Stress-rate dependency of uniaxial compressive strength of hard rock with regard to test procedure standards

Abstract

The uniaxial compressive strength test of hard rock is one of the most worldwide applied tests for characterization of hard rock in rock engineering and engineering geology. The uniaxial compressive strength as the results of this test is a basic parameter, used, for example, for the design of rock engineering structures. In the commonly applied standards, stress-controlled test procedures using constant stress rates are recommended by a wide range of stress rates varying between 2 and 60 MPa/min and/or a specific minimum test time. Though the effect of stress-rate dependency of hard rock is generally known, most investigation is focused on dynamic action behavior using high stress rates and/or fast actions. Strain-controlled test procedures are often used as well. For stress-controlled test procedures within the recommended range of stress-rates, the data base is rather poor, the effects to the results are reported by only very few researchers. The research described in this paper aims to close this gap and focuses on the stress-rate dependency of uniaxial compressive strength of hard rock. Seven different stress rates varying between 1 and 100 MPa/min on five different types of hard rock (quartzite, granodiorite, gabbro, sandstone, basalt) using five to fifteen single tests per stress rate have been executed. A significant increase of uniaxial compressive strength by increasing stress rates has been stated; the increase may not be ignored in assessing rock strength in rock engineering projects. The effect has to be considered especially when different parties are involved in the site investigation programs.