A three-phase medium model was proposed for describing wave propagation across filled rock joints in the paper. Parameters in the three-phase medium model were identified by a series of modified split Hopkinson pressure bar (SHPB) tests, where a sand or clay layer was used to represent an artificial filled rock joint. Two granitic pressure bars with the sandwiched sand or clay layer were used to represent the filled joint to simulate longitudinal stress wave propagation across such geological discontinuities. With the parameters fitted from a number of SHPB tests, the closure-pressure relation based on the three-phase medium model were compared with other test results and very good agreement was observed. Then, the three-phase filled joint model is adopted to carry out analysis of the longitudinal wave propagation through a single filled rock joint. The wave transmission coefficients were derived and compared with the test results. Finally, parametric studies with respect to the properties of filled joints and the incident wave on wave propagation through a single filled joint were carried out.
The mechanical behavior of rock mass is significantly affected by the vastly existing discontinuities, primarily joints. One of the main tasks in the fields of rock mechanics and engineering is to well understand the mechanical properties of the discontinuities and their effects on rock mass behavior, so as to ensure the stability of the rock mass and underground structures under dynamic load, which is of great interest to mining engineers, seismologists and geoscientists. In nature, besides unfilled fractures, there are also some open-mode fractures (joints) with filling materials, such as sand, clay, and other geomaterials. The static or quasi-static physical properties of some filling materials have been experimentally investigated and it has been found that they affect the stiffness and strength of the filled rock joints (Singh and Goel, 1999; Sinha and Singh, 2000). Among different filling materials, sand and clay are the most common geologic filling materials and are considered as sift or loose materials. A commonly accepted joint model in rock mechanics and engineering (Cook, 1992) is the Bandis-Barton (B-B) joint model (Bandis et al., 1983), which was originally developed from quasi-static deformation tests for natural unfilled rock joints. There are very limited studies on the mechanical properties of filled rock joints, especially under a dynamic loading condition. The filled rock joints can be considered as a complex three-phase medium consisting of rock solid particles, water and air. As a mixture, the three phases deform under different laws. At lower strain rates, the water and air are assumed to flow through the skeleton driven by the pore pressure. In contrast, at higher strain rates and pressures, such as under shock and blasting loads, water and air are trapped within the pores and the deformation of the matrix is controlled by the deformation and the volume fraction of each of the three constituent phases. Based on the multiphase mass theory of Henrych (1979).