Investigating Stress Dependency of Unbound Layers Using Falling-Weight Deflectometer and Resilient Modulus Tests

摘要

Unbound layer materials, such as base and subgrade, exhibit stress-hardening and/or softening, which can be determined from their resilient modulus values. However, unavailability of required amount of material and compaction variability between field and laboratory necessitate a need to predict stress dependency of the unbound layers by field tests. This study evaluates stress dependency of unbound layers using a falling-weight deflectometer (FWD) test. The FWD tests were conducted at multi-load levels on several pavement sections. The FWD back-calculated layer moduli are then used to determine bulk and octahedral shear stresses using the layered elastic analysis technique. Next, the regression analysis is performed to interpret the stress-dependent parameters, i.e., regression coefficients of pavement ME's resilient modulus model of nonlinear unbound material. It is observed that stress-hardening is dominant in the case of the base layer, whereas subgrade exhibits both stress-hardening and-softening. This was also confirmed by laboratory resilient modulus tests on both base and subgrade materials in this study. Therefore, it is recommended not to ignore either stress-hardening or-softening during nonlinear modeling of the unbound layers. In addition, in this study, empirical relationships are developed to perform inter-conversion of laboratory resilient and FWD back-calculated modulus incorporating the field state of stresses in base and subgrade.