Evolution of rock brittleness under mechanical and thermal effects

• Main Author: Wang, Lu
• Other Authors: Wu Wei (CEE)
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/164904
• Institution: Nanyang Technological University

Understanding rock brittleness plays an essential role in controlling the drilling and cutting efficiency in rock engineering for construction and energy applications. Numerous brittleness indices have been proposed to quantify rock brittleness under static and dynamic loads. However, rock brittleness under complex mechanical and thermal conditions has yet to be explicitly evaluated. In this thesis, a series of split Hopkinson pressure bar (SHPB) and Cerchar abrasivity index (CAI) tests are conducted experimentally and numerically to reveal the evolution of rock brittleness on rock materials under different mechanical and thermal conditions. The expansion pressure from expansive mortar is an effective and efficient solution to promote the dynamic failure of brittle rock. The experimental and numerical results from the dynamic response of expansive mortar filled rocks reveal that the expansion pressure facilitates rock fracturing surrounding the expansive mortar, accompanied by the generation of tangential and radial cracks and the attenuation of the stress wave generated during the dynamic test. The data also show that the expansion pressure is larger than the radial inertia stress and dominates crack generation in unconfined rock. The change of nominal tensile strength with the strength ratio is consistent with the failure pattern of inclusion-bearing specimens, in which the inclusion part experiences the states of being pulverized, split, and intact. The changes of loading rate and temperature can affect the nominal tensile strength to a different degree with the increasing inclusion strength. The CAI tests show that the CAI value is mainly increased during the first few millimeters of scratching distance and is strongly influenced by the stylus indentation during the subsequent scratching distance. The stylus-rock interaction is examined based on the P-wave velocity and acoustic emission, and it is suggested to evaluate the wear flat by reconstructing the intact stylus profile and limiting the scratching distance. This study implies that the rock behavior and failure pattern are inherently affected by the rock brittleness to varying degrees. The application of expansive mortar modifies the brittleness of surrounding rock under dynamic loading, resulting in changes in failure mode with more complex crack networks. For inclusion-bearing rocks, when the strength of the inclusion is comparable to or larger than that of its rock counterpart, the brittleness remains nearly constant with the increasing temperature. The brittleness is more sensitive to changes in loading rate and inclusion size than changes in temperature, especially in specimens with strong inclusion. The change in rock brittleness caused by thermal treatment essentially affects the stylus indentation. The CAI results show that a lower brittleness index corresponds to a higher CAI value and a reduced indentation stress.