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Embankment lecture 3

V
Vidhi Khokhani

The document discusses failures of earth dams, categorizing them into hydraulic and structural failures, with causes such as overtopping, erosion, seepage, and liquefaction. It emphasizes the importance of safe design criteria to prevent these failures, including proper drainage, slope protection, and structural integrity. Recommendations and preventative measures like adequate freeboard, maintenance practices, and material selection are outlined to enhance dam safety.

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EMBANKMENT DAM • LECTURE 3 • DESIGN OF HYDRAULIC STRUCTURE 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 1
FAILURES OF EARTHEN DAM Hydraulic failures .... 40% Seepage failures…….. 30% Structural failures .... 30% 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 2
Hydraulic failures .... 40% (1) Overtopping (2) Erosion of u/s slope by waves (3) Erosion of d/s slope by wind and rain (4) Erosion of d/s toe (5) Frost action 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 3
(1) Overtopping the design flood is under estimated. spillway capacity is not adequet spillway gates are not properly operated free board is not sufficient excessive settlement of the foundation and dam 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 4
(2) Erosion of u/s slope by waves The waves developed near the top water surface due to the winds, try to notch out the soil from the upstream face and may even, sometimes, cause the slip of the upstream slope. Upstream stone pitching or riprap should, therefore, be provided to avoid such failures. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 5
Wave Erosion 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 6
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 7
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 8
(3) Erosion of d/s slope by wind and rain • In the absence of proper slope protection the downstream slope of an earth dam may get eroded due to rain or high winds. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 9
• The rainwater flowing down the slope; may result in the formation of 'gullies' on the downstream slope thus damaging the dam which may generally lead to partial failure of the dam or in some cases it may cause complete failure of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 10
This can be avoided by proper maintenance, filling the cuts from time to time during rainy season, by grassing the d/s slope and by providing proper berms at suitable heights so that water has not to flow for considerable distance. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 11
Berms on d/s Slope 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 12
Toe erosion may occur due to two reasons :  erosion due to tail water  erosion due to cross currents that may come from spillway buckets. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 13
• The toe erosion can be avoided by providing a downstream slope pitching or a riprap upto a height just above the normal tail water depth. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 14
• Side walls of the Spillway (called diaphragm walls) must be of sufficient height and length, so as to prevent the possibility of the cross currents towards the earthen embankment. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 15
Frost action : • If the earth dam is located at a place where the temperature falls below the freezing point, frost may form in the pores of the soil in the earth dam. • When there is heaving, the cracks may form in the soil. This may lead to dangerous seepage and consequent failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 16
Seepage failures : Seepage failures may occur due to the following causes : (1) Piping through the foundation (2) Piping through the dam (3) Sloughing of d/s toe 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 17
Piping through the foundation : When highly permeable cavities or fissures or strata of coarse sand Or gravel are present in the foundation of the dam, water may starts seeping at a huge rate through them. This concentrated flow at a high gradient may erode the soil. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 18
• This leads to increased flow of water and soil, ultimately resulting in a rush of water and soil, thereby creating hollows below the foundation. The dam may sink into the holloWs so formed, causing its failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 19
Piping through foundation 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 20
Piping through the dam : • Piping is the progressive backward erosion which may be caused through earth dam or its foundation, by the water seeping through the dam or its foundation. It begins at a point where the water seeping through the dam emerges on downstream side of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 21
Piping through the dam : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 22
Piping in the dam may occur due to the following causes : Poor construction control - dam is not properly compacted, poor bond between the successive layers of the embankment or between the embankment material and the foundation. Burrowing animals - like crocodiles or musk rats, may dig holes in the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 23
Presence of roots, pockets of gravel or boulders in the dam. Differential settlement - cracks may develop in the dam due to differential settlement in the foundation of the dam. Soluable salts - if there are soluable salts in the soil, they get leacked out due to which hollows are created in the soil, which may lead to failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 24
3. Sloughing of d/s toe : • Sloughing is a process in which the soilmass falls down after saturation. It is a type of local slope failure. The sloughing of the d/s toe of the dam occurs under the reservoir full condition. The failure due to sloughing starts when the d/s toe becomes saturated and eroded, producing a small slump or miniature slide 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 25
• The miniature slide leaves a relatively steep face which becomes saturated by the seepage from the reservoir and slump again, forming a more unstable surface. • The process is continues till the remaining portion of the dam is too thin to withstand the horizontal water pressure, leading to the sudden failure of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 26
Structural failures : • Structural failures in earth dams are generally shear failures leading to sliding of the tents or the foundations. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 27
