K SUB R: THE RESILIENT MODULUS OF SUBGRADE REACTION

The concept of the resilient modulus of subgrade reaction (K sub R) is developed to account for the stress-dependent behavior of typical fine-grained subgrade soils. This new subgrade support parameter is defined as plate pressure or resilient deflection in an impulse plate load test simulated by using the finite-element program ILLI-PAVE. K sub R is expressed in the same units as the standard static modulus of subgrade reaction (k), but the value of the former is significantly higher. This indicates increased stiffness in response to rapidly moving loads. Factors influencing K sub R include plate size, deflection level, and subgrade type. A 30-in.-diameter plate was chosen in this study in conformity with general practice. Equations relating K sub R and deflection level are developed for four broad cohesive subgrade soil types: very soft, soft, medium, and stiff. The effect of a granular subbase on K sub R is also examined. The introduction of a granular layer increases K sub R substantially. The importance of this effect, however, diminishes rapidly as subbase thickness exceeds 8 in. The beneficial effect of a granular subbase is consistent over the range of plate pressures investigated.