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Re: Differential Settlement

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Larry,

        Your use of the equation 6EI*delta/L^2 means that you're assuming that the
mat will bend to accommodate the differential settlement.  This may not be a valid
assumption.  Let me present some alternative assumptions that you may be able to
work with.

1.)   What happens if you assume that the soil settles away from beneath the mat
leaving the mat rigid and intact but spanning the void spaces?  This would leave
you with moments in the mat similar to PL/4 or PL/8 simple and continuous beam
span moments to deal with after a seismic event.  You might have to be rather
arbitrary in deciding where the void spaces may develop (i.e. pick the worst case
you can reasonably visualize).  You might consider installing 1/2" diameter pipe
grout tubes for pressure grouting after a seismic event or drilling the holes as
needed after the event as a means of filling in the voids with grout when they
occur.

2.)   What about providing some keyed construction joints in the mat to act as
hinges and relieve the bending stresses?  This should solve your mat problem but
may transfer it to other points in the structure

        For either case you may require future pressure grouting and/or means of
levelling the building by adjusting the columns.

        Good luck with whatever you decide to do.

Regards,

H. Daryl Richardson

lrhauer(--nospam--at)earthlink.net wrote:

> 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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