Structural failures in the earth dams are of the following types : (1) u/s and d/s slope failures due to construction pore pressures (2) u/s slope failure due to sudden drawdown (3) D/s slope failure due to steady seepage (4) Foundation slide due to spontaneous liquefaction (5) Failure due to earthquake 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 28
(6) Failure by spreading (7) Slope protection failures (8) Failure due to damage caused by borrowing animals (9) Failure due to holes caused by leaching of water soluable salts 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 29
u/s and d/s slope failures due to construction pore pressures : • When an earth dam is constructed of relatively impervious soil, the drainage extremely slow. The pore water pressure develops during and immediately after construction, especially if the rate of construction is relatively fast. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 30
• The pore water pressure upto 100% of the vertical stress due to weight of soil at that point has beer observed in the field. • The slope may fail during construction if it is not designed for such a high pore water pressure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 31
(2) u/s slope failure due to sudden drawdown : The most critical condition for the u/s slope occurs during the sudden drawdown of the reservoir, when the reservoir is suddenly emptied, the hydrostatic force acting along the u/s slope at the time of full reservoir is removed, 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 32
• During this condition the pore water pressure in the soil is not fully dissipated due to low permeability of the slope and consequently the shear strength of the soil is reduced and It may fail. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 33
Sudden draw down 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 34
(3) D/s slope failure due to steady seepage : • Critical condition for d/s slope occurs when the reservoir is full and percolation is at its maximum rate. The direction of seepage forces tend to decrease stability. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 35
There are two types of d/s slides : deep slides shallow slides The pore water pressure acting on the soil below the phreatic line causes a reduction in the shear strength of the soil, the failure of d/s slope may occur. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 36
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 37
(4) Foundation slide due to spontaneous liquefaction : If the foundation consists of fine sand silt in loose condition, liquefaction may occur when the dam is subjected to vibrations. In liquefaction, a large part of the weight of the overlying soils mass is momentarily carried by the pore water pressure 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 38
there is very little intergranular (or effective) shear strength, which depends upon the intergranular pressure is reduced to almost zero and the soil flows like a liquid in the downward direction in the foundation. Hence these slides are termed as flow slider or liquefaction slides. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 39
(5) Failure due to earthquake : Most of the failures due to earthquake have occurred only with respect to those which were constructed before 1920 with inferior design details and construction methods. ' 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 40
Some of the damages which may be caused to earth dams due to earthquake as under : (a) Longitudinal cracks at the top of the dam or in the core. (b) Liquefaction of loose and saturated soil mass in the lower portion of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 41
(c)- Settlement of the crest of the dam thus reducing the freeboard and increase the possibility of overtopping. (d) Shear failure at the base of the dam. (f) Rock slide from the side hills into the reservoir. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 42
(6) Failure by spreading : • Failures by spreading have been observed only in connection with fills located above stratified deposits that contain layers of soft clays. Outstanding failures due spreading are, • Lafagets Dam in California (1928) • Marshall Creek Dam in Kansas (1927) 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 43
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 44
(7) Slope protection failures : Slopes are generally protected by riprap over a layer of gravel or filter During a heavy storm, the waves on the surface of the reservoir beat repeatedly a the slope just above the reservoir level. The wave energy is dissipated in turbulent action on and within the rocks of the riprap layer. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 45
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 46
• These waves have two effects : (a) The waves may pass through the voids of riprap and may wash away the filter layer, exposing the embankment to wave erosion. (b) 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. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 47
(8) Failure due to damage caused by burrowing animals : • Burrowing animals like muskrats burrow into embankment either to make homes or to dig passage from one pond to another. If many muskrats are involved, their holes may dangerously honeycomb a small earth dam, making it extremely weak. • Sometimes squirrels also dig holes in the embankment. Due to these holes piping failure of dam will occur. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 48
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 49
(9) Failure due to holes caused by leaching of water soluable salts : • Leaching of water soluble salts such as, gypsum, iron oxide, etc. from embankment and foundation soils may cause cavities, which may lead to the excess settlement of the dam. Moreover, these salts may get deposited in the toe filter may choke it, thus causing drainage problems. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 50
Criteria for safe Design of Earth Dam : For the safe design of an earth dam the following basic criteria should be satisfied 1. No overtopping : • The dam should be safe against overtopping during occurrence of the worst flood • An adequate free board should be provided. • A suitable allowance should be made in the height of the dam to account settlements. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 51
