modulus of subgrade reaction

[stk7879]

The soil report for one of the jobs that I am working on says the thickness and reinforcing of the mat/slab type foundation can be designed using a modulus of subgrade reaction of 350 pounds per cubic inch at the top of the structural fill. Can someone tell me what it means? The mat is supported by drilled piers and I would design as a flat slab with the superimposed load.

 

[briancpotter]

The subgrade modulus is a measure of the stiffness of the soil, as separate from soil bearing strength - the higher the number, the stiffer the soil. In practice, with higher subgrade moduli a spread footing will have a more even spread of load over the soil. There have been many discussions of the subgrade modulus on this site, for example here:

http://www.eng-tips.com/viewthread.cfm?qid=27153

Most foundation design programs will require a subgrade modulus to calculate the bearing pressure. However, if you have a foundation supported on drilled piers, you might not need it - a more typical design strategy would be to assume no soil support, and just design the cap as a beam spanning between the piers.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.