Variable Soil Stiffness

[slickdeals]

If a Geotech report suggests a soil stiffness of 27000 kN/m^3 upto a bearing pressure of 24.5 tons/m^2 and a softened stiffness of 5430 kN/m^3 for pressures greater than 24.5 tons/m^2, how does one assign the soil spring in an analysis model?

Is there a way to compute an equivalent single stiffness?

 

[csd72]

slick,

I would just use the first value and try to limit the stress to the 24.5 tons.

I dont like the idea of softened soil as this always draws parallels with plastic deformation in my mind

 

[Teguci]

Perhaps you can explain how the geotech is getting two stiffnesses for the soil? I am not familiar with any soil that would become less stiff with more load. Is this because of a deep soft clay layer (in which case the stiffness would be more of a function of footing size and not necessarily pressure)?

My initial thought would be to conservatively bracket the problem like csd suggests. Either limit your pressure to the lower value or assume the lower stiffness for deflections.

I am assuming the stiffer spring applies to all pressures up to the limit and then the lower stiffness kicks in, ie delta = P/2752 for P<24.5 and delta = 0.0089 + (P-24.5)/554 for P>24.5.

 

[dhengr]

Slick:

I’d ask the Geotech guy to explain this to you, and get him to tell you how to use these values in the design process. I suspect it has to do with the fact that at higher pressures you start to get a lateral soil shearing failure which at the extreme shows up as the soil around and above the footing actually moving (displacing) up and away from the bottom of the footing (bulging), as the footing starts to settle at a greater rate (softer spring). This is also a function of depth of the ftg. and overburden loading preventing this shearing movement, soft layers below the ftg. level, etc. I think this is called the Prandtl’s theory of rupture, and is kind of an ultimate soil capacity, not to be confused with our ultimate strength methods of design on the rest of the structure. I’ve seen this issue discussed before on this forum, search for it, ‘soil stiffness or some such,’ or hope that one of the smarter soils guys looks in on this thread.

 

[BigH]

Teguci - think about highly sensitive clay . . . - not saying that is what is at this particular site but further to your second sentence.