Modulus of Subgrade Reaction of Soil (Bowles Approach and Basic Approach)

The modulus of subgrade reaction (k) is used as a primary input for rigid pavement design. It estimates the support of the layers below a rigid pavement surface course (the concrete slab). The k-value can be determined by field tests or by correlation with other tests. There is no direct laboratory procedure for determining k-value.
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1- BASIC APPROACH Method
The modulus of subgrade reaction came about because work done by Westergaard during the 1920s developed the k-value as a spring constant to model the support beneath the slab.

The reactive pressure to resist a load is thus proportional to the spring deflection (which is a representation of slab deflection) and k.

P=k* delta
Typically, the modulus of subgrade reaction is estimated from other strength/stiffness tests, however, in situ values can be measured using the plate bearing test.

2- Joseph E. Bowles Approach Method
Bowels recommended following approximation to determine Ks based on allowable bearing capacity as suggested by geotechnical engineer.

(SI Units) Ks = 40(SF)qa kN/m3

(FPS Units) Ks = 12(SF)qa k/ft3

Where
SF = Safety Factor
qa = Allowable Bearing Pressure in ksf or kPa

Keywords
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