Water pressure on basement slab

[YP123]

Recently I was involved in a project that a Sports Hall is to be constructed on a sloping groud. In the final condition there will be a retaining wall 3.5m high above the slab level on one side of the Sports Hall, and on the opposite side of the Sports Hall, some excavation is required to reduce the external ground down to the slab level. The Sports Hall is about 20m wide. Based on the Soil Investigation Report the water table is about 1m above the slab level. I would like to hear your views on how the water pressure on the slab should be defined considering that on one side the Sports Hall the final external ground level is same as the slab level. Thanks.

 

[GeoPaveTraffic]

I would install a good drain system designed to daylight on the low side of the structure placed both beneath the floor and behind the wall. This should lower the groundwater and prevent water pressure from building up beneath the floor.

Since this system must function well over a long time, I would use graded sand and rock instead of filter fabrics and place about twice as many drain pipes as I thought were necessary.

 

[YP123]

GeoPaveTraffic, thanks. As the slab is designed as ground bearing, do you have any concern on the impact of the running water underneath the slab on soil bearing capacity, settlement, and other potential erosion related problems?

 

[GeoPaveTraffic]

Yes all of those are items that need to be considered with the specifics of the drain design. The drain system needs to be detailed by the project geotechnical engineer in light of the geometery, soil types, loads, etc.

 

[YP123]

If due to cost or other limitations the drainage solution cannot be adopted, we are still facing the problem of defining the water pressure on the basement. Will a triangular assumption be reasonable (i.e., the water pressure reduces linearly from one end to the other end)?

 

[GeoPaveTraffic]

If you are asking about the pressure on the bottom of the slab, yes a triangular assumption is reasonable. However, the pressure may be higher than the triangular assumption. (reasonable does not mean conservative or best) Depending on the soil, slab support, geometry, and water conditions the water pressure could be higher. I recommend that you discuss this with the project geotechnical engineer. It should be possible to install drains for a reasonable cost. Also the engineer could draw a flownet to determine the acutual pressure distribution beneath the slab.

 

[dmoler]

A sump with a submersible pump might work at the juncture of the retaining wall and slab (overexcavate that side to install). The pump turns on when the water level gets to a certain level.

You might want to add a compacted clay "cap" or similar water diversions on the top of the retaing wall to prevent surface water from traveling down the slope and then the retaining wall to the bottom of the slab.

 

[jheidt2543]

My vote is for GeoPaveTraffic's underdrain solution. I think, after investigation, you'll find it the least costly both first cost and life cycle cost. His solution is non-mechanical, no sump, no pump. Perforated PVC pipe is cheap, easily installed,long lasting and vertually maintanence free.