Influence of residual elastic strain on the orientation of experimental fractures in three quartzose sandstones

Abstract

This study is to evaluate the influence of residual elastic strain on the orientations of tensile and shear fractures in rock. As a first step, the three-dimensional state of residual elastic strain was measured by X-ray diffractometry. Principal strains (×10−6, expansions positive) are 0, 50, and 90 in the Tennessee sandstone, −250, −30, and 100 in the Uinta Mountain quartzite, and −130, −17, and 140 in the Tensleep sandstone. These values are converted to stresses by multiplying each by 0.89×106 bars, the elastic compliance normal to in quartz. A prediction was then made concerning the orientations of fractures that would be induced in each rock by loading normal to bedding. The prediction was based on a knowledge of the prestrain, the applied loads, the maximum tensile stress, and Coulomb Mohr criteria. Each prediction was then tested by experimentally fracturing the rocks. Tensile fractures, induced in disks of Tennessee sandstone and Uinta Mountain quartzite by point-loading normal to bedding, exhibit a marked preferred orientation. Also strongly oriented are shear fractures induced in triaxial compression tests (ė of 10−4/sec, 25°C) on cylinders of Tennessee sandstone (to 2000 bars confining pressure), Uinta Mountain quartzite (1500 bars), and Tensleep sandstone (1000 bars). In each rock the tensile fractures strike parallel to the shear fractures, and both follow the predicted trend. To identify other possible causes for the fracture anisotropy, (1) apparent grain elongation, especially the trend of long axes within the bedding plane, (2) orientation of grain boundaries, (3) crystallographic orientation of quartz grains, and (4) orientation of quartz microfractures and deformation lamellae were investigated. All aspects of the fabric are randomly oriented except the trend of apparent long axes. The induced fractures cut across the trend of the long axes in two of the three rocks. It is concluded that the state of prestrain is controlling the oriontations of the fractures. The anisotropy manifest by the prestrain persists to at least moderate confining pressures.