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Re: Modulus of Subgrade reaction at mat foundation

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Modulus of Subgrade reaction at mat foundation
• From: "Javier Encinas" <jencinas(--nospam--at)coqui.net>
• Date: Tue, 13 Sep 2005 11:37:35 -0400

```In fact the effect of a change in the modulus in the final design is
negligible, since it is elevated to the fourth root when you use it in your
calculations. That means that even a dramatic change in the modulus, say one
half or the double, will lead to a small change in the final result.

Javier Encinas, MSCE, PE

----- Original Message -----
From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, September 13, 2005 10:22 AM
Subject: Modulus of Subgrade reaction at mat foundation

A geotechnical report has included a modulus of subgrade reaction for a
mat foundation, to be adjusted based on the following equation:

K'=K*[(B+1)/2B]^2

where B= width of mat, K = subgrade modulus on 1-ft^2, and K' = design

I have seen this equation in references before, but I continue to be
baffled by its effects on the design subgrade modulus.  If I have a
20-ft to 40-ft wide mat, this results in nearly a 75% reduction in
effective modulus.  If I load one square foot of a large mat foundation,
is it really going to deform 4 times as much, just because it is in a
large mat?  If the soil's modulus of subgrade reaction on 1-ft^2 is 200
pci, is it realistic to design the mat for a modulus of only 50 pci?

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

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