2. No seepage failure : • The phreatic line should remain well within the d/s face of the dam so that sloughing of the downstream face occurs. • Seepage through the body of dam, foundations and abutments should be control! against suitable measures. • The dam and foundation should be safe against piping failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 52
• 3. No structural failure : • • The u/s slope should be safe against sudden drawdown conditions. • • The d/s slope should be safe during steadyseepage conditions. • • The d/s and d/s slopes should be safe during and immediately after construct • • The foundation shear stresses should be within the safe limits. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 53
• 4. Slope protection : • • The u/s slope should be protected against erosion by waves. • • The d/s slope should be protected against erosion due to rain and wind. • 5. The portion of the dam d/s of the impervious core should be properly draine • 6. The dam should have an economic section. As far as possible locally available mate • should be used to reduce the cost. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 54
• Section of an Earth Dam : • The design of an earth dam essentially consists of determining such a cross section • the dam which when constructed with the available materials will fulfill its required • tion with adequate safety. Thus there are two aspects of the design of an earth dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 55
• (1) to determine the cross section of the dam. • (2) to analyse the stability of the proposed cross-section. • The cross section of an earth dam depends on the following factors : • (1) Foundation conditions. • (2) Availability of materials. • (3) Physical properties of various materials. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 56
• (4) Method of construction and degree of construction control. • (5) Types of earth moving machinery. • (6) Diversion methods and construction schedule. • (7) Climatic conditions affecting the placement moisture control and subsequent • moisture changes. • Safety factor with respect to seepage. • Safety factor with respect to stability. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 57
• On the basis of these factors the cross section of an earth dam may be determined • involves the consideration of the following parameters. • I. Crest width • Free board • u/s and D/s slopes • Settlement allowance 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 58
• • • • • • Casing or outer shells Central impervious core Cut-off trench Downstream drainage system width : * crest width (i.e. top width) of an earth dam depends upon the following factors : • Nature of the embankment materials • Height of the dam 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 59
• • • • • Importance of the dam • width of the roadway on the top of the dam • Practicability of construction • Protection against earthquake forces 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 60
The following empirical formulae are commonly used to determine the top bidth (a) of the earthdam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 61
• In general, the crest width varies from 6 to 12 m, the larger values are for • higher and more important dams. In no case, the crest width should less than 4 m, for • maintenance work. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 62
• 2. Free board : • Free board is the difference in the elevation of the crest of the dam and the still • water level in the reservoir. Sufficient free board must be provided so that there is no • possibility of the dam being overtopped. • The freeboard may be classified as : • • Normal freeboard • • Minimum freeboard 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 63
• The freeboard above the Full Reservoir Level (FRL) is known as normal freeboard • The freeboard above the Maximum Water Lelvel (MWL) is known as minimui • freeboard. • According to IS : 10635-1983, the freeboard should be equal to the sum of wave • run-up and wind set-up. The normal free board as well as minimum freeboard should • not be less than 2 m. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 64
The values of freeboard, for various heights recommended by U.S.B.R. are given in Table-2.1. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 65
• 3. u/s and D/s slopes : • The upstream and downstream slopes of the dam depend upon the type of material, • foundation conditions, the height of the dam, and many other factors. No specific values • can tye given for the slopes. The general practice is to select the side slopes on the basis • of the experience gained with similar dams elsewhere and to check the stability of selected • slopes. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 66
fable-2.2 : Side slopes for Earth Dams (as per Terzaghi) 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 67
• Settlement allowance : • Settlement of an earth dam may be caused by consolidation of the soil mass in the • and in the foundation. It depends on the character of the soil in the dam and the • ition and the method of construction used. • h is the usual practice to consturct the earth dams to a somewhat greater height than • •squired height to have a suitable settlement allowance 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 68
• Generally, a provision of allowance of 1 to 2% of the height of the dam is made. However, in case • of height greater than 30 m, an extra allowance of 1% is made to account for • lement due to earthquake. The extra height is provided in the form of a longitudinal • over the top of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 69
• outer shells : • function of casing or outer shell is to impart stability and protect the core. The • pervious materials, which are not subject to cracking on direct exposure to • are suitable for casing. • 2.3 gives recommendations for suitability of soils used for earth dams as per • MB26-1978. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 70
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 71
• 6. Central impervious core : • A zoned section is usually preferred for an earth dam. The core may be vertical • inclined. An impervious core is provided at the centre of the zoned sections to contr • the loss of water by seepage through the dam. Thus the loss of head is localised in t" • core of the dam and, therfore, the phreatic line does not cut the d/s face of the dam a~ • cause sloughing of the d/s face. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 72
• The minimum safe thickness of the central impervious core depends on the follow1 • factors. • Tolerable seepage loss. • Maximum width that will permit proper construction. • Types of the materials for the core and the shells. • Design of the proposed filter layers. • Precedent on similar projects. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 73
