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Modulus of Subgrade reaction at mat foundation

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


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

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)

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