Numerical Investigation of the Influence of Bedding Plane Thickness and Friction on Cracking Pattern and Mechanical Behavior of Shale Under Unconfined Loading Condition Using the Finite-Discrete Element Method (FDEM)

Abstract

ABSTRACT: The finely bedded structure of shale makes its mechanical behavior highly anisotropic both in terms of deformation and strength. The variations in the bedding plane thickness originates from different sedimentation processes occurred during the formation of the shale. It is well known that these features act as weakness planes in the rock matrix and thus studying the mechanical behavior of shale should incorporate the influence of bedding planes. Numerous investigations have been conducted so far to understand the influence of bedding plane orientation on the deformation and strength of shale. However, there exist only a few investigations which studied the influence that bedding plane thickness and friction might have on the mechanical response and cracking pattern of the rock. The present study discusses the results of a series of unconfined compressive test simulations conducted on shale samples with different bedding plane thickness and friction values using the combined Finite-Discrete Element Method (FDEM). The bedding planes have been discretely modeled as a distribution of preferentially oriented defects inside the rock matrix. Experimental results from the shaly facies of the Opalinus Clay at the Mont Terri Underground Research Laboratory (URL) have been used as input parameters to calibrate the numerical model. Simulation results show a difference in cracking pattern and strength of the rock owing to different bedding plane thicknesses but almost no effect due to different bedding plane frictions. 1. INTRODUCTION Shale formations play an integral role in many civil, petroleum, and environmental engineering projects. Shales not only act as an unconventional reservoir for extracting hydrocarbon resources, but also as a potential host rock for underground geological storage of nuclear wastes and as caprock for Carbon Capture and Sequestration (CCS) operations. Shale is formed through the sedimentation and subsequent hardening of fine sediments and thus it features a finely laminated and bedded structure. Platy particles tend to deposit in distinct orientations during the sedimentation of the shale causing an anisotropic texture at the macroscopic scale (Wenk et al., 2008). The layered structure of the shale makes its deformation and strength characteristics highly anisotropic at different scales, for example, at a larger scale it is attributed to the presence of bedding planes (Saroglou and Tsiambaos, 2008). The temporal variations in the sediment supply and flow velocity causes the thickness and properties of the bedding planes to differ along the deposition axis (O’Brien, 1996). These bedding planes act as weakness planes inside the rock matrix and thus understanding the shale mechanical behavior should incorporate the influence of bedding planes (Woo et al., 2021). In addition, the existence of physical discontinuities in the rock such as natural fractures or tectonic features contributes to the anisotropy at the rock mass scale.