One interesting aspect of how engineers decide on how deep piers must be dug is based on soil conditions. These recommendations are usually based on small soils samplings taken by a soils engineer.
Once those tests are done the structural engineering is based on those results. Now in our case we have specced some pretty deep peers partly because we are ignoring the top 4 feet of soil as being expansive clay. Once the site is excavated you can clearly see the composition of the layers and how deep they are. Based on what we actually see now we're going to try and see if we can ammend those recommendations because the clay appears to be very shallow. So shallow that almost all the clay is gone from the initial excavation work. I'm hoping this can save us some time and money for the foundation work. We're having the soils engineer come out to take a look. Wish us luck!
The picture below shows a cross section of depth of the unmolested hill. the dark dirt is clay the light is sandstone. note how the small roots are above the sandstone layer.
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