This study examines how the petrologic differences between a suite of sandstones and embedded calcareous concretions from Vancouver Island, British Columbia create different fracturing patterns observed in the field. Microscopic examinations, porosity measurements, static moduli calculations, and strength tests on rocks from the study area reveal a marked contrast in material properties between the concretions and sandstone matrix. In thin section, the concretions differ in composition and structure from the surrounding sandstone in that calcite has filled the pore spaces, sometimes even separating pre-existing mineral grains. Stress-strain tests for elastic moduli demonstrate that calcite cement increases the Youngs value. Large variations in uniaxial strength exist both between concretions and sandstones, and between different segments of the study area. Microstructural heterogeneity is used to explain the field observation that concretions contain closely-spaced, non-interacting fractures that are in the same orientation as one or more joint sets. An analytical solution for a spherical inclusion in an infinite medium demonstrates that the stress concentration within the concretions due to differences in elastic properties is 1.15 to 1.33 times the sandstone stress. This stress concentration is not always great enough to cause preferential fracturing of the concretions in either tension or compression based on laboratory strength tests; however, it is shown that compression-driven fracturing is more likely responsible for the internal fractures. Tensile strengths obtained from brazilian tests are input into a finite difference code and demonstrate that under the applied stresses necessary for tensile fracture in the concretions, a large enough driving stress reduction is created near the fracture to prevent more than one crack from forming in the concretion. Compression-driven extensile fracturing can explain the debonding fractures that occur along the contact between the concretion and sandstone as apparent indentation fracturing (characterized by pore compaction and peripheral extensile cracking). A solution to the Barenblatt (1962) expression for the stress field around a crack with a constant opening displacement is used to explain the compression-driven mode I fracturing mechanism, and a numerical simulation demonstrates that the compressive stress shadow decreases with fracture length.
Geologists have successfully argued for the formation of joint sets by tensile fracturing mechanisms. What is less well understood are the conditions that can induce compression-driven extensile fracturing. This process is inhibited geologically for two reasons: first of all, laboratory compressive strengths are an order of magnitude larger than tensile strengths; secondly, a negative stress intensity factor imposed by far-field compressive stresses acting normal to the crack can arrest compression-driven joints that grow without the aid of a tensile component. Researchers address these concerns by suggesting that saturated rock under geologically higher temperature conditions and lower strain rates may be significantly weaker than laboratory-determined values (Lorenz, 1993). Also, extensile fracturing models demonstrate that compressive loads can induce local tensile stresses and propagate fractures in rock (Kemeny and Cook, 1987). Geologists have successfully argued for the A field site on Vancouver Island, British formation of joint sets by tensile fracturing Columbia, offers evidence for geologic compression- mechanisms.
