Settlement/Collapse Potential in Silty Sand
Settlement/Collapse Potential in Silty Sand
(OP)
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!
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!





RE: Settlement/Collapse Potential in Silty Sand
RE: Settlement/Collapse Potential in Silty Sand
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.