• It should be noted that the shear strength of core materials is always lesser than • rest of the embankment. Hence a thinner shell is preferable from the point of vieW • stability. However, a thick core has more resistance to piping. The governing criteria • that the thickness of the core at any elevation is not less than the height of the embanl • at that elevation so that average hydraulic gradient is less than unity. The width of the • at the crest of the dam should be a minimum of 3 m to permit economical placement 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 74
• compaction of impervious embankment material by construction equipments. The top • of the core should be atleast 1 m above the MWL to prevent seepage by capillary syphon: • • Suitability of Material for Core : • The soils having high compressibility and higher liquid limit are not suitable as; • are prone to swelling and formation of cracks. Soils having organic contents are also • suitbale. Table 2.3 gives the suitability of soils for impervious core of zoned earth 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 75
• Table 2.4 gives recommendations based on IS : 8826-1978 regarding suitability of soils • for construction of core for earth dams in earthquake zones 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 76
Table-2.4 : Suitability of Soils for Construction of Core of Earth Dam in Earthquake Zones 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 77
• Cut-off trench : • To reduce seepage through the foundation and to avoid piping failure, a cut of wall • usually provided when the foundation is pervious to moderate depth. The centre line • the cut-off wall is kept parallel to the centre line of the dam. The side slopes of the • :h are usually 1.5 : 1 to 1 : 1 [Fig. 2.19(a)] 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 78
• Cut-off is required to • (1) reduce loss of stored water through foundation and abutments * • (2) Prevent sub surface erosion by piping. • As per IS : 8826-1978 recommendations : • (1) The alignment of the cut off trench should be fixed in such a way that its central • line should be within the u/s base of the impervious core and it should be keyed • into rock. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 79
• (2) The bottom width of cutoff trench may be fixed taking following factors into • considerations : • • Provide sufficient working space to carry curtain grouting, • • Provide sufficient working space for compaction equipment. • • Provide safety against piping. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 80
• Cut-off may be provided in the following ways : • • by providing concrete cutoff wall • • by providing cutoff trench filled with impervious material • • by driving sheet pile • • by curtain grouting 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 81
• Sheet pile cut-off : • A sheet pile cut-off can be used in place of a cut-off trench [Fig. 219(a)]. However, • if the foundation strata consists of soil mixed with boulders, the sheet piles would bend • and will not be effective. • . 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 82
• Impervious blanket : • If the foundation is impervious to a very large depth, a positive cut-off wall up • the impervious stratum is not practicable. In that case, an u/s impervious blanket is provi • on the river bed as an expansion of the central impervious core of the dam 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 83
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 84
• 8. Downstream drainage system : • Filter zones are invariably provided in all earth dams. They are constructed of pervious • materials. Provision of filter zones at d/s side serves the following functions : • • to drain off the water seeping through earth dam safely. • • To reduce the pore water pressure in the downstream portion of the dam. • • To keep the phreatic line within the body of the dam. • • To check the piping by checking migration of the particles. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 85
• • • • Usually following types of filters are provided : (1) Toe filter (2) Horizontal drainage filter (blanket) (3) Chimney drains 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 86
• Iter criteria : • Generally, a multilayered filter is provided in which each subsequent layer becomes • asingly coarser than the previous one. Such a filter is sometimes known as inverted • r or reverse filter. The soil to be protected is known as base material. Basic • luirements of filter material are : • (i) The filter material should be sufficiently fine and properly graded so that the • voids of the filter are small enough to prevent base material particles from • penetrating and clogging the filter. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 87
• (ii) The filter material should be sufficiently coarse and pervious compared to the • base material so that the incoming water is rapidly removed without any • appreciable build up of seepage forces within the filter. • (iii) The filter material should be coarse enough not to be carried away through the • drainage pipe openings. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 88
• (iv) The filter layer should be sufficiently thick to provide a good distribution of all • particle size throughout the filter and to be able to carry the seepage discharge. • The Filter material should satisfy the following two criteria. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 89
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 90
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 91
1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 92
• 2.9 Cross-section of Earth Dam for Different Situations : • The section of a zoned earth embankment dam should be so selected that av • materials are utilized to the maximum and limited quantity of other required materi • imported to the site. • • • Available materials for dam : • 1. Gravel or coarse sand is available along with clayey silt • 2. Only fine gravel or coarse sand is available , • 3. Only silty clay is available 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 93
• General Foundation Conditions : • (a) Foundations impervious to a large depth • (b) Foundation pervious to moderate depth, after with impervious strata is av * • (c) Foundations pervious to large depth. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 94
Case-1 : Gravel or coarse sand is available along with clayey silt : (a) Foundations impervious to a large depth : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 95
