Correct Answer - Option 3 : 1, 2 and 3
Hydraulic structures such as weir, dams, barrages, head regulators, cross drainage works, etc. may either be founded on an impervious solid rock foundation on a pervious foundation. Whenever, such a structure is founded on pervious foundation, it is subjected to seepage of water beneath the structure, in addition to all other forces to which it will be subjected when founded on an impervious rock foundation.
1. Failure by piping or undermining:
When the seepage water retains sufficient residual force at the emerging downstream end of the work, it may lift up the soil particles. This leads to increased porosity of the soil by progressive removal of soil from beneath the foundation. The structure may ultimately subside into the hollow so formed, resulting in the failure of the structure.
2. Failure by direct uplift:
The water seeping below structure, exerts an uplift pressure on the floor of the structure. If this pressure is not counter-balanced by the weight of the concrete or masonry floor, the structure will fail by a rupture of a part of the floor.
3. Rupture of floor due to section caused by standing wave.
4. Scour on the upstream and downstream of the weir.
5. Erosion of apron protection.
6. Deterioration of cutoff and subsequent loss of containment.