EMBANKMENT LECTURE 10

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STABILITY OF SLOPES SEEPAGE CONTROL MEASURES AND SLOPE PROTECTION:

STABILITY OF SLOPES SEEPAGE CONTROL MEASURES AND SLOPE PROTECTION

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Stability of Slopes of Earth Dam : The stability of slopes of earth dam is tested under the following conditions 1. Stability of downstream slope during steady seepage. 2. Stability of upstream slope during sudden drawdown. 3. Stability of upstream and downstream slopes during and immediately after construction.

1. Stability of Downstream slope during steady seepage ::

1. Stability of Downstream slope during steady seepage : On the upstream slope, the seepage forces are directed inwards and hence increase the stability of slope. However, on the down-stream slope the direction seepage forces is such that they decrease the stability. The steady seepage condition therefore critical for the downstream slope. The critical condition of downstream slope occurs when the reservoir is full is at its maximum rate. The pore water pressure acting on the soil mass below the center line reduces effective stress responsible for mobilizing shearing resistance. The safety in this case is given by

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The pore water pressure may be determined by drawing the flow net. The pore water pressure at various points along the slip surface are obtained by measuring at each of connections with an equipotential line, the vertical distance between the point of action and the point where the equipotential line meets the phreatic line. The pore water pressures represented by the vertical distances so obtained are plotted to scale in section normal to the slip surface at the respective points of intersection.

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A smooth curve is drawn through the plotted points to obtain the pore water pressure distribution The area of U-diagram is measured with the help of planimeter . If ε u is the area of diagram (cm2) then,

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In absence of a flow net, the factor of safety of the d/s slope can be approximate calculated from the equation,

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where the normal components N' are to be calculated on the basis of buoyant unit weight y' of the dam while T components are to be calculated on the basis of saturated unit weight ( ysat ).

2. Stability of upstream slope during sudden drawdown ::

2. Stability of upstream slope during sudden drawdown : For the upstream slope of an earth dam the most critical condition occurs when the reservoir is suddenly emptied without allowing appreciable drainage from the saturated soil mass. This condition is known as sudden drawdown. When sudden drawdown take place, the weight of water which is still present in the soil tends to cause sliding of the wedge, as the water pressure which was acting on the u/s slope to balance this weight has been suddenly removed. According to another interpretation, the shearing resistance of the soil is considerably reduced due to pore pressure existing in the soil, whereas the disturbing force due to saturated unit weight of the soil remains the same.

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To take into account this fact in stability analysis the pore water pressures determined along the portion of the slip surface which lies below the water surface obtaining the net effective resisting forces and the actuating (disturbing) forces are calculated by taking the saturated weight of soil mass which lies below water surface, this case also the pore water pressure may be determine by drawing the flownet .

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pore water pressures at various points along the slip surface are obtained by measuring each of its intersections with an equipotential line, the vertical distance between the point of intersection and the point where the equipotential line meets the free surface which in this case is the u/s slope of the dam.

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The pore water pressures determine at various points along the slip surface are plotted to scale in the direction normal to slip surface at the respective points of intersection. A smooth curve is drawn through plotted points to obtain the pore water pressure distribution diagram. The area of these diagram is measured with the help of planimeter and the value of ZU is obtained as mentioned earlier.

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The factor of safety is calculated from the equation

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In the absence of flownet , the factor of safety of the u/s slope can be calculated from the expression,

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where, the normal components N' are to be caleualted on the basis of submerged unit weight y' of the u/s dam material while T components are to be calculated on the basis of saturated unit weight ( ysal ).

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(3) Stability of u/s and d/s dopes during and immediately after construction : When an earth dam is built of the soil of low permeability, excess pore pressures develop in the air and water voids due to compaction carried out during construction due to its own weight. The pore pressure developed depends upon the placement water content, method of compaction, weight of the overlying layers and the rate of the dissipation of pore pressure during construction. The high pore pressure developed during constrction continue to exists even after the completion of the dam for the first few years of life of the dam. Since during construction and immediately after completion of the there would be no water load on the upstream slope of the dam, this condition would critical for both die u/s and d/s slopes.

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The construction pore pressure may be estimated by Hilf's equation :

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A plot is then made between the effective stress a' and percent consolidation A, the results of consolidation tests conducted on the samples. The values of u are obtain from the above equation for different values of A. The total stress is given by

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A plot is then made between the total stress o and the pore pressure u. This is used for the determination of pore pressures at various points in the dam during construction from the values of the total stress a. [Fig. 2.40] In order to find total stresses, the mid heights z1, z2, z3 etc. of the strips (slices) measured and total stresses ct,, ct2, a 3 etc. are calculated from the relation a = y • Z.

