Water pressure in basement design
Water pressure in basement design
(OP)
Hi,
I wonder whether anyone can help me on the following issue:
When we design a retaining wall we need to consider the worst waterpressure situation, i,e, the worse case between site measured water table and assumed water table (roughly about 1m below ground level), but do we need to adopt the same approach in the basement slab design, for example, the uplifitng pressure? I have checked several British Standards, but could not find a clear answer.
I wonder whether anyone can help me on the following issue:
When we design a retaining wall we need to consider the worst waterpressure situation, i,e, the worse case between site measured water table and assumed water table (roughly about 1m below ground level), but do we need to adopt the same approach in the basement slab design, for example, the uplifitng pressure? I have checked several British Standards, but could not find a clear answer.





RE: Water pressure in basement design
RE: Water pressure in basement design
Assuming the basement is 3m below ground level and the water table is 0.5m above basement level. When we design the retaining wall we usually assume the watertable is 1m below ground level to work out the water pressure on the wall, but should we use 0.5x10= 5kN/m2 or 2.5x10 = 25kN/m2 for the basement design to resist the uplifting?
RE: Water pressure in basement design
RE: Water pressure in basement design
I have seen severe damage due to water pressure on slabs. In one case in a residential structure, several of the interior stanchions heaved up and made the floor joists above deflect and popped out the hardwood floor!
RE: Water pressure in basement design
RE: Water pressure in basement design
RE: Water pressure in basement design
As for when it can't be used - likely when you have so much water, you can't keep pumping - or you have no gravity outlet and don't want to use a sump for some reason (perhaps a need to always make sure it is working). Uplifting pressure is water head x 62.4pcf - uniformly on the basement slab. You indicated only 1.8ft of water pressure normally - so this isn't that much - about the same as a 9 inch slab - dead weight. Don't think your slab would heave on this. But always best to put in positive measures (active).
RE: Water pressure in basement design
My client wants to build a small residence (@ 110 sq. m footprint) in a coastal zone with a high water table. Soil borings show standing water at - 1.5 m and a seasonal high water table at - 0.9 m. He wants a full cellar and is willing to live with a bit of dampness if necessary. Local land use regulations would put the top of the floor slab no higher than 1.2 m below grade (otherwise the "cellar" becomes a "basement" and must be counted as a "story").
I have never designed a cellar with a floor below seasonal high water or so close to the water table. My calculations suggest that by excavating right down to the standing water and then placing @ 8 mil rubber membrane, .15 m of coarse gravel and .15 m concrete slab, we will have a slab that will not "float" under worst case scenario. I would also place two independent drain tile systems, one under the slab, and one a bit higher outside the footings, both piped to two sump pits and pumped to daylight. The foregoing, however, is purely academic. Does anyone have some real world experience in this area that they would be so kind as to share?
Thank you all.
RE: Water pressure in basement design
The biggest problem with dampness is mold. It is becoming a hot issue in the States.
It sounds as though you are going about it the right way though. You may have problems during construction if you have an abnormally wet period. You may need dewatering to construct the foundations and slab.