PERENNI AL PROBLEM OF EARTHEN BUND OF W ELLI NGDON RESERVOI R. - PowerPoint PPT Presentation

PERENNI AL PROBLEM OF EARTHEN BUND OF W ELLI NGDON RESERVOI R. ANALYSI S OF CAUSES AND REMEDI AL MEASURES A CASE STUDY. By Er. S. Muthu Parvatha Vardhini, Asst. Executive Engineer, (Designs) Water Resources Organisation, Tamil Nadu. Er. Amelia. Balraj, Executive Engineer, (Designs) Water Resources Organisation, Tamil Nadu.

Willingdon Reservoir

HYDRAULIC PARTICULARS Year of 1913 – 1923 384 cumecs Maximum Flood construction Discharge 27664 acre Ayacut (11200 R.L +72.67m MWL hectares) FRL R.L +72.54m TBL R.L +75.10m Length of the bund 4300m Sill level of surplus R.L +70.84m regulator 30, Nos No & size of vents (3.05m of surplus regulator x1.7m) Capacity of the 2580Mcft (or) 73.06Mm 3 Reservoir

ORIGINAL SECTION OF THE EARTH DAM  Design or construction - Details not traceable.  Techniques used - Not  Data on the foundation strata available.  Bund could have been - Locally available soil - Homogeneous - Absence of Filter - No foundation cutoff

PERFORMANCE OF THE BUND Distress noticed in the earth dam right from inception  Slippage of slope  Sinking of Top of bund  Longitudinal cracks  Flow of mud d/s of the dam  Upheaval on the d/s beyond the toe

VARIOUS CAUSES ATTRIBUTED IN EARLY YEARS AND MEASURES SUGGESTED

VARIOUS CAUSES ATTRIBUTED IN EARLY YEARS AND MEASURES SUGGESTED

MEASURES SUGGESTED BY DSRP During May 1994, to 1998 -The fact referred to the DSRP (Dam Safety Review Panel) for Tamil Nadu . Suggestions  Paving on the upstream slope in vulnerable reaches.  Flatten the upstream and downstream slopes to 3:1 by placing soil with good strength parameters, brought from a different source over filter material (sand) with berms on the downstream.  A rock toe and toe drain on the downstream.  A heel support in the form of a rock toe, of 5m or more width on the end of the upstream slope.

MEASURES SUGGESTED BY DSRP Even while carrying out these measures the bund continue to slip.

ADDITIONAL MEASURES SUGGESTED BY DSRP DSRP sugges tions during 2002,  Relief wells on the downstream slope, upto hard strata between the chainage 1950 and 2350 at 15m intervals with the spacing reduced to 8m in the reach LS2200 to LS 2260m.  To provide 1.5m deep toe drain at a distance of 8m from the end of rock toe of the downstream slope. 6 holes of 30cm diameter to be bored in the bed of the drain upto hard strata. The bore holes shall be back filled with cleaned crushed stone.  To install compacted clay-quick lime columns on the lowermost berm of the downstream slope.

DISTRESS OBSERVED EVEN AFTER CARRYING OUT SUGGESTIONS OF “DSRP”

REM EDIAL M EASURES AND RESUL TS Rehabilitation measures done (1995-1996) • Measures implemented • Reservoir level was raised gradually. At water level was at EL + 72.60m , • subsidence, • longitudinal cracks • slope slippage. • shearing the relief wells, • uplifting filter between the rock toe and toe drain • displacing the quick lime columns.

DETAILED INVESTIGATIONS TO FIND OUT ROOT CAUSE OF THE PROBLEM  Tests on Bund soil Shear Parameter Undrained Drained Condition Condition 0 ◦ 30 ◦ Angle of Internal Friction  Bore Holes drilled- At 10m d/s of toe @ LS1850m, LS1900m, LS 2000m, LS1200m, LS2200m, LS 2300m, LS 2400 m and LS 2500 m

BORE HOLE DETAILS Soft Rock with Hard rock Weathered Clayey Sand lime rock Clay with lime Fractured Rock Gravelly and sand sand

INFERENCES BASED UPON INVESTIGATIONS Condition of the Embankment  Poor quality of soil of the embankment which looses its shear strength when partially or fully saturated with water  Hence it is not a slope stability problem but that of poor quality of the soil and flattening the slope to ay value will not provide a stable embankment . Sub surface condition  The bed soil beneath the bund had a clay layer underlain by weathered rock and thereafter by fractured rock , hence causing seepage of water

CAUSES OF FAILURE Sinking of bund upheaval Mudflow Clay zone Piping uplift Fractured rock  Absence of cut off and high head on the upstream, caused Seepage through the clay and fractured rock zone . Piping near the toe. Erosion of weathered rock and clay overlying it thus resulting in subsidence.  Since residual pressures are large near the toe, caused Upheaval due to uplift.  High uplift pressure of seepage, caused Net downward force at the contact plane reduce resulting in very poor shear strength almost equal to zero and mud flow

CAUSES OF FAILURE  Seepage through the embankment caused Low shear strength of embankment soil subsidence and cracking of bund Sinking of bund Mudflow Longitudnal cracks Clay zone Piping Fractured rock  The foundation failure accelerated by the properties of soil of the bund which also has a very low shear strength, has been the ultimate reason for the distress.

ONE SOL UTION TO TACKLE ALL POSSIBLE CAUSES Going for total replacement of bund in the reach where, it is badly affected instead of making piece meal proposals for slips and settlements. Formation of PARALLEL BUND in a new alignment between LS 1700m to 2500m considering the following  New alignment downstream of the toe of existing bund where good foundation exists  Treating the foundation  Design of all components of embankment, Cutoff ,filter etc to the present standards.  Suitably connecting the new alignment to the exiting bund at the ends  Removing of existing bund in this reach .

ALIGNMENT OF NEW BUND Centre Line of Existing Bund Boundary of PWD Land Existing Bund Toe Existing Bund Heel Proposed Bund Toe Proposed Bund Heel

CROSS SECTION OF NEW BUND – LS 1900

PRESENT STATUS OF BUND Parallel Bund formed during 2009-2010  Initially water was Impounded upto sill of regulator  Then water raised in stages of 0.3m during inflow  Water stored upto 71.94 (0.6m below FRL) No evidence of distress minimal flow observed in toe drain.

CONCLUSION A parallel bund designed as per present standards over a good foundation strata was one solution to address all possible causes. NO DISTRESS was noticed in the reconstructed and other portions of the earth dam AND BELIEVED that there will be NO DISTRESS FURTHER. TODAY THE RESERVOIR STANDS WITH A NEW LEASE OF LIFE

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