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Confused Effect on Bearing Capacity Due to Water

LONG1 (Structural)
5 Nov 02 23:32
During design of shallow pad on medium/dense sand for a two storeys house, I was told to reduce design bearing capacity to about half to account for water level which might rise up to the founding level. But I learnt from books that it is settlement, not bearing failure, normally controls. Settlement appears not seriously affected by presence of water. PLEASE HELP!
CarlB (Civil/Environmental)
6 Nov 02 2:53
I also learned that bearing capacity is reduced in sands when the groundwater table rises to with a depth of b below a footing of width b; the bearing capacity linearly varying from the 'normal' value to 1/2 normal for water level at or above bottom of footing. Allowabe bearing pressure is limited by bearing capacity failure for narrow footings, and by settlement limited to 1" for larger footings. For a 2-storey house footing is probably not large, thus settlement doesn't control, so allowable baearing pressure should be reduced. I believe you are correct that if limited by settlement, should be no reduction in allowable bearing pressure. Refer to standard curves in a soils text, charts showing allowable B.C. for varying d/b, to see if your situation is B.C. or settlement limited.
RamiT (Civil/Environmental)
6 Nov 02 5:54
I have another opinion regarding this. I believe that the bearing capacity of the foundation when governed by settlement limitation should still be reduced if the foundation sandy soil is submerged. There is no question regarding the reduction of bearing capacity of the soil if there is a rise of water table except that the linear variation is from the normal value when groundwater is at a depth B below the footing of width B, to 1/2 this value when water level reaches the surface of the surrounding soil (not the bottom of the footing). The overburden weight is accounted for in the BC calculation through the term qNq and all of it has to be submerged to have a reduction of 0.5 (assuming of course an average soil unit weight twice that of water).

Now, such a rise of water table, which reduces the effective pressures within the sand to roughly half their original values, also correspondingly reduces the stiffness of the sand. Hence, the footing pressure required to produce a settlement of 1", if the water level is at the surface of the surrounding soil,is only about half that required to produce a 1" settlement if the water is at a depth B or more beneath the footing.

The correction factor can be expressed as follows :
Cw = 0.5 + 0.5Dw/(Df +B)

Where
Dw is the depth of water from the surface of surrounding soil
Df is the depth of the footing base from the surface of the surrounding soil

This is retrieved from the textbook "Foundation Engineering" by Peck, Hansen & Thornburn (2nd. Ed.)
Check-out this reference for further details.

So if let's say you calculate your allowable BC for an estimated settlement of 1" using Bowles formula based on SPT results, you would have :

qall = N x Kd x Cw / F1   (B<=1.2m)

But you would need to recalculate your settlement based on the obtained qall to verify that it is suitable.

Hope this can be helpful.


Rami
Focht3 (Geotechnical)
6 Nov 02 12:14
Two points:

1. Ramit is correct (although I skimmed his calcs and did not review his response in detail) - submerged sands have a reduced bearing capacity in comparison to drained sands.

2. Settlement and bearing capacity are inextricably linked - the term "allowable bearing capacity" is based on a perceived maximum acceptable foundation settlement.  The idea is to prevent the soil from experiencing shear failure which results in a sudden increase in settlement.

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