Foundation Settlement, Sand over Silt

[pbruins1]

I have a project where I need to estimate settlement due to a six story structure. The general soil profile is 20 feet of medium sense sand over 80 feet of stiff silt. Best method? Thanks!

 

[fattdad]

hooks law works. Just need modulus values for the various soil layers. I'm assuming that the silt is fairly non-plastic (i.e., it's not some normally-consolidated elastic silt layer).

Modulus-based compression typically occurs as the load is applied. Post construction compression can be an additional 20 to 30 percent of the calculated value, depending on the design life.

f-d

ípapß gordo ainÆt no madre flaca!

 

[pbruins1]

What would the implication of a groundwater table at 10 feet be?

 

[oldestguy]

If you can find it this paper ties settlement with split spoon blow count in sand, etc.

It has been found quite accurate by Calif DOT, in non saturated soil but under estimates it about 50% in saturated soil.

"Compressibility as the Basis for Soil Bearing Capacity"., B.K. Hough
ASCE Journal of the Soil Mechanics and Foundation Division, August 19+59 , Number SM 4, 2135

 

[BigH]

First off - is your structure on a raft/mat foundation? or separate spread/strip foundations? If the latter, do they interact with each other? That is, are they close enough so that within a depth of 2B to 4B (4B for strip and 2B for spread)the pressure bulbs would interact (can use a 2V:1H pressure spread for preliminary assessment). If they don't you will have isolated footings and the settlements would depend on the width of the footing. As fattdad says, you will need (1) the elastic estimations for settlement and (2) if the pressures extend down into the stiff silt, you might need to know the preconsolidation pressure (assuming the silt is acting cohesively - you can estimate this from the overburden/undrained shear strength ratio. The water table comes into effect in that the settlements are based on effective overburden pressures (existing ground pressures) onto which your added structural pressures will be added. This is basic text book stuff - and you can get from various sources - which I am hoping this isn't a homework problem. Then by various methods - elastic for sands and consolidation/elastic for the stiff silts, you can estimate the settlements - don't forget that the consolidation settlements will take time whereas the sands will be rather instantaneous. Therefore you may wish to determine, given various degrees of consoldiation to develope a settlement vs time curve. And, if you can estimate the settlements to within 25% or so of the actual settlement you will have had a good day.