Stress Distribution beneath a foundation

[SuperSandman]

Hi Guys,

I trust you are all well,

I have a project where the soil horizon is loose and compressible (high settlements calculated) and I am recommending ground improvement which comprises excavating and removing some loose soils down to a certain depth and replacing with good granular soils compacted to a dense consistency (settlements now accepted by client).

To save time, the client wishes to use a low strength mass conrete/soilcrete (3MPa to 7MPa) as a replacement to my recommended recompacted soils option. I agree with the client that this method will save a lot of time and would certainly provide a good bearing medium for the foundations.

However, I also believe that using such concrete/soilcrete will merely transfer the foundation loads directly onto the insitu soils (loose) beneath the concrete/soilcrete. From my understanding of stress distributions, the applied foundation stresse will decrease in the recompacted soils with depth. However, the applied foundation stress will be directly transferred through low strength concrete/soilcrete and the foundation stress will now act directly on the insitu soils beneath the concrete/soilcrete. Is my assumption correct? Or am I missing some very important point in the way stresses are transferred through compacted soils and higher strength materials?

If i am correct and I carry out some settlement calculations, the insitu soils beneath the proposed ground improvement exhibit high settlements which are intolerable.

Thanks for the input.

 

[Ron]

A few thoughts....

First, what type of foundations and what is the loading?

Flowable fill or "controlled low strength material" can be used for fill in lieu of compacted soil; however, consider that the unit weight of the material will likely be higher than the compacted soil. The trade-off for settlement though, is that the shear strength of the material will be significantly higher than the compacted soil, which serves to attenuate a lot of the foundation load and spread it over a larger area. You'll have to do some iterative calculations to optimize this and keep settlements within tolerable levels.