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Knowing IU, the factor of safety of the u/s and d/s slope of dam is calculated the equation

Slope protection :

Slope protection The seepage control measures are necessary to prevent adverse effects of water percolating through embankment and its foundation. The seepage control measures can be divided into two groups : A. Embankment seepage control 1. toe filter 2. Horizontal drainage filter 3. Protective filter d/s of the toe 4. D/s coarse section 5. Chimney drain

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(B)Foundation seepage control 1. Impervious cutoff 2. U/s impervious blanket 3. D/s seepage berms 4. Drainage trenches 5. Relief wells

1. Toe filter ::

1. Toe filter :

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At the d/s end of an earth dam a rock toe is provided by arranging 15 to 45 cm size stones. The height of rock toe is sually kept 30 to 40% of the reservoir Rock toe keeps the phreatic line well within the section and also provide drainage. It also prevent failure of d/s of the dam due to piping or sloughing.

Horizontal drainage filter ::

Horizontal drainage filter :

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The horizontal drainage filter may extended from 25 to 100% of the distance downstream toe to the centre line of the dam. •Functions of horizontal filter : (1) It keeps the phreatic line well within the embankment. (2) It gives greater leakage because of shorter seepage path. (3) It also provides drainage for foundation. (4) It accelerates consolidation.

3. Protective filter d/s of the toe:

3. Protective filter d/s of the toe

D/S coarse section ::

D/S coarse section :

Chimney drain ::

Chimney drain :

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When there is high degree of embankment stratification the horizontal permeability is greater than the vertical, resulting in greater horizontal spread of seepage. A correctly built vertical drain can completely intercept embankment seepag

Foundation seepage control ::

Foundation seepage control : 1. Impervious cutoff : The various cutoff may consists of : (1) cutoff trench (2) grout curtain (3) Sheet pile

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2. U/S impervious clay blanket : The impervious layer provided u/s of the dam reduce seepage of water and thus r educe uplift pressure on the dam.

(3) D/s seepage berms ::

(3) D/s seepage berms : The d/s seepage berms serves the following functions : (1) They provide additional weight to resist uplift pressure below the top stratum. (2) They afford some protection against possible sloughing of the d/s the dam as a result of seepage.

4. Drainage trenches ::

4. Drainage trenches :

5. Relief Wells ::

5. Relief Wells :

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The preliminary purpose of relief well is to reduce the sub-stratum pressure d/s of the dam which otherwise would cause formation of sand possibly sub-surface piping. They intercept the seepage" and control the out seepage.

Slope Protection ::

Slope Protection : 1. Protection of u/s slope : The u/s slope of the earth dam is protected against the erosive action of waves. In some instances provision must be made against burrowing animals. Following are the various materials used for slope protection : (1) Rock riprap (2) Concrete pavement (3) Bituminous pavement (4) Precast Concrete blocks

Rock riprap ::

Rock riprap : Riprap consists of stones placed on the face of the embankment. It is generally of two types : ( i ) Dumped or randon riprap (ii) Hand placed pitching ( i ) Dumped or randon riprap : In this type of riprap, large sized stones are dumped in place from trucks or tossed in place by hand. The individual stones should be sufficiently heavy so that they-can resist displacement by wave action. The thickness of thfe dumped rock should be about 1 m and should be placed over a gravel filter of about 0.3 m thickness. The filter prevents the washing of fines from dam into the riprap. The dumped riprap is more effective than an equivalent layer of hand placed pitching.

(ii) Hand placed pitching ::

(ii) Hand placed pitching : The hand packed riprap requires a lesser thickness aNd proove more economical if suitable rock is available only in limited quantity. However when provided in smaller thickness (i.e. single layer), it is more susceptible to damage. The thickness of the hand placed pitching is usually about 45 cm. the minimum size of the stone used should be about 30 cm long, 30 cm wide and 7.5 cm thick.

(2) Concrete pavement ::

(2) Concrete pavement : Concrete pavement (slab) may also be laid over the u/s slope of the earth dam.When such slabs are constructed, they must be laid over a filter bed and weep holes should be provided so as to permit escape of water when the reservoir is draw down. If the filter is not provided, the fines from the embankment may get washed away from the joints creating hollows beneath the slab and causing the consequent cracking and failure of the slab under its own weight.

2. Protection of d/s slope ::

2. Protection of d/s slope : The d/s slope of the earth dam is protected against the erosive action of waves upto and slightly above the tail water depth, in a similar manner as is explained above for u/s slope.

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Moreover, the d/s slope should be protected against the erosive action of rain and its run-off by providing horizontal berms at suitable intervals say about 15 m or so, to intercept the rain water and discharge it safely. Attempts should be made so as to grass and plant the d/s slope, soon after their construction.

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