Compression tests were performed at room temperature on solid circular cylinders of medium‐grained granodiorite from the Hardhat Event conducted at the Nevada Test Site. Confining pressures ranged from 1 to 7000 bars. A piston‐cylinder apparatus equipped with an internal axial load cell and manganin coil was used. The confining fluid was a low‐viscosity oil. Pre‐detonation specimens were cut 0°, 45°, and 90° to the drill core axis and were taken from 195‐, 225‐, and 255‐meter depths. Post‐detonation rock was sampled at 23 and 27.5 meters from ground zero, which was located at a depth of 290 meters and experienced a calculated peak shock pressure of approximately 20 kb. Specimens mechanically shocked in the laboratory (35 to 40 kb) were from the 195‐meter depth pre‐detonation sample. No orientational effects were observed. The failure of granodiorite under compressional stresses was found to be adequately described by a modified Coulomb maximum shear stress theory. Under uniaxial compression, the mode of failure was brittle shear. Under triaxial compression, a quiet slip failure was observed at low confining pressures and brittle shear at high confining pressures. A possible explanation for the occurrence of slip failure may be the presence of a fluid pore pressure within the specimen. Strain measurements were made on several specimens. At atmospheric pressure, the stress histories of the rock were clearly apparent. Under confining pressure, shock effects rapidly disappeared, and, at 4.5 kb, the behavior was essentially that of normal granodiorite. Young's modulus at atmospheric confining pressure for unshocked rock was found to be 0.64 ± 0.26 × 106 bars, which in the upper limit agrees with the reported dynamic value. The average value for rock shocked by nuclear blast is 0.18 × 106 bars and that for mechanically shocked rock is of the order 0.01 × 106 bars.

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