## Confused Effect on Bearing Capacity Due to Water

## Confused Effect on Bearing Capacity Due to Water

(OP)

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!

## RE: Confused Effect on Bearing Capacity Due to Water

## RE: Confused Effect on Bearing Capacity Due to 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

## RE: Confused Effect on Bearing Capacity Due to Water

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.