sliding resistance - adhesion

[dan00]

I got this question from a structural engineer regarding a structure supported on shallow foundation bearing on clayey soil. Can additional sliding resistance be obtained from adhesion at the contact between clay and vertical faces of the shallow foundation? As far as I know, it is common practice to calculate it along footing base. He intends to use (cohesion) x (surface area of the vertical faces parallel to loading direction). One reason not to use it is the shrinkage of the clay soils and the lack of contact between soil and foundation (similar to ignoring upper few feet for pile foundations in clay). Any suggestions? Thank you.

 

[BridgeSmith]

He's trying use use what we would normally consider the skin friction between a drilled shaft and clay embedment material. The values we get for that are typically low and we only assume we can count on it because 1) it is surrounding the foundation and the clay itself is confined and 2) it's deep enough that it will remain moist. You can't really count on confinement pressure or it not drying out on a shallow foundation. Plus, if it's a sustained load, clay creeps. The side face of the footing would have to be notched to provide interlock, and even then I would be hesitant to count on it. On the bottom of the footing, it can't shrink away from the footing; it may settle, and the footing may sink, but it will remain in contact with the footing.

Adding a shear key (concrete lug) on the bottom of the footing could help, unless as I stated above, it's a sustained load, in which case the footing will continue to move.

 

[dan00]

Thank you HotRod10.

 

[LRJ]

The sliding resistance of a shallow foundation relates to horizontal movement. You therefore aren't really mobilising skin friction at all at the vertical faces, but rather the passive earth pressure. Inclusion of passive earth pressure is possible if the foundation is sufficiently embedded (e.g. skirted shallow foundation).

 

[MotorCity]

If the structural engineer is relying on the perceived benefit of adhesion in his/her shallow foundation design, I'd say its time to re-design the foundation. Even if the resistance offered by adhesion is quantifiable and reliable, it is not a common practice. In my neck of the woods, sliding resisted is only resisted by friction or a key.