Q. What do you know about Shallow Piers?
In case of some of the existing bridges, piers may be shallow and grip may be found to be inadequate. Protection round piers is then required to be given for their safety. Minimum grip essential for safety is worked out as 1/4 the total height of existing pier from bottom to design H.F.L. At this level bed scour is required to be arrested by providing stone apron with adequate quantity to fully cover scoured face. Launching slope in sandy bedded rivers is assumed as 2 horizontal to 1 vertical. Size and thickness are assumed same as in case of apron of the guide bunds. If one man stone is found to be of insufficient weight to be stable, concrete blocks or stone crates are required to be used. Placing stone round piers at unnecessary higher level is inadvisable as it is liable to be displaced and washed down more easily. At the same time it is not a safe and feasible proposition to wait till river bed scours to just the safe level of 1/ 4 height to ensure minimum grip length. Dumping of stone has, therefore, to be resorted to a little earlier before the lowest safe level is reached.