EARTH DAMS

EARTH DAMS

Contents -INTRODUCTIONTYPES OF EARTH DAMSFAILURE OF THE EATH DAMSDESIGN OF EARTH DAMPHREATIC LINE IN EARTH DAMSTABILITY ANALYSIS

INTRODUCTIONEarth dam is most common type of dam and generally built of locally available materials in their natural state with a minimum of processing Earth dam are composed of fragmental materials which maintain their individual identityThese fragments have spaces and voids between them and drive their strength from their position, friction and cohesionThe foundation recruirements of earth dams are less stringent than for the other types of dams Earth dams are of considerable size

TYPES OF EARTH DAMSAccording to method of constructionTypes of earth damsHYDRAULIC- FILL DAM-The material is excavated, transported and placed by hydraulic methodsFlumes are laid at a suitable falling gradient along the outer edge of the embankment. The mixed material is pumped into these flumesThe slush is discharged through the outlets in the flumes at suitable interval along their lengthThe slush flow towards the centre of the bank so coarse material of slush settle at the outer edge while the fine materials settle at the centreNo compaction is done

Types of earth damsROLLED-FILL DAM-The embankment is constructed in successive, mechanically compacted layersThe suitable materials are transported from the borrow pits to the construction site by earth moving machineryMaterial is spread by bulldozers , and sprinkled to form layers of limited thickness having proper water contentThese layers are compacted and bonded with preceding layers by power operated rollers of proper design and weight

Types of rolled-filled earth damHOMOGENEOUS EMBANKMENT TYPE-A purely homogeneous earth dam is composed of single kind of material and used only for low to moderate heightsThese dams are usually composed of impervious or semi-impervious soils to provide adequate barrier.The upstream slope is kept flatter to make the dam safe during sudden drawdown conditionA purely homogeneous section has been replaced by modified homogeneous section

Types of rolled-filled earth damZONED EMBANKMENT TYPE-Dam is made of more than one materialIn this dam section a central impervious core is flanked by zones of more pervious materialThe outer shells are made of freely draining pervious material . The shells give stability to the central impervious fill and distribute the load over a larger area in foundationSometimes a semi-pervious zone is provided as a transition between outer shell and central coreA drainage system is provided at d/s side.

Types of rolled-filled earth damDIAPHRAGM EMBANKMENT TYPE-This is modification over the homogeneous embankment typeIn this bulk of embankment is constructed of pervious material & a thin diaphragm of impermeable material is provided to check the seepageDiaphragm is placed either centre of the section as a central vertical core or at the u/s face as a blanketThe diaphragm may be of impervious soil, cement concrete, bituminous concrete any other impervious material

CAUSES OF FAILURES OF EARTH DAMSOn the basis of investigation reports on past failure, types of earth dam failures are categories into three classes-Hydraulic failures : 40%Seepage failures : 30%Structural failures : 30%

Hydraulic failure-Hydraulic failureOvertopping-The earth dam may get overtopped if the design flood is under-estimated or if the spillway is of insufficient capacityInsufficient free board or settlement of foundation and embankment may also lead to overtoppingToe erosion-Toe erosion may occur due to tail water and due to cross currents that may come from spillway buckets Diaphragm walls of sufficient length & height should be provided to check the cross flow towards the damA thick riprap on the d/s slope is provided to avoid toe erosion upto a height slightly above the tail water levelHydraulic failureWave erosion-The effect of the wave is to notch out earth from the u/s slope in absence of proper slope protection Rollers are developed in the waves which try to scoop out the earth. Wave can also cause u/s slip

Gullying-D/s slope may fail due to formation of gullies by heavy downpourProper berms, turfing & drainage system is provided to prevent from failure

SEEPAGE FAILURESPiping-The seepage of water through the body and foundation of the dam may lead to piping or progressive erosion of concentrated leaks

CONT.Water seeping through the earth dam may have four bad effect-Seeping water generates erosive forces which dislodge particles from the soil structure and causes rearrangement of fines to voids between larger grainsThe flow associated with differential pore pressure can lift portion of soil mass causing sand boilingInternal erosion of soil mass leads to the formation of an open conduit through the soilInternal pressure in the soil water reduce the soil strength and thereby lead to weakening of soil mass and even failure by shear