b) Foundation pervious to a moderate depth, after which impervious strata is available : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 96
(c) Foundation pervious to a great depth : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 97

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Embankment lecture 3

  • 1. EMBANKMENT DAM • LECTURE 3 • DESIGN OF HYDRAULIC STRUCTURE 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 1
  • 2. FAILURES OF EARTHEN DAM Hydraulic failures .... 40% Seepage failures…….. 30% Structural failures .... 30% 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 2
  • 3. Hydraulic failures .... 40% (1) Overtopping (2) Erosion of u/s slope by waves (3) Erosion of d/s slope by wind and rain (4) Erosion of d/s toe (5) Frost action 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 3
  • 4. (1) Overtopping the design flood is under estimated. spillway capacity is not adequet spillway gates are not properly operated free board is not sufficient excessive settlement of the foundation and dam 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 4
  • 5. (2) Erosion of u/s slope by waves The waves developed near the top water surface due to the winds, try to notch out the soil from the upstream face and may even, sometimes, cause the slip of the upstream slope. Upstream stone pitching or riprap should, therefore, be provided to avoid such failures. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 5
  • 9. (3) Erosion of d/s slope by wind and rain • In the absence of proper slope protection the downstream slope of an earth dam may get eroded due to rain or high winds. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 9
  • 10. • The rainwater flowing down the slope; may result in the formation of 'gullies' on the downstream slope thus damaging the dam which may generally lead to partial failure of the dam or in some cases it may cause complete failure of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 10
  • 11. This can be avoided by proper maintenance, filling the cuts from time to time during rainy season, by grassing the d/s slope and by providing proper berms at suitable heights so that water has not to flow for considerable distance. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 11
  • 12. Berms on d/s Slope 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 12
  • 13. Toe erosion may occur due to two reasons :  erosion due to tail water  erosion due to cross currents that may come from spillway buckets. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 13
  • 14. • The toe erosion can be avoided by providing a downstream slope pitching or a riprap upto a height just above the normal tail water depth. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 14
  • 15. • Side walls of the Spillway (called diaphragm walls) must be of sufficient height and length, so as to prevent the possibility of the cross currents towards the earthen embankment. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 15
  • 16. Frost action : • If the earth dam is located at a place where the temperature falls below the freezing point, frost may form in the pores of the soil in the earth dam. • When there is heaving, the cracks may form in the soil. This may lead to dangerous seepage and consequent failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 16
  • 17. Seepage failures : Seepage failures may occur due to the following causes : (1) Piping through the foundation (2) Piping through the dam (3) Sloughing of d/s toe 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 17
  • 18. Piping through the foundation : When highly permeable cavities or fissures or strata of coarse sand Or gravel are present in the foundation of the dam, water may starts seeping at a huge rate through them. This concentrated flow at a high gradient may erode the soil. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 18
  • 19. • This leads to increased flow of water and soil, ultimately resulting in a rush of water and soil, thereby creating hollows below the foundation. The dam may sink into the holloWs so formed, causing its failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 19
  • 20. Piping through foundation 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 20
  • 21. Piping through the dam : • Piping is the progressive backward erosion which may be caused through earth dam or its foundation, by the water seeping through the dam or its foundation. It begins at a point where the water seeping through the dam emerges on downstream side of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 21
  • 22. Piping through the dam : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 22
  • 23. Piping in the dam may occur due to the following causes : Poor construction control - dam is not properly compacted, poor bond between the successive layers of the embankment or between the embankment material and the foundation. Burrowing animals - like crocodiles or musk rats, may dig holes in the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 23
  • 24. Presence of roots, pockets of gravel or boulders in the dam. Differential settlement - cracks may develop in the dam due to differential settlement in the foundation of the dam. Soluable salts - if there are soluable salts in the soil, they get leacked out due to which hollows are created in the soil, which may lead to failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 24
  • 25. 3. Sloughing of d/s toe : • Sloughing is a process in which the soilmass falls down after saturation. It is a type of local slope failure. The sloughing of the d/s toe of the dam occurs under the reservoir full condition. The failure due to sloughing starts when the d/s toe becomes saturated and eroded, producing a small slump or miniature slide 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 25
  • 26. • The miniature slide leaves a relatively steep face which becomes saturated by the seepage from the reservoir and slump again, forming a more unstable surface. • The process is continues till the remaining portion of the dam is too thin to withstand the horizontal water pressure, leading to the sudden failure of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 26
  • 27. Structural failures : • Structural failures in earth dams are generally shear failures leading to sliding of the tents or the foundations. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 27
  • 28. Structural failures in the earth dams are of the following types : (1) u/s and d/s slope failures due to construction pore pressures (2) u/s slope failure due to sudden drawdown (3) D/s slope failure due to steady seepage (4) Foundation slide due to spontaneous liquefaction (5) Failure due to earthquake 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 28
