Particle Crushing of a Filled Fracture During Compression and Its Effect on Stress Wave Propagation

Filled fractures commonly exist in the earth medium and occur at all scales, such as filled joints and gouge-filled faults. The compression behavior of a filled fracture is important for understanding its seismic response, although this has rarely been studied. In this study, laboratory tests were conducted to investigate the compression behavior of a simulated filled fracture under different stress states. It was found that the simulated fracture was compacted and experienced strain-hardening deformation under low compressive stress. Its compression behavior was described by the Bandis-Barton (B-B) model. When the compressive stress was high, numerous particles in the fillings were crushed and the filled fracture weakened, while its stiffness abruptly decreased. This process was related to the strain-softening deformation. As the compressive stress increased further, fillings were compacted again and some particles were crushed. The unloading deformation of a filled fracture had an obvious hysteresis characteristic. A modified B-B model was proposed to characterize the deformation behavior of the filled fracture under high stress states. It was proven that the modified B-B model can characterize the effect of the particle crushing on the deformation behavior of the filled fracture and also has the capability to capture the seismic response of the filled fracture under high-amplitude stress waves. The results of this study indicate a new and attractive form of dynamic weakening of the granular gouge, which resulted from particle crushing. They can be used to understand the dynamic weakening mechanisms of a gouge-filled fault under high-amplitude stress waves. Plain Language Summary Fractures commonly exist in the earth medium, which are discontinuous geological interfaces and occur at different scales such as the joint and the fault. A natural fracture is often filled with granular materials, which is the weakest planes in a rock mass, and is the location where sliding, opening, and compression are initialized. The filled fracture such as gouge-filled fault often weakens by dynamic stressing like seismic waves, leading to the decrease of the stiffness and frictional strength. In this paper, laboratory tests were conducted to investigate the compression behavior of a simulated filled fracture under different stress states. It was found that particles in granular fillings were crushed under high compressive stress, causing the abrupt decrease in the stiffness of the filled fracture. After the experimental investigation, the constitutive model was proposed to characterize the compression behavior of the filled fracture under different stress states. The results of this study indicate a new and attractive form of dynamic weakening of the gouge-filled fault, which resulted from particle crushing in fillings. They can be used to understand the dynamic weakening mechanisms of a gouge-filled fault under high-amplitude stress waves.