Volume Changes in Models of Jointed Rock

by Edwin T. Brown,

Serial Information: Journal of the Geotechnical Engineering Division, 1976, Vol. 102, Issue 3, Pg. 273-276

Document Type: Journal Paper


A number of authors have pointed to the significance of volume increase or dilation in failing rock masses. Although several experimental studies of the compressive and shear strength and failure characteristics of models of jointed rock have been reported in this Journal and elsewhere, only in the case of the biaxial compression or plane stress tests of John and Rosenbland have lateral strains been recorded. Because of the lack of existing experimental data, it seemed that a useful contribution could be made by extending previous work to include measurements of volume change during deformation. Accordingly, a preliminary series of triaxial compression test was carried out on idealized models of jointed rock prepared from gypsum plaster using previously described techniques. Four specimen types were tested. All specimens were rectangular prisms 8 in. high by 4 in. square. Tests were carried out in triplicate on each specimen type at confining pressures of 50 psi and 250 psi. Axial loads were applied using a closed-loop electro-hydraulic testing system. This permitted control of fracture propagation and investigation of the progressive post-peak break-down of specimens. The method used for measuring volume changes in test specimens was developed by Crouch, and depends on the fact that the volume of a fluid-filled pressure vessel must be adjusted to compensate for lateral expansion of the specimen if the fluid is to be kept at constant pressure. In the present case, this pressure was manually controlled by a pressure intensifier with a threaded plunger 0.50 in. in diameter. The displacement of the plunger was transferred to a 40-turn potentiometer by a belt drive, and the output was recorded. Because the cross-sectional area of the ram and specimen differed, an axial strain-dependent correction, determined in calibration tests using an aluminum block, had to be applied to the recorded volume change to obtain the volume change in the specimen.

Subject Headings: Compression tests | Volume change | Pressure vessels | Structural models | Rocks | Rock masses | Case studies | Compressive strength

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