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Differential Settlement

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I am engineering a 3 story building which requires a mat foundation due to the
soil conditions, (the bearing value is okay- 1500 psf, but there is a layer of
dry sands btwn 15 to 40 ft below grade which are susceptible to seismic
induced settlement). The soils report is requiring designing the mat
foundation for 1.7 inches of differential settlement for a span of 20 feet.

Using the equation M=(6EI x defl.) / L x L, I get very large moments for the
potential differential settlement, particular if the mat is considered
uncracked, (24? thick). And this is not even considering the moments due to
gravity loads. It really doesn?t seem practical, and maybe the solution is to
get through the layer of sand with end bearing caissons to eliminate the
differential settlement.

Anyone have any experience or thoughts on this problem?

Thanks in advance.

Larry Hauer, S.E.

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