The resilient modulus is a measure of the ability of a soil or granular base to resist
permanent deformation under repeated loading. Many soils are stress-dependent. As
the stress level increases, these soils will behave in a nonlinear fashion. Fine-grain
soils tend to be stress-softening, whereas granular soils tend to be stress-hardening.
Laboratory procedures for determining resilient modulus have been published by
AASHTO as test method T307, or NCHRP as test method 1-38A. A typical setup for
the laboratory test is shown in Fig. 3.9. The stress due to the repeated load applied
through the load actuator is the deviator stress and is intended to duplicate the effect
of loads passing over a section of pavement. The confining stress within the chamber
is intended to duplicate the confinement of the soil within the subgrade. A typical
load-response curve is shown in Fig. 3.10. As shown, the resilient modulus (MR) is the
ratio of deviator stress to strain in the elastic range.