Net pressure to Soil

[Balageotech]

Dear Expert,

Can we allow deduction of overburden pressure from raft pressure (superstructure) for estimating settlement?? if depth of excavation is high. Type of soil is pure clay.

ie, Pressure on soil = Footing pressure (from building) - overburden pressure.

Regards
Bala

 

[L0k]

yes, certainly

 

[Balageotech]

Thanks. L0K,

If the water table is at surface, should I take submerged density or saturated density.

Regards
Bala

 

[BigH]

This is the underlying concept of the "floating raft" foundation. See original Winterkorn and Fang - Golder wrote a chapter on this. First really developed in Chicago in the mid 1880s by an architect - Burnham & Root, it allowed "skyscrapers" to be first constructed.

http://www.wbez.org/series/curious-city/building-skyscrapers-chicagos-swampy-soil-111658

 

[DougHole]

recommend taking the buoyant unit weight,

here is a worked example to help you...

unit weight of soil = say 18kN/m3 for "pure clay:
buoyant unit weight = 8 kN/m3 (18-10)
depth to underside of proposed raft = say 3m
water at surface as per your thread

so take current overburden pressure = 3m x 8 kN/m3 = 24 kPa

if you're assessing settlement, then recommend taking the dead load only + the live load but don't; factor them up (for calculating the "gross structural bearing pressure")

lets say the gross structural bearing pressure comes out at 50 kPa

The net bearing pressure will be 50 - 24 = 26kPa.you can use this for the calc of settlement.


Doug Hole
Junior Geotechnical Engineer