  • 29. (6) Failure by spreading (7) Slope protection failures (8) Failure due to damage caused by borrowing animals (9) Failure due to holes caused by leaching of water soluable salts 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 29
  • 30. u/s and d/s slope failures due to construction pore pressures : • When an earth dam is constructed of relatively impervious soil, the drainage extremely slow. The pore water pressure develops during and immediately after construction, especially if the rate of construction is relatively fast. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 30
  • 31. • The pore water pressure upto 100% of the vertical stress due to weight of soil at that point has beer observed in the field. • The slope may fail during construction if it is not designed for such a high pore water pressure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 31
  • 32. (2) u/s slope failure due to sudden drawdown : The most critical condition for the u/s slope occurs during the sudden drawdown of the reservoir, when the reservoir is suddenly emptied, the hydrostatic force acting along the u/s slope at the time of full reservoir is removed, 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 32
  • 33. • During this condition the pore water pressure in the soil is not fully dissipated due to low permeability of the slope and consequently the shear strength of the soil is reduced and It may fail. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 33
  • 34. Sudden draw down 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 34
  • 35. (3) D/s slope failure due to steady seepage : • Critical condition for d/s slope occurs when the reservoir is full and percolation is at its maximum rate. The direction of seepage forces tend to decrease stability. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 35
  • 36. There are two types of d/s slides : deep slides shallow slides The pore water pressure acting on the soil below the phreatic line causes a reduction in the shear strength of the soil, the failure of d/s slope may occur. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 36
  • 38. (4) Foundation slide due to spontaneous liquefaction : If the foundation consists of fine sand silt in loose condition, liquefaction may occur when the dam is subjected to vibrations. In liquefaction, a large part of the weight of the overlying soils mass is momentarily carried by the pore water pressure 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 38
  • 39. there is very little intergranular (or effective) shear strength, which depends upon the intergranular pressure is reduced to almost zero and the soil flows like a liquid in the downward direction in the foundation. Hence these slides are termed as flow slider or liquefaction slides. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 39
  • 40. (5) Failure due to earthquake : Most of the failures due to earthquake have occurred only with respect to those which were constructed before 1920 with inferior design details and construction methods. ' 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 40
  • 41. Some of the damages which may be caused to earth dams due to earthquake as under : (a) Longitudinal cracks at the top of the dam or in the core. (b) Liquefaction of loose and saturated soil mass in the lower portion of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 41
  • 42. (c)- Settlement of the crest of the dam thus reducing the freeboard and increase the possibility of overtopping. (d) Shear failure at the base of the dam. (f) Rock slide from the side hills into the reservoir. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 42
  • 43. (6) Failure by spreading : • Failures by spreading have been observed only in connection with fills located above stratified deposits that contain layers of soft clays. Outstanding failures due spreading are, • Lafagets Dam in California (1928) • Marshall Creek Dam in Kansas (1927) 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 43
  • 45. (7) Slope protection failures : Slopes are generally protected by riprap over a layer of gravel or filter During a heavy storm, the waves on the surface of the reservoir beat repeatedly a the slope just above the reservoir level. The wave energy is dissipated in turbulent action on and within the rocks of the riprap layer. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 45
  • 47. • These waves have two effects : (a) The waves may pass through the voids of riprap and may wash away the filter layer, exposing the embankment to wave erosion. (b) 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. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 47
  • 48. (8) Failure due to damage caused by burrowing animals : • Burrowing animals like muskrats burrow into embankment either to make homes or to dig passage from one pond to another. If many muskrats are involved, their holes may dangerously honeycomb a small earth dam, making it extremely weak. • Sometimes squirrels also dig holes in the embankment. Due to these holes piping failure of dam will occur. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 48
  • 50. (9) Failure due to holes caused by leaching of water soluable salts : • Leaching of water soluble salts such as, gypsum, iron oxide, etc. from embankment and foundation soils may cause cavities, which may lead to the excess settlement of the dam. Moreover, these salts may get deposited in the toe filter may choke it, thus causing drainage problems. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 50
  • 51. Criteria for safe Design of Earth Dam : For the safe design of an earth dam the following basic criteria should be satisfied 1. No overtopping : • The dam should be safe against overtopping during occurrence of the worst flood • An adequate free board should be provided. • A suitable allowance should be made in the height of the dam to account settlements. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 51
  • 52. 2. No seepage failure : • The phreatic line should remain well within the d/s face of the dam so that sloughing of the downstream face occurs. • Seepage through the body of dam, foundations and abutments should be control! against suitable measures. • The dam and foundation should be safe against piping failure. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 52
