Junru Li

Experimental Study on Wave Propagation Across a Rock Joint with Rough Surface

Joints are an important mechanical feature of rock masses. Their effect on wave propagation is significant in characterizing dynamic behaviors of discontinuous rock masses. An experimental study on wave propagation across artificial rock joint was carried out to reveal the relation between the transmission coefficient and the contact situation of the joint surface. The modified split Hopkinson pressure bar apparatus was used in this study while all the bars and specimens were norite cored from the same site. One surface of the specimens with a number of notches was adopted to simulate the artificial rough joint. Two strain gauges were mounted on each pressure bar at a specific spacing. The incident, reflected and transmitted waves across the joints were obtained using a wave separation method. Comparisons of the transmission coefficients were made under two different conditions: with the same joint thickness but different contact area ratios, and with the same contact area ratio but different joint thicknesses. The results show the effects of contact area ratio and thickness of joints on wave transmission.

Direct Tension Test for Rock Material Under Different Strain Rates at Quasi-Static Loads

Mechanical properties of rock material under dynamic tension are important in evaluating the damage characteristics of rock structures such as rock caverns, rock slopes as well as rock foundations following explosion or earthquake. In this arena, since the 1960s, many experimental works have been conducted. Among these works, indirect dynamic tensile test methods, such as the Brazil and the split Hopkinson pressure bar (SHPB) test methods, were mainly employed to study the strength and deformation properties of rock material under dynamic tension. For example, through dynamic Brazil tests for dolerite and limestone, Price and Knill (1966) suggested that the tensile strength of both rocks generally increases with the increase of the loading rate. For dolerite, the tensile strength at the highest loading rate is 17 % greater than that at the lowest loading rate. For limestone, the tensile strength at the highest loading rate is 44 % greater than that at the lowest loading rate. Wu and Liu (1996) conducted Brazil tests to study the variation of the tensile strength, the Young’s modulus and the failure strain for Longman limestone at loading rates from 10 to 10 MPa/s. The tensile strength, the Young’s modulus and the failure strain for the rock were reported to increase with the increase of the loading rate. The three-point bending and Brazil test methods were employed to study the dynamic tensile properties for Bukit Timah granite by Zhao and Li (2000). They found that the tensile strength of granite obtained by the two methods is conformably increased with increasing loading rate. When the loading rate rises by one order from 10 to 10 MPa/s, the increment of the tensile strength is about 10 %. In addition, the tensile strength of the rock by the three-point bending method was tested to be 2.5 times that obtained from the Brazil test method at the same loading rate. Based on SHPB tests for quartz monzonite rock, Birkimer (1971) pointed out that the dynamic tensile strength of the rock increases with a cube root of the strain rate when the strain rate ranges from 10 to 10 s. By SHPB tests, Cho et al. (2003) studied the variation of the tensile strength for Inada granite and Tage tuff at strain rates ranging from 1 to 10 s. It was indicated that the strengths of both rocks at the strain rate range are significantly higher than that under static load and increase notably with increasing strain rate. In addition, the fracture processes, the generation and interaction of microcracks, which contribute to the rate dependency properties of rock tensile strengths, were analysed by a proposed finite element method. By the split Hopkinson tension bar and a hydro-pneumatic machine, Cadoni (2010) studied the dynamic tensile strength of Onsernone Orthogneiss for loading directions 0 , 45 and 90 with respect to the schistosity at three different strain rates, i.e. 0.1, 10 and 100 s. It was observed that the dynamic tensile strength of the rock increases with increasing strain rate, and the dynamic tensile strength of the rock is up to about two H. Li J. Li (&) B. Liu J. Li X. Xia State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China e-mail: jcli@whrsm.ac.cn

Analytical Study of Underground Explosion-Induced Ground Motion

A theoretical approach is presented to study the ground motion induced by an underground tunnel explosion. The ground motion is caused by two coupled stress waves, i.e., the reflected body wave and the secondary surface wave or Raleigh wave. Based on the principle of conservation of momentum at the wavefronts, the reflected body waves along the ground surface are derived. The interaction of the body wavefront and the ground surface induces the secondary surface wave which transfers outwards on the ground. The particle velocity and particle acceleration on the ground surface are subsequently derived. The analytical results are compared with results from numerical simulations and empirical formulae with different material damping ratios. The effects of the loading density and the material damping on the ground motion are investigated. Finally, the limitations of the proposed theoretical approach for ground motion prediction are discussed.