Soil Stiffness
Soil Stiffness
(OP)
I am designing a raft foundation using finite element analysis software. I need to insert a value for the stiffness of the bedding material (ie the soil). Could anyone give me some advice on how I calculate this value? I have heard that it is the allowable bearing pressure divided by teh amount the raft will settle, (ie 150kN/m2 / 0.01m = 15000 kN/m3) Does this make sense to anyone?






RE: Soil Stiffness
RE: Soil Stiffness
RE: Soil Stiffness
Sandy soils
Loose sand 4,800 - 16,000 kN/m^3
Medium dense sand 9,600 - 80,000
Dense sand 64,000 - 128,000
Clayey medium dense sand 32,000 - 80,000
Silty medium dense sand 24,000 - 48,000
Clayey soils
q < 200 kPa 12,000 - 24,000
200 < q < 800 kPa 24,000 - 48,000
q > 800 kPa > 48,000
Bowles, J.E., Foundation Analysis and Design, McGraw Hill Book Co, 1997
RE: Soil Stiffness
When using a high end stiffness for the soil, the soil will be relatively stiff compared with the concrete. This will tend to give you small deflections in the soil, and higher concentrations of pressure on the soil.
When using a low end stiffness for the soil, the soil will be relatively less stiff than the concrete. The concrete will then show higher stresses, moments, etc. and the soil pressure will be more uniform across the section.
This then gives you a sort of lower bound and upper bound solution - and by using both to create your design, assures you of a more confident design.
RE: Soil Stiffness
RE: Soil Stiffness
That is a great tip regarding an "envelope" approach to mat design, Thanks.
Auce98
RE: Soil Stiffness
RE: Soil Stiffness
I don’t quite understand the 0.01m. Modulus of subgrade is the slope of the stress-displacement curve. If the displacement (or settlement) of the subgrade happens to be 0.01m when the stress reaches to a value 150kPa (it doesn’t matter the stress is allowable or not, once it is significant enough), then the secant modulus is 150/0.01=15000kPa. Am I right?
RE: Soil Stiffness