barragt,. Alogical method for the deshrn of raft is to treat it as Hexibleso that maximum pressure .inter,sities occur near the piers,thus reducir:g considerably the bending moments in themiddle of slab which w0uld occur if the raft was consideredrigid and load distribution imiform along the entire spanbetween piers. Reasonably accurate results may be obtainedby soil line method which assume linear distribution ofpressure at the base.

It shall be seen that hydraulic structures on permeablestrata are subject to uplift pressures as well as superimposedloads. After evaluating these forces, the design of various

~tructures is carried out by the accepted principles ofstructural analysis.

6.3.4 Effect of Three Dimensional Seepage Flow on tileDesign of Barrage Floor

The impervious floor of hydraulic structures, founded onpervious stratum, has to, withstalld forces due to seepage flowunderneath. In actual practice the seepage flow tinder suchstructures is invariably three dimensional. In case ofbarrages or weirs the effect of three dimensional seepage flowbecomes predominant in the bays adjacent to canal headregulator, due to seepage from offtaking channel. Such threedimensional seepage flow is not amenable to theoreticalsolution. Also no dependable fleld. data about the. uplift

pr

108 BARRAGES AND SEDIMENT EXCLUDERS VEIR SECTION DESIGN 109

(6.13)mb"'SD/2D = -/- + P

The values of the parameters viz. c, H. he, / and L dependupon the layout of the entire barrage complex and its designon the basis of surface flow. Initially the floor profile isworked out with surface flow consideration and the thicknessesof the floor on the basis of two dimen sional seepage flowtheory. When the layout of the whole complex is ready,the ratio SD!2D is worked out at the toe of glacis and atthe end of the floor. The ratio SO!2D for any otherpoint in between can be computed by linear interpolation.After calculating the SD/2D ratios, the uplift pressures canbe modified corresponding to three dimensional seepage flowand the floor thickness provided according to the newrevised uplift pressures.

The value of "'SD/

110 BARRAGES AND SEDIMENT EXCLUDERS DESIGN OF A BARRAGE 111

(iv) Permissibleafllux = 1.5 m

Fig.6.19 Uplift pressures at the toe of glacis for L/C andhelh value

The ratio so obtained when multiplied by the uplift pressureworked out by two dimensional approach gives the true upliftpressure eorresponding to three dimensional seepage flow.

6.4 DESIGN OF A BARRAGEDesign Example 6.1Design a barrage for the follo winz data .-

(D Design discharge(a) For finding waterway: 50 years

frequencyflood' = 7200 mS/s(b) For design of structures and for protection

works: 100 years frequency flood = 11400 mS/s(ii) Average bed level of the river = 130.0 m

(iii) Flood level before construction of barrage= 135.801 m (nOOm3/s)

408 m

-127.75 m130.0 m128.75 m131.0 m

= 90 m= 270 m= 45 m

3 m

Total

= 2.0.8 x 2.08 = 0224 m-2x 9.81

= 136.0 + 0.224 = 136.224

Velocity head

TEL d.s.

Assumed Ie velsd.s. floor level in sluice baysCrest level of sluice baysd.s. floor level of 0 ther baysCrest level of other bays

1.0 No concentration and no retrogressionMax. flood level downstream = 136.0 mLacey's waterway 4.83 v' 11400 = 517.0 mFlood intensity q = 114001517 = 22 cumec/mLacey's scour depth = R = 1.35 (222/n1/s = 10.6 mVelocity of flow = 22/10.6 = 2.08 mls

(a) Underslu!ce BaysAssumed afflux = 2 mu.s. TEL = 136.224 + 2.0 = 138.224 mHence u.s. max. flood level = i 38.224 .- 0.224 = 138.0

SolutionLacey's waterway = 4.83V7200 = 410 mBays will be kept as below:5 undersluice bays each 18 m wide15 other bays each 18 m wide18 piers each 2.5 m thick1 divide wall 3 m thick

(v) Pond levels 135.0 m (canal discharge 510 cumec)135.6 m (canal discharge 650 cnmec)136.0 m (canal discharge 765 cumec)

