Correct Answer - Option 3 : Both safe exit gradient and scour depth
Design Principles for Weirs on Permeable Foundation:
(i) To counteract uplift pressure it is necessary to provide a suitable thickness of the floor at different points.
(ii) The exit gradient can be kept within permissible limit by providing suitable depth of the sheet pile at the downstream end of the floor.
(iii) Since the scour holes can occur on the upstream or the downstream side of the weir sheet piles are essentially required at upstream as well as downstream end of the floor to prevent failure of the weir by slipping off the subsoil into the scour holes by simple earth pressures.
(iv) The pressures under the downstream floor increase as the depth of the downstream sheet pile increases. The upstream pile line has little effect in reducing these pressures as the spacing between the two is generally more.
(v) The intermediate sheet pile neither prevents undermining nor alters pressure distribution substantially to reduce uplift pressure under the downstream floor. The main purpose of providing an intermediate sheet pile is to create a second line of defence in case of failure of either upstream or downstream sheet piles.
(vi) The depth of the downstream sheet pile is determined in such a way that in addition to its depth reaching normal flood scour depth for that section of the work it gives a safe exit gradient in conjunction with a suitable length of the floor.
(vii) Commonly adopted practice in respect of intermediate pile is to provide a single and deep intermediate sheet pile under the crest of the weir.
(viii) Upstream and downstream of the weir/barrage provision of guide banks are essential to direct the flow through the waterway.