Experimental study on the competing effects of strain rate and water weakening on compressive strength of saturated rocks

Measuring the rock compressive strength under combined effects of strain rate and water weakening is of great importance and at the same time a major issue for engineers. In this study, we explore the ability to detangle the contributions of strain rate and water-weakening effects on rock compressive strength within a wide static and dynamic strain rate range. To this end, we conduct a series of compression tests and acoustic emission (AE) tests on two types of rock with specific mineral compositions (i.e., limestone and granite). The correlations between rock compressive strength, including static compressive strength acs and dynamic compressive strength acd, and water-weakening factors fw with strain rate are derived. The experimental AE waveform data is analyzed using a new methodology named statistical analysis of dominant frequency, from which we infer the intrinsic rock progressive failure and evaluate the competing effects of water weakening and strain rate. Our results show that water saturation causes a drastic loss in the compressive strength of limestone and a small reduction in the compressive strength of granite. The strain rate effect governs the changes in the compressive strength of dry rocks, characterizing a reduction in low dominant frequencies of AE waveforms with increasing strain rates. Whether the strain rate effect or water-weakening effect plays a dominant role depends largely on the rock types and loading regimes. We proposed that the primary effects on acs of saturated limestone are strain rate under relatively low static strain rate (lower than 8.33 x 10-5 s- 1) and pore water pressure under relatively high static strain rate (beyond 8.33 x 10-5 s- 1), respectively, and that on sigma cd of saturated limestone is strain rate. The variations of acs and acd of saturated granite are both determined by the strain rate effect within the strain rate range investigated. Additionally, the exertions of various water-weakening effects of limestone and granite at different strain rates, which are responsible for their fw variations, are evaluated in detail.