Effective Length of Foundation Walls and Piers for Shallow Foundations Confined by Compacted Backfill

[Structural.1997]

What do you typically use as the effective length of your foundation walls and piers? Do you consider the stabilizing effects of the confining soil?

I have always neglected the contribution of the soil and treated the piers as if they were columns above grade. Much of my work is in Canada where frost depths typically result in shallow foundations over 3m deep. Completely neglecting the lateral bearing capacity of the soil to prevent buckling and bending of the piers and foundation walls sometimes this seems over conservative.

Assuming the pier acts as an unbraced cantilever column would give k=2, am I able to reduce this by considering the effect of the soil compacted around the pier and do you recommend any references relating to this?

Thank you!

 

[haynewp]

If you compare them to foundation elements such as drilled piers and piles, then we usually assume the adjacent soil braces the member for axial buckling. Unless the soil is of a very poor type or undergoes liquefaction. I think the question is how stiff is the soil that the foundation wall or pier is embedded into as it relates to the stiffness and strength required to prevent buckling of the element under the axial load demand. We typically assume the soil braces short piers for the depths and dimensions of the piers, the soil types, and the loads we often have. Doing some L-Pile runs may give you a better feeling for a particular case.

 

[Structural.1997]

@haynewp For these it would always be good engineered fill material backfilling over the foundation and around the pier/wall. So it seems that it's safe to assume that the soil will prevent buckling under axial loading.

Can the soil also be considered to contribute to the bending strength of the pier? ex. for a pier in a braced bay with significant side loading.

Unfortunately i don't have access to L-pile.

Thank you

 

[haynewp]

I wouldn't count on the soil at the unloaded pier face to help resist bending unless it has been justified with L-Pile or some model that accounts for the soil as springs. We usually get the geotech to run L-Pile for us in my area for a small fee, but that may not be as common for them to do it in other regions.