CAUSES OF FAILURE OF WEIRS ON PERMEABLE SOILS AND THEIR REMEDIES

The weir failure may take place due to the following reasons

1. Piping or undermining.

2. By uplift pressure.

3. By suction due to standing wave.

4. By scour on the U/S and D/S of the weir.

1. Piping or undermining.

• When water seeping through the permeable foundation emerges out at the D/S end of the impervious floor of the weir, at
• hydraulic gradient or exit gradient greater than the critical value for the foundation soil, the soil starts boiling at the exit point.
• Boiling of the soil indicates lifting of the soil against gravity and it happens only when exist gradient of seeping water is greater than the safe limit for the foundation soil.
• The soil gets wash out with boiling water.
• With washing out of some soil from D/S side, the exit gradient increase and boiling of soil start with even more vigor.
• This process of erosion of soil from below the foundation, progressively, works backward towards the U/S.
• This process ultimately develops a channel or pipe, below the foundation of the weir and causes failure of the weir.
• Such a failure can be avoided by adopting the following preventive measures.

(i) Provide deep cut-off piles at U/S and D/S ends.

(ii) Provide sufficient length of the pucca floor so as to maintain the exit gradient below the critical limit, for the foundation soil.

2. By uplift pressure.

• Seeping water through the foundation, exerts uplift pressure, on the floor.
• The uplift pressure is maximum at the point, just D/S of the weir wall or crest wall, when water is full up to the top of the gates and there is no water on the D/S side.
• Hence critical section the floor is just at the D/ S side of the weir’s chest wall.
• If the thickness of the floor is insufficient, its weight would be inadequate to resist the uplift pressure.
• This may ultimately lead to bursting of the floor and thus failure of the weir may occur.
• Actually, once the floor is burst due to uplift pressure the effective length of seepage is very much reduced which causes a further increase in the exit gradient and consequent failure by piping.

Such a failure of the weir can be avoided by adopting the following measures:

(i) Provide sufficient length of the impervious floor.

(ii) Provide impervious floor of sufficient thickness.

(iii) Provide a cut-off pile at the U/S end so as to reduce the effect of uplift throughout the foundation.

3. By suction due to standing wave.

• Standing waves of hydraulic jump formed at the D/S of the weir cause suction.
• This suction increases the effect of uplift.
• If floor thickness is inadequate to sustain the combined effect of uplift and suction, it will fail.
• Such failures occurred at Marala weir on the Chenab and Rasul weir.

Remedial measures may be the following:

(i) Provide increased thickness of floor to resist the additional effect of the standing wave.

(ii) Floor should be laid in one layer of concrete, instead of several, thin layers of masonry.

4. By scour on U/S and D/S of the weir.

• The beds of alluvial rivers are scoured to considerable depths especially during floods.
• This scouring may take place on the U/S side also, but it is most likely on the D/S side of pucca floors.
• If this scour is allowed unchecked, it will form scour holes underneath the pucca floor.
• These holes may slowly progress towards the main weir and cause its failure. This

failure can be prevented by adopting the following measures:

(i) Providing deep piles at U/S and D/S ends of pucca floor. The piles should be taken much below the calculated scour depth.

(ii) Suitable lengths of launching aprons, both at U/S and D/S of impervious floors should be provided.