Water pressure in basement design 1

[YP123]

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.

 

[BigH]

If you have a basement - and you have high natural ground water levels - certainly you will need to design for uplift pressure - unless you relieve the pressure via underdrain and sumps (near continous-running).

 

[YP123]

BigH, thanks for your reply. Probably I need to put my question this way by giving an example:
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?

 

[BigH]

First off - in the scenario you give, I would put a drain at the bottom of the basement wall to bring down the water table so that you don't have any hydrostatic pressures against the wall - hopefully you will be able to drain this by gravity. Similarly, if properly located and you use drains below the slab on grade - drainage blanket with some slightly deeper trench drains leading to your perimeter drainage - and gravity outlet; you should be okay. If you can't get gravity outlet, then you'd need to estimate the seepage quantity and design a sump to handle this. In clayey soils, this may not be onerous and you can use a collection pit and sump from it. Obviously with the groundwater level so close to the basement bottom, you should try to drain, in my view - at least for the wall situation.

 

[eric1037]

If you go with a sump system to drain, make sure you have some kind of battery or generator backup. Also I would install a backup pump. These items could save alot of damage if the power or primary pump fail.

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!

 

[BigH]

eric - agreed!

 

[YP123]

BigH and Eric, thanks, that is useful. What you described is to avoid the problem (I will term this as active approach). I would like to hear your views when the active approach cannot be adopted and we have to design the basement to resist the uplifting water pressure (i.e, the passive approach). In this scenario how do we define the uplifitng pressure?

 

[BigH]

YP123 - my company had a standard drawing showing the measures to take - you can also see this from Tschebotarioff (1951) Fig 14.3, page 410.

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).

 

[Birchy]

Hi all!

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.

 

[eric1037]

Birchy:

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.