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Determining Subsidence

Determining Subsidence

Determining Subsidence

(OP)
If I'm digging a shallow trench for some minor construction, and then refill it with the excavated soil, how can I determine the rate at which that soil will settle?  

The trench will only be a few feet deep and well above the water table.  In addition, there will be no load placed on top of the fill, so I don't think the equations for 1 dimensional soil consolidation apply.   

RE: Determining Subsidence

Provided the contractor compacts the material to at least 95% of optimum dry density determined by an "appropriate" test procedure (ASTM D698 or D1557 in the U.S.), then post-construction settlement should be small.  While you could use consolidation theory to evaluate this problem, that's effectively trying to "fine tune with a coarse knob."  The fill isn't uniform enough, and you can't adequately predict the likely variations with sufficient accuracy - at least for a 2 ft deep trench.  

"Rules of thumb" are more useful in your circumstance: properly compacted clay fill will shrink ("settle") about 2 to 4 percent of it's thickness, while sands will shrink about 1 to 2 percent.  Crushed rock will settle about 1/2 to 1 percent.  Again - these are rules of thumb, and should be used with caution.  Deeper fills require more testing and analysis, particularly if settlement is a major concern.

Why are you worried about settlement?



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