basement design - predicting water table level

huangyhg

basement design - predicting water table level
i have moved this thread from the geotechnics forum
my projects often involve undeground basements, usually for carparks of up to 3 underground levels.
my query is that as some of you may know bs 8102 recommends that water table levels for basement design should be as follows:
for basements up to 4.0m deep the design water level should be 1.0m above the existing water table or 2/3's the depth of the basement i.e. 2/3's x 4 = 2.66m head of water.
for basements over 4.0m deep the design water level should be taken as 1.0m below ground level in all cases.
i believe these valuse to be conservative in certain situations considering soil types e.g. in sands/gravels water will not build up behind a retaining wall as could happen with impermeable clay.
there is also the issue of rising water levels which can be a problem in certain areas but if evidence is availale that water levels are not rising this may not be a problem.
any opinions on this would be greatly appreciated as  taking the water table at 1.0m below ground level for every basement means that large hydrostatic pressures will have to be designed for resulting in very deep basement floor slabs.


my answer's the same as the other string.
mike mccann
mccann engineering