Depth of influence for settlements

[Mccoy]

This is something which is really not too clear. The condition is the one where a rigid layer is not found near surface. In some textbooks, the depth of influence for calculating settlements Z_I is assumed to be the one where the load surcharge drops to 0.1 times the value of overburden. Somewhere else, they go up to Delta_Q= 0.3 or even 0.5 overburden.

Sometimes Z_I is assumed to be 2B, sometimes though especially in very wide foundations, ZI=B or event 0.5 B.

Bowles gives values of 5B, Burland & Burbidge give different values, Schmertmann says 1.5 B, chaos rules supreme. Apparently.

What do you guys use as a depth for the influence of loadings, either to calculate settlements or to figure out a weighted value of the modulus of subgrade reaction for foundations. Literature references are very much welcome.

 

[GeoPaveTraffic]

Mccoy,

I think part of what we are seeing is, as you pointed out, a difference in the structural and geotechnical way of thinking that usually doesn't cause a problem. I say usually because most times soils become better with depth and usually soils across a mat foundation are not so variable that the mat can't compensate for the local softer areas, and usaully the mat has more thickness and more steel than is really necessary, and usually the column loads are less than predicted. When you add up all of that usually, it is unlikely to get a mat that doesn't perform ok.

All that being said, I believe that a complete analysis of a mat foundation would need to consider the effects of soil to at least a depth of 2B to account for long term settlement of portions of the mat. Long term differential settlement will cause loads to shift in the mat as the load is attracted to the stiffer portions of the matt. If the mat is perfectly rigid, then everything will be fine. However, few rigid mats are constructed. The more flexible the mat the more deflection will occur in the matt and therefore the high the resulting moments will be in the mat.

I believe the only way to truely analize this situation is with a finite element analysis where the entire soil profile and structural design of the mat are modeled. Not something that is likely to happen on a project in the US where structural and geotechnical analysis are performed by different firms at different times often with little real communication. It is a good thing all those usuallys are there to keep everyone out of trouble.

Mike Lambert