ABSTRACT: This paper presents the results of laboratory measurements on the mechanical and acoustic properties of weakly cemented granular rock. Artificial rock samples were fabricated by cementing sand and glass beads with sodium silicate binder. During uniaxial compression tests, the rock samples showed stress strain behavior which was more similar to that of soils than competent rocks, exhibiting large permanent deformations with frictional slip. The mechanical behavior of the samples approached that of competent rocks as the amount of binder was increased. For very weak samples, acoustic waves propagating in these rocks showed very low velocities of less than 1000 m/sec for compressional waves. A borehole made within this weakly cemented rock exhibited a unique mode of failure that is called "anti-KI mode fracture" in this paper. The effect of cementation, grain type, and boundary conditions on this mode of failure was also examined experimentally.
Rocks with weak intergranular cementation are often encountered during drilling and excavation in poorly consolidated, young, geological formations. Because the strength of such materials is low, they can be a potential threat to the stability of boreholes and other stress-bearing rock structures. They are, however, difficult to characterize because their mechanical behavior is intermediate between that of rock and soil.
One of the fundamental differences between granular rock (sandstone) and soil (sand) is the strength of intergranular cohesion. Due to the lack of cohesion, sand under differential stress exhibits significant permanent deformation due to intergranular slip and rotation. A number of theories have been developed to predict the nonlinear, hysteretic stress-strain behavior of sands from only the grain packing geometry and frictional properties between grains (e.g., Mehrabadi et al., 1992; Nemat-Nasser & Balendron, 1992). In contrast, competent sandstone (such as Berea) shows permanent deformation that is only a small fraction of total deformation during differential stress loading. Most deformation is attributed to the non-linear elastic deformation of grains and grain contacts that is essentially stress-path independent (Nihei et al., 2000). Intergranular cohesion allows the transmission of macroscopic tensile stress in the medium that can lead to the formation of tensile fractures. Weakly cemented granular rocks are expected to show intermediate behavior between sand and competent sandstone, exhibiting strongly hysteretic stress-strain behavior and the ability to sustain intergranular extensile fractures.
The objective of this study is to understand the effect of the micromechanical properties of weak granular rock such as intergranular cohesive strength and porosity on macroscopic properties including load-displacement response, ultimate strength, acoustic wave velocities, and failure made. To this end, we conducted laboratory experiments upon synthetic samples of weakly cemented sandstone with a range of micromechanical properties. The synthetic rock samples were fabricated of pure silica sand with smooth, elongated grain geometry. The grains were cemented by sodium silicate. A series of laboratory load-displacement tests and acoustic wave propagation tests were performed both on core samples and rectangular bricks containing a single simulated borehole. The primary micromechanical parameter used for a series of experiments was the strength of intergranular cohesion. Porosity of the samples was also varied but its range was limited to relatively high values (32-42%). For the brick samples containing a simulated borehole, samples were also made of glass beads (nearly perfectly spherical) to examine the effect of gr