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

Maximum Flood Discharge 384 cumecs MWL R.L +72.67m FRL R.L +72.54m TBL R.L +75.10m Length of the bund 4300m Sill level of surplus regulator R.L +70.84m No & size of vents

• f surplus regulator

30, Nos (3.05m x1.7m) Capacity

• f

the Reservoir 2580Mcft (or) 73.06Mm3 Year

• f

construction 1913 – 1923 Ayacut 27664 acre (11200 hectares)

HYDRAULIC PARTICULARS

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
• f 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.

DSRP suggestions 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. ADDITIONAL MEASURES SUGGESTED BY DSRP

DISTRESS OBSERVED EVEN AFTER CARRYING OUT SUGGESTIONS OF “DSRP”

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.

REM EDIAL M EASURES AND RESUL TS

DETAILED INVESTIGATIONS TO FIND OUT ROOT CAUSE OF THE PROBLEM

• Bore Holes drilled- At 10m d/s of toe
• Tests on Bund soil

Shear Parameter Undrained Condition Drained Condition Angle of Internal Friction 0◦ 30◦ @ LS1850m, LS1900m, LS 2000m, LS1200m, LS2200m, LS 2300m, LS 2400 m and LS 2500 m

BORE HOLE DETAILS

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

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
• f

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

• 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

upheaval Piping Clay zone Fractured rock Mudflow Sinking of bund uplift



The foundation failure accelerated by the properties

• f soil of the bund which also has a very low shear

strength, has been the ultimate reason for the distress.

CAUSES OF FAILURE

• Seepage through the embankment caused

Low shear strength of embankment soil subsidence and cracking of bund

Piping Clay zone Fractured rock Mudflow Sinking of bund Longitudnal cracks

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.

ONE SOL UTION TO TACKLE ALL POSSIBLE CAUSES

ALIGNMENT OF NEW BUND

Centre Line of Existing Bund Existing Bund Toe Proposed Bund Toe Existing Bund Heel Proposed Bund Heel Boundary of PWD Land

CROSS SECTION OF NEW BUND – LS 1900

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.

PRESENT STATUS OF BUND

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

THANK YOU !