Cont.Leaks in the embankment may also lead to piping failureMost of the serious trouble from piping has resulted from progressive backward erosion of concentrated leaks which develop through or under the dam

SEEPAGE FAILURESSloughing-Under the full reservoir condition the d/s toe remains saturated and may erode and produce small slump and miniature slideThis miniature slide leaves a relatively steep face, which become saturated by seepage from the reservoir and a slump again, forming a slightly higher and more unstable faceThis raveling process can continue till the remaining portion of the dam is too thin to withstand the water pressure and complete failure occurs suddenly as the reservoir breaks throughStructural failuresStructure failure may be due to following reasons:u/s and d/s slope failure due to construction pore pressuresu/s slope failure due to sudden drawdownd/s slope failure during full reservoir conditionFoundation slide : spontaneous liquefactionFailure by spreadingFailure due to earthquake Slope protection failureFailure due to damage caused by burrowing animalsDamage caused by water soluble materials

Structural failuresu/s and d/s slope failure due to construction pore pressures:When a dam is built of impervious compressive soil, the drainage is extremely slow and excess pore pressure develop during and immediately after constructionWhen the permeability is law there may be no drop in pore pressure in central zone of dam by end of constructionThere are two types of construction slides Slide occurs slowly and continues at a uniform rate for a period usually of two to four weeksSlide occurs rapidly and suddenly

Structural failuresu/s slope failure due to sudden drawdown:For u/s slope , the critical condition is when the reservoir is suddenly emptied without allowing any appreciable change in water level within the saturated soil mass. This is known as sudden drawdown.When the u/s slide occurs due to sudden drawdown, the pore pressure along the surface of slide are dissipated to a large extent. Hence there is lesser tendency for the continued sloughing and sliding.u/s slides does not cause complete loss or failure of water from the reservoir.

Structural failuresd/s slope failure during full reservoir condition:For d/s slope the critical condition occurs when the reservoir is full and percolation is at maximum rateThe direction of seepage force tend to decrease the stabilityPore water pressure acting on soil mass below the saturation line reduces the effective stress responsible for mobilizing shearing resistance There are two types of d/s slides-Deep slidesShallow slides

Structural failuresFoundation slide : spontaneous liquefaction:When the earth dam has foundation of fine silt or soft soil it can slide whollyExcess water pressure on confined sand and silt seams in the foundation may also cause unbalanced condition causing foundation failureSometimes a soft and weak clayey seam exists under the foundation and the dam can slide over it causing failureExpansion of soils on saturation may cause lifting of the slope and thus cause failure

Structural failuresFailure by spreading:These are observed only in connection with fills located above stratified deposits that contain layers of soft clayFor example : Marshall Creek Dam in Kansas (1927)

Structural failuresFailure due to earthquake :Most serious damage and failures of dam may be due to following effect due to earthquake:-Cracks in core of dam leading to leakage and piping failureSettlement of crest due to compression of foundation thereby reducing free board and causes overtopping Shaking of reservoir bottom causing slow waves hence failure due to overtoppingLiquefaction of sand below foundationAcceleration forces on embankment may cause shear slide of appreciable point of the slope of damFault movement causing reduction in the reservoir capacity and consequent overtopping

Structural failuresSlope protection failure:Slopes are protected by riprap over a layer of gravel or filter blanketDuring the heavy storm, the waves on the surface of the reservoir beat repeatedly against the slope The wave energy is dissipated turbulent action on land and within rocks of riprap layer.

Cont.Turbulent action have two effect:The wave may pass through the voids of riprap and may wash away the filter layer, setting the riprap layer and exposing the embankment to wave erosion If the average size of rock comprising riprap is not heavy, it may be washed out of the layer by the hydraulic forces generated by the waves

Structural failuresFailure due to damage caused by burrowing animals:Burrowing animals may cause piping failure of small damAnimals like muskrats burrow into embankment either to make homes or to dig passes from one pond to anotherIf many muskrats are involved , their holes may dangerously honeycomb a small earth dam, making it weakGround squirrels normally dig only in dry soil and stop at the point where seepage is encountered

