Determining Subsidence

[cmichaelson]

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.

 

[Focht3]

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&quot 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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