  • 53. • 3. No structural failure : • • The u/s slope should be safe against sudden drawdown conditions. • • The d/s slope should be safe during steadyseepage conditions. • • The d/s and d/s slopes should be safe during and immediately after construct • • The foundation shear stresses should be within the safe limits. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 53
  • 54. • 4. Slope protection : • • The u/s slope should be protected against erosion by waves. • • The d/s slope should be protected against erosion due to rain and wind. • 5. The portion of the dam d/s of the impervious core should be properly draine • 6. The dam should have an economic section. As far as possible locally available mate • should be used to reduce the cost. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 54
  • 55. • Section of an Earth Dam : • The design of an earth dam essentially consists of determining such a cross section • the dam which when constructed with the available materials will fulfill its required • tion with adequate safety. Thus there are two aspects of the design of an earth dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 55
  • 56. • (1) to determine the cross section of the dam. • (2) to analyse the stability of the proposed cross-section. • The cross section of an earth dam depends on the following factors : • (1) Foundation conditions. • (2) Availability of materials. • (3) Physical properties of various materials. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 56
  • 57. • (4) Method of construction and degree of construction control. • (5) Types of earth moving machinery. • (6) Diversion methods and construction schedule. • (7) Climatic conditions affecting the placement moisture control and subsequent • moisture changes. • Safety factor with respect to seepage. • Safety factor with respect to stability. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 57
  • 58. • On the basis of these factors the cross section of an earth dam may be determined • involves the consideration of the following parameters. • I. Crest width • Free board • u/s and D/s slopes • Settlement allowance 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 58
  • 59. • • • • • • Casing or outer shells Central impervious core Cut-off trench Downstream drainage system width : * crest width (i.e. top width) of an earth dam depends upon the following factors : • Nature of the embankment materials • Height of the dam 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 59
  • 60. • • • • • Importance of the dam • width of the roadway on the top of the dam • Practicability of construction • Protection against earthquake forces 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 60
  • 61. The following empirical formulae are commonly used to determine the top bidth (a) of the earthdam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 61
  • 62. • In general, the crest width varies from 6 to 12 m, the larger values are for • higher and more important dams. In no case, the crest width should less than 4 m, for • maintenance work. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 62
  • 63. • 2. Free board : • Free board is the difference in the elevation of the crest of the dam and the still • water level in the reservoir. Sufficient free board must be provided so that there is no • possibility of the dam being overtopped. • The freeboard may be classified as : • • Normal freeboard • • Minimum freeboard 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 63
  • 64. • The freeboard above the Full Reservoir Level (FRL) is known as normal freeboard • The freeboard above the Maximum Water Lelvel (MWL) is known as minimui • freeboard. • According to IS : 10635-1983, the freeboard should be equal to the sum of wave • run-up and wind set-up. The normal free board as well as minimum freeboard should • not be less than 2 m. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 64
  • 65. The values of freeboard, for various heights recommended by U.S.B.R. are given in Table-2.1. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 65
  • 66. • 3. u/s and D/s slopes : • The upstream and downstream slopes of the dam depend upon the type of material, • foundation conditions, the height of the dam, and many other factors. No specific values • can tye given for the slopes. The general practice is to select the side slopes on the basis • of the experience gained with similar dams elsewhere and to check the stability of selected • slopes. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 66
  • 67. fable-2.2 : Side slopes for Earth Dams (as per Terzaghi) 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 67
  • 68. • Settlement allowance : • Settlement of an earth dam may be caused by consolidation of the soil mass in the • and in the foundation. It depends on the character of the soil in the dam and the • ition and the method of construction used. • h is the usual practice to consturct the earth dams to a somewhat greater height than • •squired height to have a suitable settlement allowance 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 68
  • 69. • Generally, a provision of allowance of 1 to 2% of the height of the dam is made. However, in case • of height greater than 30 m, an extra allowance of 1% is made to account for • lement due to earthquake. The extra height is provided in the form of a longitudinal • over the top of the dam. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 69
  • 70. • outer shells : • function of casing or outer shell is to impart stability and protect the core. The • pervious materials, which are not subject to cracking on direct exposure to • are suitable for casing. • 2.3 gives recommendations for suitability of soils used for earth dams as per • MB26-1978. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 70
  • 72. • 6. Central impervious core : • A zoned section is usually preferred for an earth dam. The core may be vertical • inclined. An impervious core is provided at the centre of the zoned sections to contr • the loss of water by seepage through the dam. Thus the loss of head is localised in t" • core of the dam and, therfore, the phreatic line does not cut the d/s face of the dam a~ • cause sloughing of the d/s face. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 72
  • 73. • The minimum safe thickness of the central impervious core depends on the follow1 • factors. • Tolerable seepage loss. • Maximum width that will permit proper construction. • Types of the materials for the core and the shells. • Design of the proposed filter layers. • Precedent on similar projects. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 73
