Influence of Loading Rate on Rock Response

Static and dynamic laboratory tests were conducted on three igneous and two sedimentary rock types to determine the response of the rocks to various loading rates. The dynamic loading was accomplished using a ram loader, a shock tube, and an air gun from which projectiles made from the same rock were fired into the test specimens. The laboratory test results were compared with available field data to determine which of the laboratory test methods best described the in situ response. Thin sections of the rock specimens were examined under a petrographic microscope, before and after loading, to determine the amount of microfracturing induced by the shock wave. The rock responses were influenced by several factors which included type of rock, grain bond strength, amount of weathering, porosity, and loading rate. Laboratory data were related to field data using corresponding particle velocities to obtain a peak-strain comparison. A relationship between laboratory-determined modulus and strain rate was also obtained and the validity was spot checked by in situ field testing.