Modulus of Subgrade Reaction

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 PCC 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.

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 (Figure 1).

The reactive pressure to resist a load is thus proportional to the spring deflection (which is a representation of slab deflection) and k (Figure 2):

The value of k is in terms of MPa/m (pounds per square inch per inch of deflection, or pounds per cubic inch - pci) and ranges from about 13.5 MPa/m (50 pci) for weak support, to over 270 MPa/m (1000 pci) for strong support. 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.

Related Tests

Plate Load Test

@misc{pavint-modulus-of-subgrade-reaction,
  author = {{Pavement Interactive}},
  title = {Modulus of Subgrade Reaction},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/design/design-parameters/modulus-of-subgrade-reaction/}},
  note = {Accessed [date]}
}