Holes made by muskratsStructural failuresDamage caused by water soluble materials:The leaching of natural deposits of water soluble materials, such as gypsum create troubleWhere considerable foundation leakage occurs, leaching of gypsum of the foundation may cause excessive settlementThe deposition of soluble material previously leached from the natural soil may tend to plug specially designed filtersDESIGN OF EARTH DAMCriteria for safe design of earth dam:The embankment must be safe against overtopping during occurrence of inflow design flood by provision of spillwayThe dam must have sufficient free board so that it is not overtopped by wave actionThe seepage line should be well within d/s face so that no sloughing of slope take placeSeepage flow through the embankment, foundation and abutments must be controlled by suitable design provision so that no internal erosion take placeThe portion of the d/s of impervious core should be properly drained

Cont.The u/s and d/s slope should be designed so that it is safe during and immediately after constructionThe d/s slope should be designed so that it is safe during steady seepage case under full reservoir conditionThe u/s slope should be stable during rapid drawdown conditionThe u/s and d/s slopes of dam should be flat enough so that shear stress induced in the foundation is enough less than the shear strength of the material in foundationThe dam as a whole should be earthquake resistant DESIGN OF EARTH DAMSection of an earth dam:- The preliminary design of an earth dam is done on the basis of past experience and on the performance of the dam built in past.Top width- The crest width of an earth dam depends on the following consideration-Height of structureImportance of the structureWidth of the highway on the top of the damPracticability of constructionProtection against earthquake forcesNature of embankment material Cont.Empirical expression for top width b of the earth dam-

b =Z/5 +3 : Applicable for low damsb = 0.55Z1/2 +0.2Z : Applicable for dams lower than 30mb = 1.65(Z+1.5)1/3 : Applicable for dams higher than 30m

where Z is height of the damAccording to the Indian Standard should not have crest less than 6 mFree board:-Free board is the vertical distance between the horizontal crest of the embankment and the reservoir levelNormal free board is the difference between top of the embankment and normal reservoir levelMinimum free board is the difference in the elevation between the crest of the dam and the maximum reservoir water surface that would result should inflow design flood occur and should outlet works and spillway function as planned

Casing or outer shells:-The function of casing or outer shells is to import stability and protect the core Pervious materials, which are not subjected to cracking and on direct exposure to atmosphere are suitable for casing

As per IS:1498-1970 & IS:8826-197833Side slope for design of earth dams according to Terzaghi:

Preliminary dimensions of earth dam according to Strange:

Central impervious core:- The minimum safe thickness of the central impervious depends on the following factors-Tolerable seepage loss Maximum width that will permit proper constructionTypes of materials available for core and shellsDesign of the proposed filter layersPrecedent on similar projects

Shear strength of the core materials is always lesser than the rest of the embankment. The thickness of the core at any elevation is not less than the height of the embankment at the elevation so that the average hydraulic gradient is less than unity

Cont. The width of the core at the crest of the dam should be a minimum of 3m to permit economical placement and compaction of impervious embankment material by construction equipmentThe top level of the core should be at least 1 m above the maximum water level to prevent seepage by capillary syphoningSize and range of impervious core in zoned embankment-

Suitability of material for core-

As per IS: 8826-1978Cutoff trench:- Following are the Indian standard recommendations for cutoff trench (IS : 8826-1978)The alignment of cutoff trench should be fixed in such a way that is central line should be within the u/s base of the impervious core and it should be keyed into continuous impervious strataThe positive cutoff should be taken atleast 1m into continuous impervious stratumThe bottom width of cutoff trench may be fixed taking following factors-Provide sufficient working space for compaction equipmentProvide sufficient working space to carry out curtain grouting Provide safety against piping Minimum bottom width of 4m is recommendedDESIGN OF EARTH DAMDOWNSTREAM DRAINAGE SYSTEM: Filter zones are provided in all earth dams. Provision of filter zones at downstream side serves two purposes-It reduces the pore water pressure in the d/s portion of the dam , hence increase stabilityIt checks the piping by checking the migration of particles.Generally three types of drains are provided in the earth dams:Toe drains Horizontal blanket drainsChimney drains Toe drains:-Toe drains were installed in oldest homogeneous dams in effect to prevent softening to which was observed at the d/s toe

Horizontal blanket drains:-Horizontal drainage blankets are widely used for dams of moderate height Advantage of provision of this drain results from the fact of stratification of embankment material having permeability in horizontal direction more than vertical

Chimney drains:-In horizontal drain the seepage water may flow horizontally at top of relatively impervious layer and discharge on d/s face cause surface sloughing.To prevent trouble with stratification and to intercept seepage water before it reaches the d/s slope , dam is constructed with inclined or vertical chimney drains.

