De-watering
De-watering
(OP)
I have to design / specify foundations for a house. The ground is soft at the surface and has stone below. The stone is in a firm clay and the depth to this layer varies across the house footprint from 1.4m to 2.1m (one corner). This stratum would be adequate for bearing but at the deeper corner the is a large inflow of water (and a stream near by - approx. 10m).
If we specify foundations taken down to this stratum and dewatering with a sump and pump should be be looking at the effect the flow of water in the trenches will have on the stratum. What precausions should we take? Would it be adequate to scrap of the top layer in the trench prior to pouring concrete?
If we specify foundations taken down to this stratum and dewatering with a sump and pump should be be looking at the effect the flow of water in the trenches will have on the stratum. What precausions should we take? Would it be adequate to scrap of the top layer in the trench prior to pouring concrete?





RE: De-watering
If there is a fair amount of water pressure; not sure why this would be, but don't want to neglect the potential. Or the clay has a lot of sand and voids, and therefore relatively permeable, water flow could be a problem. Also, the amount of time the excavation will be left open prior to construction of the basement and backfilling will have some impact. The longer it is left open, the more time the water has to seep out and beggin to create stability problems.
Water should not be allowed to pond on the bottom of the excavation. It will only seep into the clay and softening it. Workers then will walk over this creating a 'disturbed' bottom where the foundation or basement floor will be placed over. In which case additional excavating would be needed to obtain an undisturbed native soil. And besides, concrete should not be poured over water anyway (unless specifically designed to do so).
Hope this helps, I am looking at this how we would for our area, here we excavate below the water table quite frequnetly. Sometimes the clays are highly impermeable and the excavation is open only a short time, so not much difficulty. Other times a relatively permeable soft sandy clay or silt that will open for a long period of time, additional measures need to be recommended.
Hopefully others will give there input.
RE: De-watering
Also, becuase this is more of a soils question, you should ask on the 'Soil Testing' forum or the Earthwork Grading' forum.
Good luck.
RE: De-watering
RE: De-watering
Hope this helps. . . .
f-d
¡papá gordo ain’t no madre flaca!
RE: De-watering
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RE: De-watering
RE: De-watering
RE: De-watering
I read a peat layer in your thread. Also, a stream that is now a river only 10m away. Do you have subsurface boring information? Sounds like an experienced local geotechnical engineer should be consulted to confirm the depth to a suitable bearing strata, to identify the need for permanent/temporary dewatering and to identify settlement issues or problems. A future homeowner will not be too happy with a new house that has water or settlement problems.
RE: De-watering
RE: De-watering
I do this type of work, just not in cyberland.
f-d
¡papá gordo ain’t no madre flaca!