Dynamic fracture failure of rocks subjected to static hydrostatic confining pressure is commonly encountered in deep underground rock engineering. The static fracture behavior of rocks under hydrostatic stress has been well studied in the literature. However, it is desirable to investigate the dynamic fracture failure of rocks under various pre-loads and hydrostatic confining pressures. In this study, a split Hopkinson pressure bar (SHPB) system is modified to measure the dynamic fracture toughness of rocks under three pre-loads and five hydrostatic confining pressures. Pulse shaping technique was used in all dynamic tests to facilitate dynamic force equilibrium in the specimen. Three groups of NSCB specimen are tested under a pre-load of 0, 37 and 74% of the maximum static load and five groups of NSCB specimens are tested under a hydrostatic confining pressure of 0 MPa, 5 MPa, 10 MPa, 15 MPa and 20 MPa. The results show that under a given pre-load or a certain hydrostatic confining pressure, the dynamic fracture toughness of rock increases with the loading rate, resembling the typical rate dependence of materials. Furthermore, the dynamic rock fracture toughness decreases with the static pre-load at a given loading rate, implying that the dynamic load-bearing capacity of the engineering structure is reduced under static pre-load. However, the dynamic fracture toughness increases with the hydrostatic confining pressure at the similar loading rate due to the closure of microcracks in rocks, indicating that dynamic loading capacity of rocks is significantly enhanced under static hydrostatic confining pressure. Empirical equations are proposed to represent the effect of loading rate and pre-load force/hydrostatic confining pressure on the rock dynamic fracture toughness, and the results show that these equations can depict the trend of the experimental data well. The experimental results are of great significance to underground engineering design and assessment.

1 Introduction

With the further development of the mining industry and the increasing depth of underground engineering, rocks are naturally subjected to dynamic impact (such as blasting and seismicity) under static pre-load or static hydrostatic confining pressure. For example, in underground mining processes, tectonic stress and gravity are typical static pre-loads and hydrostatic confining pressures which act on underground rock engineering structures. In addition, these structures may be exposed to dynamic loads due to production blasting nearby or rock bursts or earthquakes. Under these circumstances, the dynamic response of rock subjected to static pre-load or hydrostatic confining pressure is crucial to the safety of workers, equipment and engineering facilities (Xia and Yao 2015).

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