  • 74. • It should be noted that the shear strength of core materials is always lesser than • rest of the embankment. Hence a thinner shell is preferable from the point of vieW • stability. However, a thick core has more resistance to piping. The governing criteria • that the thickness of the core at any elevation is not less than the height of the embanl • at that elevation so that average hydraulic gradient is less than unity. The width of the • at the crest of the dam should be a minimum of 3 m to permit economical placement 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 74
  • 75. • compaction of impervious embankment material by construction equipments. The top • of the core should be atleast 1 m above the MWL to prevent seepage by capillary syphon: • • Suitability of Material for Core : • The soils having high compressibility and higher liquid limit are not suitable as; • are prone to swelling and formation of cracks. Soils having organic contents are also • suitbale. Table 2.3 gives the suitability of soils for impervious core of zoned earth 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 75
  • 76. • Table 2.4 gives recommendations based on IS : 8826-1978 regarding suitability of soils • for construction of core for earth dams in earthquake zones 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 76
  • 77. Table-2.4 : Suitability of Soils for Construction of Core of Earth Dam in Earthquake Zones 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 77
  • 78. • Cut-off trench : • To reduce seepage through the foundation and to avoid piping failure, a cut of wall • usually provided when the foundation is pervious to moderate depth. The centre line • the cut-off wall is kept parallel to the centre line of the dam. The side slopes of the • :h are usually 1.5 : 1 to 1 : 1 [Fig. 2.19(a)] 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 78
  • 79. • Cut-off is required to • (1) reduce loss of stored water through foundation and abutments * • (2) Prevent sub surface erosion by piping. • As per IS : 8826-1978 recommendations : • (1) The alignment of the cut off trench should be fixed in such a way that its central • line should be within the u/s base of the impervious core and it should be keyed • into rock. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 79
  • 80. • (2) The bottom width of cutoff trench may be fixed taking following factors into • considerations : • • Provide sufficient working space to carry curtain grouting, • • Provide sufficient working space for compaction equipment. • • Provide safety against piping. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 80
  • 81. • Cut-off may be provided in the following ways : • • by providing concrete cutoff wall • • by providing cutoff trench filled with impervious material • • by driving sheet pile • • by curtain grouting 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 81
  • 82. • Sheet pile cut-off : • A sheet pile cut-off can be used in place of a cut-off trench [Fig. 219(a)]. However, • if the foundation strata consists of soil mixed with boulders, the sheet piles would bend • and will not be effective. • . 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 82
  • 83. • Impervious blanket : • If the foundation is impervious to a very large depth, a positive cut-off wall up • the impervious stratum is not practicable. In that case, an u/s impervious blanket is provi • on the river bed as an expansion of the central impervious core of the dam 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 83
  • 85. • 8. Downstream drainage system : • Filter zones are invariably provided in all earth dams. They are constructed of pervious • materials. Provision of filter zones at d/s side serves the following functions : • • to drain off the water seeping through earth dam safely. • • To reduce the pore water pressure in the downstream portion of the dam. • • To keep the phreatic line within the body of the dam. • • To check the piping by checking migration of the particles. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 85
  • 86. • • • • Usually following types of filters are provided : (1) Toe filter (2) Horizontal drainage filter (blanket) (3) Chimney drains 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 86
  • 87. • Iter criteria : • Generally, a multilayered filter is provided in which each subsequent layer becomes • asingly coarser than the previous one. Such a filter is sometimes known as inverted • r or reverse filter. The soil to be protected is known as base material. Basic • luirements of filter material are : • (i) The filter material should be sufficiently fine and properly graded so that the • voids of the filter are small enough to prevent base material particles from • penetrating and clogging the filter. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 87
  • 88. • (ii) The filter material should be sufficiently coarse and pervious compared to the • base material so that the incoming water is rapidly removed without any • appreciable build up of seepage forces within the filter. • (iii) The filter material should be coarse enough not to be carried away through the • drainage pipe openings. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 88
  • 89. • (iv) The filter layer should be sufficiently thick to provide a good distribution of all • particle size throughout the filter and to be able to carry the seepage discharge. • The Filter material should satisfy the following two criteria. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 89
  • 93. • 2.9 Cross-section of Earth Dam for Different Situations : • The section of a zoned earth embankment dam should be so selected that av • materials are utilized to the maximum and limited quantity of other required materi • imported to the site. • • • Available materials for dam : • 1. Gravel or coarse sand is available along with clayey silt • 2. Only fine gravel or coarse sand is available , • 3. Only silty clay is available 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 93
  • 94. • General Foundation Conditions : • (a) Foundations impervious to a large depth • (b) Foundation pervious to moderate depth, after with impervious strata is av * • (c) Foundations pervious to large depth. 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 94
  • 95. Case-1 : Gravel or coarse sand is available along with clayey silt : (a) Foundations impervious to a large depth : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 95
  • 96. b) Foundation pervious to a moderate depth, after which impervious strata is available : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 96
  • 97. (c) Foundation pervious to a great depth : 1/9/2014 PREPAREDBY V.H.KHOKHANI, ASST. PROFESSOR, DIET 97