(vi) Lacey's silt factor 1.0(vii) Safe exit gradient for fiver bed soil 115(viii) Concentration 20%

54'"

:;:-;,\L POTeNTIAL

#~ ""------ ~------_!@:::.:< /W ..?OOV~~,

'~

112 BARRAGES AND SEDIMENT EXCLUDERS DESiGN OF A BARRAGE 113

L

He = Head over crest = 138.224 -1300 = 8.224 m. h. = Head difference = 138.224- 136.0"" 2.224 m

d = 136.0 - 127.75 = 8.25 mh./He = 2.224/8.224 = 0.272; (hd + d)/He = 10.474/8.224

= 1.27Hence reducticn in C. = 9% from USBR cnrvesActual C. ~ 0.91 x 1.7 = 1.55q ~ 1.55 X (8.224)1'5 ~ 36.4 cumec/m

q (outside piers) = 36.4 X (90/100) = 32.76 cumec/mV = 32.4/8.25 = 3.92 m/sv2n = 3.92 x 3.92 = 0 79

g 2>

114 BARRAGES A1'."'D SEDIMENT EXCLUDES DESiGN OF A BARRAGE us

8.25 17.2' "of A

Fig 6.20 Design example - definition sketch

.v3~1

Actual G works out 0 177 Le. I in 5.65, hence okSimilarly exit gradient downstream of filter can be worked OJt.

Downstream sheet piled = 127.75 - 120.55 ' 7.2 m; b . 54 m

1150,,124.0 1000

., -j1j-6750 1 33000 1,,120.55----54,0001----

5.0 Floor Thickness(a) Static conditionsWater level u.s. ~ 136.0 mWater level d.s. 127.75 (undersluice bays);

128.75 (barrage bays)

(I) Underslnice bays (Fig 6.20)d = 130 - 124.0 ' 6 m .. b = 54 m1/' - d/b = 6/54 = 1'9 ~ 0.111v 100-19.5 80.5'/. :. 0=100 29 71'\'.

Correction for floor thickness =1.15 (80/ - 71' ~ 1.82 (+ vel

Interference of d.s. sheet pile :D = 128.85 - 120.55 8.30 md = 128.85 - 124.0 = 4.85 mb = b' = 54 m

correction = 19J 83 x 83 +-/.85 = 1.82(+ve)54 5

corrected 40 = 71 0 + 1.82 + 1.82 74.64%

d = 127.75 - 120.55 = 7.2 mH = 136.0 - 127.75 = 8.25

H I IGE ~. (j' ,,'" T or 5.5

-i- = 0.1585".. A

From Khosla's curve (Fig. 6.13), " = 7.2b = total length of lIoor = 7.2 X 7.2 = 51:84,

provided 54 m, hence safeActual exit gradient on floor level

GE = 87, ; / 1 = O. I761 Le. I in 5.67. "1+41+,,2 ok

'1/-2-

Similar design can be found for downstream of sluice baysand on the u.s. and d.s. of other bays.

4.0 Exit Gradient (at the level of lIoor)(I) Under sluice bays

(iI) Other barrage baysH = 136.0 - 128.75 = 7.25 m ., d = 6.3 mI 7.25 1 1

5.5 = 63' ,," A or.of A = 0.158From Khosla's curve" = 7.1Hence floor length = 7.1 x 6.3 = 45 m; provided 47.23 m

the thickness of stones ~ 1.8 m (every dimension of stonebigger than 30) mm and weight plus 40 kg)

Hence length oflaunching apron =2.8D/1.8 = (2.8 x 12.82)/1.8 = 19.55 m say 20 m

Over 600 mm thick stones, two layers of blocks of size0.6 x 0.6 x 0.6 m be laid, with 50 mm jhiri in between twoblocks would suffice.

Hence length of blocks = 16 x 0.6 + 16 X 0.05 ~ 104m.The balance length of 9.6 m can be laid in 1.8 m loose thickstones.

