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), Wang et al.
Due to difficulties associated with sample gripping in direct tension, indirect methods are commonly used to quantify the tensile strength of rocks. In this work, an indirect tensile test method - semi-circular bend (SCB) is used to investigate the tensile (flexural) strength of Laurentian granite (LG). The static tests are conducted with a servo-controlled material testing machine and the dynamic experiments are carried out using a split Hopkinson pressure bar (SHPB) system. For dynamic tests, pulse shaping technique is adopted to achieve dynamic force balance on both ends of the sample. The dynamic force balance eliminates loading inertial effect in the sample and thus enables quasi-static stress analysis. Momentum-trap technique is also employed in SHPB to ensure single-pulse loading. Finite element method is used to relate the failure load to the strength of the sample. Rate dependence of the strength is observed. The value of the flexural strength is higher than the tensile strength measured using Brazilian disc method. We rationalize this difference using the non-local failure theory. Furthermore, a coupled Discrete element - Finite element method (Fem/Dem) in conjunction with the smeared crack model is utilized to simulate the fracture process of a dynamic SCB test. The fracture pattern obtained from the simulation agrees with that of the recovered sample.
Rocks are considerably weaker in tension than in compression. Understanding of tensile strength of rocks thus bears important engineering and geophysical applications. Due to the difficulties of experimentation in direct tensile test, various of indirect methods have been proposed and developed as convenient alternatives to measure the tensile strength of rocks; some examples are Brazilian disc test (Bieniawski and Hawkes, 1978; Coviello et al., 2005; Hudson et al., 1972; Mellor and Hawkes, 1971), ring test (Coviello et al., 2005; Hudson et al., 1972; Mellor and Hawkes, 1971), and bending test (Coviello et al., 2005). These methods aim at generating tensile stress in the sample by far-field compression, which is much easier in instrumentation than direct tensile tests.
Existing attempts to measure rock tensile strength are mostly limited to quasi-static loading, primarily due to the difficulties in the dynamic experimentation and subsequent data interpretation. However, in many mining and civil engineering applications, such as quarrying, rock cutting, tunneling, rock blasts, and rock bursts, rocks are stressed dynamically. Accurate characterizations of tensile strength over a relative wide range of loading rates are crucial. Direct dynamic tensile testing is rare (Goldsmith et al., 1976), and research efforts have rather concentrated on extending the indirect methods from quasi-static to dynamic loading. Zhao and Li (2000) measured the dynamic tensile properties of granite with the Brazilian disk and three point bending techniques, and the loading was driven by air and oil. Most researchers used the standard dynamic testing facility, split Hopkinson pressure bar (SHPB), to characterize dynamic tensile strength. For examples, conventional SHPB tests were conducted on Brazilian disk and flattened Brazilian disk specimens of marble (Wang et al., 2006) and on Brazilian disk specimens of argillite (Cai et al., 2007).