Filter criteria:The dimension and permeability of filter drains must be chosen in such a way that the drainage system can carry away the anticipated flow with an ample margin of safety.Generally a multilayered filter provided in which each subsequent layer becomes increasingly coarser than the previous one. Such filter known as inverted filter or reverse filter.According to Terzaghi the filter material should fulfill the following criteria-

42The criteria given by Terzaghi have been further modified as-

, provided that filter does not contain more than 5% of material finer than 0.074mm (IS sieve no. 200)

CONT.Drainage of earth damThe filter or the rock toe discharges the water into a network of drains. The drains may consist of open trenches or filled with rough stone.The number of drains and interval depends on-Nature of soil on which dam is foundedNature of the groundType of material used for dams

Phreatic line in earth damPhreatic line- Phreatic line or seepage line is defined as the line within a dam section below which there are positive hydrostatic pressure in the dam. The hydrostatic pressure on phreatic line is atmospheric or zero.The phreatic line can be located by Analytical method Graphical methodExperimental method

In case of homogeneous earth dam with horizontal filter the phreatic line can be located by Casagrande methodCasagrande method-For homogeneous earth dam with horizontal filterCasangrande assumed the phreatic line to be a base parabola with its focus at F , the starting point of the filter FE.

Procedure for locating the phreatic line graphically-B is the u/s face. Let horizontal projection be L BC= 0.3L and point C is starting point of base parabolaTo locate the position of directrix of parabola let C is the centre and CF as radius. Draw an arc to cut the horizontal line through CB in D

Cont.Draw a vertical tangent to the curve F at D. Evidently, CD = CF, hence DH = DirectrixThe last point G on the parabola will lie midway between F and HTo locate the intermediate points on parabola use principle that its distance from focus and directrix must be equal . e.g. locate any point p , draw vertical line QP at any distance x from F. Measure distance QH. With F as centre and QH as radius , draw an arc to cut the vertical line through Q in point PJoin all these points to get base parabola. Correction should be made at every point.The phreatic line must start from B , not from CCONT.Phreatic line is a flow line and must start perpendicularly to the u/s face AB which is 100% equipotential line.The base parabola should also meet the d/s filter perpendicularly.The discharge through the body of the dam , through the vertical section PQ is given by-

Where : k = coefficient of permeability D and H = Co-ordinate of C w. r. t. focus F

Phreatic line for dam with no filterGeneral solution by Casagrande :-

The focus in the dam with no horizontal drainage filter will be the lowest point F of the d/s slope and the base parabola BJG will evidently cut the d/s slope at J and extent beyond the limit of dam.According to the exit conditions , the phreatic line must emerge out at some point K , meeting d/s tangentially. The portion KF is known as discharge face.

Cont.The correction a for various value of slope by which the parabola is shifted downward , is found by value- = value from graph a+a = JF (from fig.)

Cont.The slope can even exceed the value of .Exit condition for various slope -

Stability analysisSwedish circle method of slope stability:-In this method the potential failure surface is assumed to be cylindrical.Factor of safety against sliding is defining as the ratio of the average shearing strength to the average shearing stress Procedure

Slip circle method

Cont.To test stability of slope, a trial slip circle is drawn, and soil material above assumed slip surface is divided into a number of vertical slices.The forces between slices are neglected and each slice is assumed to act independently as a column of soil of unit thickness and width b The weight W of each slice is assumed to act at its centreIf the weight of each slide is resolved in normal (N) and tangential (T) components. Normal component will pass through centre of rotation OTangential component cause driving moment = Tr r = radius of slip circle Normal component does not cause driving momentCont.The resisting force from coulomb's eq.- = c.L+ N.tan for entire slip surface AB Driving moment : Resisting moment : where: c = unit cohesion L = curved length T = Sum of all tangential components N = Sum of all normal components

Cont.Hence factor of safety against sliding is-

T and N are determined by N-curve and T curve by making ordinates of these diagrams equal to N and T value for different strips and joining them by smooth curveThe area of these diagrams can be measured by planimeter and T and N can be computed.

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