~.~

~

116 BARRAGES AND SEDIMENT EXCLUDERS DESIGN OF A BARRAGE 117

TABLE 6.6Flo"r thickness of barrage bays

.-lI.125.7 1000-111-6750 I 30,500 I 122 C;

I 47,250 ..2. A .Fig. 6.21 Design Example

ActualDistance Per cent of required floorfrom floor Uplift head, m flood thicknessend'm' uplift thickness provided(m) (m)

v3II

.10

v136.0.........

TABLE 6.5

d/b => 7.2/54 = 0.1333" D1~ 23%; 81 = 83%Correction for floor thickness = 1.9(3$.;23) => 2.64(-ve)

Correction for n.S. sheet pile interferenceD => 125.85 - 124.0 => 1.85 md = 125.85 - 120.55 = 5.3 mb => b' = 54m

Correction = 19.J 1;:5. 1.85 ~ 5.3 =.0.464 (-ve)Corrected .pE = 33 - 2.264 - 0.464 = 29.895 or 29.9 %Pressure drop per metre length of floor =

(74.64-29.9)/54 = 0.83 %Total static uplift = 136.0 - 127.75 = 8.25 m

Floor thickness of nnderslnice bays

(i) Barrage bays (Fig. 6.20Working in the same way as above, we get pressures

and floor thickness as below :

uplift pressure \FIoor length Actual floorin m thickness thickness

col. 3";-1.24 provided

----3--- 4 5

distance oflper centpoint from up upliftfloor end

I I 2

0 29.933 57.339.75 62.9

40.75 63.8

2.474.73[5.19 (130 127.75)]=> 2.945.26-2.25= 3.01

1.993.82

2.37

2.43

2.153.95

2.55

0 30 2.17 1.75 1.9030.5 57.8 4.2 3.39 3.6037.25 64.0 4.64-(l3t -128.75)=2.39 1.93 2.0538.25 64.8 4:72- 2.25=2.47 1.99 2.1039.75 66.2 4.82-2.75=2.07 1.67 1.85

6.0 Profile of Hydraulic Jump and Unbalanced Pressore(Fig. 6.22)

Calculations for barrage bays are given herein. Similarcalculations can be done for undersluice bays.

Q = 9500 m3/s ; H = 138.663 - BI.O = 7.063 mq = Cv V2g (Zo + H--=

118 BARRAGES AND SEDI\fENT EXCLUDERSR.C.C. RAFT DESIGN FOR BARRAGE 119

The uplift pressures and floor thickness are worked out(for 9500 mls in Table 6.8

Table 6.8Uplift pressores and floor thickoesses (Q = 9500 comec)

6 II 5 4 I 3 2-,...~--I~ I I '--11-1 _-v13,IOI I!~~~ de

Zc 2iiOo I 1 If I Q 1-4000i-400~

I 10,000 I 6750-+ 34500- --lSection

"'.hed s.water

levelm

level ofjump proprofile

m

Uplift pre- Required Givenpressure, m floor floor

thickness thicknessm (5+2) m m

Fig. 6.22 Design Example

30 2d" 8Fl ~ ..r9 81 x2.8 ~ 2.06d2 ~ 6.88 for momentum formula

Normal water depth = 135.25 - 126.75 = 650 mHence hydraulic jump can form on the bed stilling basin

(3024 113

water depth at section 6 = yo = (d2:g)1/3 => 9.;n) = 4.54 m

water depth at section 5El = dl + V12/2g = 138 C63 - 130.08 = 7.983 m

830.242 d" + 46.7 .

7.9 3 = dl + 2x 9.81 Xd12 = 1 dlS . dl=3.IOm

W~ter depth at section 4El = 138.063 - 129.42 => 8.643 m = dl + 46.7/dls

dl ~2.83mHence the jump profile in the critical range (Fig. 6.16), istabulated (Table 6.7)

Table 6.7. Hydraulic jump profile

X-'dl y/dl x(m) y(m)

2 1.85 5.6 5.18

4 2.30 15.26.44

6 2.55 16.8 7.14

8 2.68 22.4 7.50

10 2.70 28.57.55

1-1 05737x2.6~1.49 135.63 131.55 2.71 4.2 3.39 3.602-2 0.5367 x 2.6 ~ 1.40

"133.25 1.59 2.99 2.41

"3 3 o4989x2 6~1.30"

134.23 0.93 2.23 1.79"4-4 o5917x2 6~1 54

"132.25 2.26 3.8 3.05

"5-5 0.6107 x2.6 ~ 1.59"

133.18 1.63 2.22 1.8"6-6 0.6357x 2.6~1.65

"135.54 006 1.71 1.38

Full US. flvu~ ...,.~. J. U"~O;;U. ......1 c ......"',U.

