Volume changes of a granite, a marble, and an aplite were measured during deformation in triaxial compression at confining pressure of as much as 8 kb. Stress‐volumetric strain behavior is qualitatively the same for these rocks and a wide variety of other rocks and concrete studied elsewhere. Volume changes are purely elastic at low stress. As the maximum stress becomes one‐third to two‐thirds the fracture stress at a given pressure, the rocks become dilatant; that is, volume increases relative to elastic changes. The magnitude of the dilatancy, with a few exceptions, ranges from 0.2 to 2.0 times the elastic volume changes that would have occurred were the rock simply elastic. The magnitude of the dilatancy is not markedly affected by pressure, for the range of conditions studied here.
For granite, the stress at which dilatancy was first detected was strongly time dependent; the higher the loading rate the higher the stress. Dilatancy, which represents an increase in porosity, was traced in the granite to open cracks which form parallel with the direction of maximum compression.
This report describes a program which was directed toward developing a capability suitable for generating engineering data on the high pressure mechanical properties of geologic materials. To provide the greatest possible contribution, this effort concentrated on three of the most important deficiencies in high pressure experimental technology: (1) development of a system capable of accepting relatively large samples so that more-nearly representative data may be obtained than with the smaller samples used previously, (2) development of a system suitable for monitoring the strains of highly deformable geologic materials, and (3) development of an encapsulating technique for soils. Each of these objectives was attained to a certain degree. Specimens up to three inches in diameter were subjected to fluid pressures of several kilobars. The deformation of soil was measured with a potentiometric slidewire device. An encapsulating technique was developed for soils which was suitable up to about two kilobars. Further refinements are necessary, but it appears that the more important of the limitations of past studies have been overcome.
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