The shear strength of rocks is one of the significant parameters for designing geo-structures such as rock slope, tunnels, and underground space. This parameter has been widely applied for simulating rock fracture problems, which adopt a frictional failure criterion. In order to apply a shear failure model for simulating a dynamic fracture problem, it is essential to obtain the dynamic shear strength with increasing confining pressure. This paper presents the dynamic tri-axial punch shear test for evaluating the dynamic shear strength of rocks under confining pressure. In this method, a modified Hoek tri-axial pressure cell, which is applicable to split Hopkinson pressure bar (SHPB) system, was utilized to acquire confining pressure dependent dynamic shear strength. The investigation of dynamic shear strength was carried out with increment of constant confining pressure and loading-rate. It was found that dynamic shear strength increases with increasing confining pressure increment and considerably increases with increasing loading-rate. The value of cohesion also increased with loading-rate increment, while the internal friction angle remained constant. Finally, based on the experimental results, loading-rate dependent frictional dynamic failure criterion of rock was suggested.

1 Introduction

The shear strength is an important material parameter for rocks. This parameter has been widely applied to simulate rock fracture problems, which apply a frictional failure criterion.

There are several suggested methods for quantifying the shear strength of rocks and concrete, such as direct shear-box test and block punch index (BPI) test under static-loading (Schrier van der 1988; Sulukcu & Ulusay 2001).

However, rock-material may be subjected to dynamic loadings in various rock engineerings such as rock drilling, blasting, and projectile penetration. Therefore, it is necessary to come up with a method for determining the dynamic shear strength of the rock.

Thus, several methods have been developed to perform the dynamic shear tests for geo-material, which is subjected to dynamic loadings. A dynamic punch test method utilizing a split Hopkinson pressure bar (SHPB) system was suggested to quantify the dynamic shear strength of rocks (Huang et al. 2011). However, this method was incomplete as the dynamic shear strength is obtained with the condition of the confining pressure.

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