120 BARRAGES AND SEDIMENT EXCLUDERS R.C.C. RAFT DESIGN FOR BARRAGE 121

settlement in subgrade due to the load transmitted throughthe raft. The magnitude of the settlement, of conrse,depends on the type of subgrade, and this variable is accountedfor by the modulus of wbgrade readion (kY which representsthe resisrance of the subgrade to displacement undcr load. Onthe other hand, the raft beam must deflect to the extent ofsettlement. The soil reaction between the end of span and thecentre due to loads transm;tted through piers will depend on

(6.14)calledy~Yl+Y2

This expresses the characteristics of the -beam. -and isthe heam line equation.

The soil reaction below the raft at the end is w + qw andtherefore the settlement at the end points is Y! t qw and at the

the modulus of suhgrade reaction (k), which is given by (w/S),where w is the load and S the settlement. If we consider thefloor slab with concentrated loads at pier points acted on bysoil reaction (proportional to the settlement at each point) thesoil reaction at any point is k.y., where y is the settlement

Also the deflection of beam is proportional to E~~:Y.Solving these equations it is theoretically possible toobtain the varying. soil reaction across the slab span. Thephysical effect of taking into consideration the deflection andthe variation in the soil pressure would be to allow for thedisc-shaped settlement which actually occurs in raft beamsunder uniform loading.

In average raft beams designed on the basis of udform soilreaction, the moments work O\,t of such a high order so as tocause a deflection of 25 mm to 75 mm in the raft beam.But due to resistance of subgrade to settlement, the momentsare considerably rednced. Hence to provide reinforcementfor such moments without taking into account the deflectionsand variation in soil pressure, is extremely irrational anduneconomical.

6.5.2 Theory of SoH line According to the soillille method,the variation of soil reaction between the piers and the centreis taken to be straight line and therefore, can be expressed asw+qw at the end of span ar,d w-- qw at the middle, where wis the average soil reaction and q being the variation constantof soil reaction. The value of w consists of average value ofsoil reaction due to the loads transmItted through the piers anduniform loads like uplift pressure. The soil reaction variesfrom maximum at the ends to the minimum at the centre ofspan as shown in Fig. 6.14.

Let Yl = deflection due to the average soil reaction w, andY2 -. deflection due to the varying element of q.w. The netdeflection

-,

.'

~-0oil

r;:;

*'~t:'lI]

~a.

g!~.,"''!:\~Y,

_~~ 11..,;r--

.J ~ -I-~1; T1~ ~ 1}~, ~ ti~'1 a; 1

LU I;: ..,.SO!

p;w'r:t"'"-

'-

122 BARRAGES AND SEDIMENT EXCLUDERSDESIGN OF RAFT ON SOIL LINE THEORY 123

'" L------_

Fig 6.24 Variation of Soil Reaction

The sign' of Y2 being ~ ve, the net deflection y = Yl - y2I 7 q W 1.4 )

". _. (w+qw) L4 - 1920 - a-' {6.186.5.3 Load Conversion Factor The load under the piers areassumed not to generate any bending moments and tlteuniformly distributed loads being transmitted tltrough the piersare not converted in to the loads causing moments. In

(6.20)

= 200 cumec= 302.60 m.=3m= 299.60 m= 1070

= 1/5

Design Example 6.3The foundation explorations below a head ragulator {data

given below} indicate tbat river sand is available upto R.L.293.0 m only below which there is impervious clay stratum.Experiments also indicate that there would be no excessiveuplift pressure if the depth of concrete floor is restricted toR.L. 298.50m. Design the floor as raft using soil line theory.Assume the value of the modulus of subgrade reaction (k)equal to 2