Potential Settlements from heavy Rain

[Ibeam]

This is a question about overly wet soils:

We’re putting in a cantilevered retaining wall for secondary storage around AST’s for Fuel storage in Sacramento, CA. This is a new site, with no other structures in the immediate area. The ground was very flat with a water table around 11 ft. below existing grade.

Excavation in the tank storage area (including the footprint of the retaining wall) was done from existing 20 ft. grade, through loose fine to medium silty sand, down to 18.5 ft. grade, where soil was firm with an allowable bearing of 2000 psf. This soil is a stiff clay with some sand and silt, mostly non-expansive.

Then the rains started. Another 1.5 ft was lime treated and compacted. This would be down to 17 ft. elev. Then the rains became worse and turned the whole construction site into a swamp. Some places have 8 inches of standing water. The area in question is a swimming pool with the lime treated bottom, and they’ve been pumping it out to work.

There’s a lull in the bad weather right now so they want to start the wall footing. The bottom of the 5.5 ft. wide continuous footing is at 17.83 ft. (6 to 10 inches) above the bottom of the lime treated layer and a foot below existing rough grade.

My question: Is the soil below the footing still firm with so much saturation and will there be settlement when it dries out? The undisturbed lime layer, which will only be 6 inches thick should be ok I’d think, but then what of the undisturbed soil below that.

 

[Ibeam]

I asked the geotech to pay of site visit and give some recommendations. He had 9 test pits dug 3 ft. deep. The soil (silty clay and some clayey silt) below the lime treatment was competent enough for normal operations to proceed. The lime did a good job of protecting it from flooding, and the clays kept it from permeating lateral to a good extent.