I am working on a site where we are adding several feet of fill material. The underlying soils are saturated clayey sand soils (SC). The fill material is a slightly silty sand (-200=12 to 15%) with a modified proctor of 119 pcf and 11% moisture. We have two feet of fill in above the wet clayey soils (SC). The fill material was tested to be at 96 to 98% compaction of the modified proctor maximum dry density with an in-situ moisture of about 10%. However, poritons of the building pad were yielding and pumping under the weight of a human (i.e. me walking on the site). The fill was placed and compacted with a vibratory roller. I believe the vibratory roller caused the underlying satrurated clayey sand soils (SC) to lose shear strength. I believe the underlying clayeys soils will stabilize once the excess pore water pressures built up from the vibratory roller have had a chance to dissipate. The contractor has obviously passed the fill compaction requirement of 95% MPMDD, but the pumping subgradein the buidling pad is not acceptable. What to I need to do or what should I tell the contractor? Should they continue to place fill to see if they can bridge the underlying soft soils? Thanks for your help!

Compaction and stability are two different animals. You can achieve compaction without stability, but usually not the other way around.

Yes, the soil will stabilize when the pore pressures dissipate; however, load should not be placed on the soil until that happens. If you place fill on top of the existing, make sure it is dry of optimum, which will tend to pull moisture from the underlying soils and help dissipate the excess pore pressure.

Use static compaction only at this point.

With respect to Ron's response, I would question whether the underlying fill will "heal" when pore water pressures drop. Soil instability represents shear failure, and that changes the soil structure and fabric, and represents a reduction in the in-place strength when compared to the shear strength of stable, compacted material. The question is will the fill ever achieve the strength intended by the design? If so, bridging is the easiest way to get your fill constructed.

Your situation just highlights the issue of just specifying compaction density and moisture content ranges. Add in the requirement that fill must be stable, and define what you'll accept with respect to defining stable fill.

Finally, if you accept the fill construction, you'll also accept future problems with improvements - the contractor will certainly remember you accepted the construction of the fill as is. Proceed carefully!

Update - The pumping fill was allowed to sit idle for four days during which we did not receive any additional rainfall. The site was re-proofrolled and the subgrade was observed to be firm and unyielding indicating the stability issue was most likely caused by porewater pressure build up caused by a vibratory roller within close proximity to saturated soils. Once the excess porewater pressure built up by the vibratory roller was allowed to dissipate the subgrade became stable.

grabens...as expected. Thanks for the update.

Good one, Ron.

Stability should almost always overrule compaction testing which is a tool to determine maximum stability. I use both and the stability never fails unless conditions change.