Evaluating the resilient behaviour of crushable subgrade soil

Resilient behaviour forms the basis of mechanistic pavement design frameworks, where the resilient modulus M r is widely used to characterise the elastic response of unbound pavement layers under repeated traffic loading. The resilient modulus is strongly influenced by changes in matric suction associated with seasonal moisture fluctuations, and numerous models have been proposed to incorporate suction effects into the stiffness prediction. Most existing resilient modulus models, however, assume that the soil skeleton remains mechanically stable throughout cyclic loading, with material response governed primarily by stress state, moisture condition, and load repetitions. However, with increasing demand for sustainable pavement construction using marginal geomaterials such as volcanic soils, the influence of particle breakage can no longer be neglected. In crushable granular subgrade soils, progressive particle breakage alters gradation, reduces interparticle friction, and weakens the load-carrying fabric, leading to stiffness evolution that conventional formulations do not adequately capture. This study investigates the resilient behaviour of crushable and non-crushable subgrade soils under multi-staged and single-staged cyclic triaxial loading, evaluates the applicability of existing resilient modulus prediction models, and proposes rational modifications incorporating particle breakage effects. The results demonstrate that progressive particle breakage alters stiffness evolution in crushable volcanic subgrade soil, leading to systematic bias in conventional suction-dependent formulations. Incorporating breakage-sensitive parameters improves prediction reliability and provides a more mechanistically consistent framework for the pavement design using marginal geomaterials.