Triaxial compression data on nuclear explosion shocked, mechanically shocked, and normal granodiorite from the Nevada Test Site

Abstract

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.

A review of rock behavior to shear over approximately 100 KB

“…The stored elastic strain energy at failure is defined as the work energy of disruption. When the relationship dW = VS de is used, where W is work energy, e is elastic strain, V is sample volume, and S is shear stress at failure ke, where k is the elastic constant at failure, and integration is performed, The initial region of low shear strength is similar to that found earlier in triaxial tests where a true pore fluid pressure did exist [Giardini et al, 1968]. (In the published triaxial test work the presence of a pore pressure was suspected but not experimentally proven.…”

“…In both cases confining pressure was continuously increased at a rate linear with the applied torque. The data given in Figure 2 The initial rise in torsional shear strength over the first 15 kbar of pressure corresponds to the region of brittle shear failure established earlier in triaxial tests [Giardini et al, 1968]. The rate of torsional strength increase is less than that obtained from triaxial tests, but this difference is attributed to experimental difficulties inherent in the torsional method used [Giardini and Abey, 1972].…”

“…Dual disk-shaped bulk samples, each 1.27 cm in diameter and 0.25 cm high, were used. The granodiorite was of the same origin as that used earlier by Giardini et al [1968] in triaxial tests to about 6 kbar and by Riecker and Rooney [1966a] in torsional tests to about 60 kbar.…”

Mechanical properties of Nugget sandstone

“…The ex perimental techniques used have been [4][5][6][7][8] described in detail elsewhere. All stresses are referred to a Cartesian coordinate system with a-, or, and ^3 the maximum, intermediate, and minimum principal stresses, respectively.…”

Understanding and constructively using the effects of underground nuclear explosions

“…The compressibility measurements presented in Figures 4 and 5 were made at static conditions in an assembly in which the sample is subjected to approximately hydrostatic (uniform sSress distribution) pressures up to 40 kb [Stephens, 1964;Stephens and Lilley, 1966]. Triaxial tests (nonuniform stress distribution), performed with a different tess assembly, give information on the compressire strength and the stresses at which compressire failure or crushing occur [Giardini et al, 1968]. The pressures at which rocks vaporize are on •he order ooe megabars; thereoeore, low-stress, static tests muss be supplemented with labora5ory dynamic tests [Lombard, 1961; van Thiel, i966] tha• approach but do not attain such high pressures.…”