Dynamic shear strength of rock joints subjected to impulse loading

The shear testing of rock joints under impulse loading is studied in the present paper. Following the description of never shear equipment which allows the shear tests to be carried out under both static and dynamic loading conditions, the results of rests on saw-cut surfaces are given, with attention being paid to the mechanical behaviour of rock discontinuities prior to peak strength in a stage where no irreversible shearing has yet occurred. The objective is to describe the initial dynamic strength of a rock discontinuity versus the shearing stress rate applied during the test, It is shown that: (i) under low normal stress and no initially mobilized strength, the dynamic shear strength is greater than the corresponding static value and is greater in relation to the higher shearing stress rate applied (ii) the dynamic to static shear strength ratio has a tendency to decrease as the mobilized shear strength prior to loading increases (iii) with the normal stress increasing, the dynamic influence on the shear strength reduces.