The stress-strain behavior of granular soils is related largely to their grain-size distributions and density. If sand particles are crushed under load, some basic physical and mechanical properties of sands will be changed to a certain extent, and the traditionally defined critical state is not unique in this crushing process. To describe the complex effects of particle breakage, the volumetric and shearing responses of crushable sands to compression-shear loading are attributed to the rearranging and crushing state (RCS) of sand particles. The interrelation between rearrangement and breakage of sand particles was analyzed phenomenologically as the physical basis of this study. To characterize the RCS over a large stress range, a special curve named the RCS curve is defined in the e-ln p plane and quantified through a specific loading path. A new breakage model is proposed to correlate the crushing stress with the breakage index and to control the evolution of the RCS-curve. To account for the state-dependent dilatancy of sand particles, a new state parameter called the RCS parameter is introduced into the plastic potential function. An elastoplastic model for crushable sands was established based on the evolution of the RCS and verified by relevant triaxial test data of three representative crushable sands with an initial confining pressure ranging from 50 kPa to 68.9 MPa. The stress-strain behavior, excess pore-water pressure, and accumulated particle breakage of these crushable sands were simulated satisfactorily. In addition, procedures for calibrating the model parameters are suggested to make the established model more reliable.

This paper investigates the response of Ottawa sand to cyclic loading using virtual oedometer tests and the level-set discrete element method. We study both the macroscopic and the micromechanical behavior, shedding light on the grain-scale processes behind the cyclic response observed in crushable sand, namely stress relaxation under strain control and ratcheting under stress control. Tests without particle breakage first show that asymmetrical frictional sliding during loading-unloading induces these cyclic-loading effects. Then, tests considering particle breakage reveal more pronounced stress relaxation and ratcheting, which decrease in rate over cycles, accompanied by increased frictional sliding and reduced particle contact forces. It is found that the broken fragments unload the most and promote an enhanced cushioning effect. These micromechanical processes contribute to a decrease in breakage potential as the cycles progress, implying that cyclically loaded materials may become more resistant to breakage when compared to the same material loaded monotonically at the same strain level. These new insights highlight the main contributions of the present work, factoring in real particle shapes from 3D X-ray tomography and notably contributing to the existing literature on the topic, where most studies rely on idealized particle shapes and rarely consider crushable grains.