Modulus of Subgrade Reaction for Footings and Mats.

      a. The modulus of subgrade reaction can be determined from a plate load test (para 4-6) using a 1- by
1- foot plate

The equation above is valid for clays and assumes no increase in the modulus with depth, which is incorrect,
and may give k1,  which is too large.
For footings or mats on sand:

For a rectangular footing or mat of dimensions of B x mB:

       (  15m5)(10-3)

with a limiting value of ksf = 0.667ksl

      b. ks may be computed as ks = 36qa. (kips per square foot) (10-4)  which has been found to give values about as reliable as any method.  This equation assumes qa (kips per square foot) for a settlement of about 1 inch with a safety factor,

F ≅ 3.  A typical range of values of ks is given in table 3-7.

An Engineering Evaluation of Shales
Table 3- 7.  An Engineering Evaluation of Shales

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