Settlement/Collapse Potential in Silty Sand 1

[geotech_ben]

Hello World -

First time question, long time reader. Big, big fan of all the helpful information on this forum Here is my situation:

We have a 7' swimming pool being constructed in 17' of loose(bpf of 4-7) silty sand to sandy silt (borderline, a typical wash indicates 52% sand) of low unit weight (moist 89 pcf) overlying a dense sandy gravel. We performed consolidation testing on what we believe were undisturbed shelby samples. (Good recovery, care, but they were shipped very well packaged FEDEX next day) We added water to one sample at around 6.7 ksf stress and had a drop of 4.8 percent strain. The other sample showed 0.9 percent strain at 3.4 ksf. Collapse anyone?

The geology maps indicate predominately lacustrine deposits but we may have aeolian (loess) deposits in some depressions. From what I have read in Peck Hanson and Thorburn, 1974, an aeolian transitional material between sand dunes and loess consists of a silty sand. This material will have a very low density and has a potential for collapse.

We are preparing to perform additional consolidation testing where our loading increments are lower and the water is added at a lower stress value to mimic our site conditions. Good idea? Is consolidation testing the best idea? Our total strain for the our expected loads is around 4 percent. Does this seem reasonable?

Thanks!

 

[oldestguy]

You may be working with undisturbed samples, but who knows. What size tube? Did the test get run on the sample in the tube? Did any tube samples get extruded for examination? Considering what you have provided, I can hardily believe sampling, shipping and testing were all OK. Any chance of going on site, setting up a plate loading test and then saturate the ground? Or something similar? Plate at least 12" square. Those blows don't seem to fit with the low density calculated. If no site visit, is there a nearby testing firm?

 

[aeoliantexan]

Some loess is quite sandy. You didn't list a dry unit weight or water content, but at the moist density you reported, it may well be collapsible. If the saturated water content is greater than the liquid limit, it probably is collapsible. Unsaturated loess has a tendency to compress during sampling, so the actual density may be even lower than your test result.

4 percent collapse is modest for a low density loess.

I you want more accurate consolidation test results, I would suggest had trimming block samples from test pits.

If I were making recommendations for a swimming pool in collapsible loess, I would probably want to overexcavate and place several feet of low permeability compacted fill under and around the pool, and be very careful with the plumbing. The excavated material may make a suitable compacted fill; you could compact some and run permeability tests. Properly compacted, the fill should not collapse, and will greatly slow infiltration into the underlying soil. I would also avoid filling the construction area above the natural grade.