Laboratory Investigation on Relations Between Characteristic Impedance, Energy Transformation and Dynamic Stress Induced Faults in Rocks

Abstract

ABSTRACT: In this study, relations between characteristic impedance (product of sonic wave velocity and density), energy transformation and dynamic stress induced faults in rock specimen were investigated. To achieve this, peridotite and pyroxenite from Boliden Kevitsa mine in Sodankylä, Finland was prepared to length and diameter ratio of 1:1 in accordance with recommendations of the International Society for Rock Mechanics (ISRM) for dynamic tests. This was followed by measurement of characteristic impedance of specimen under cyclic dynamic stress until breakage occurred. Results reveal that dissipated energy increased with incident energy under cyclic dynamic loading. However, the ratio of incident energy and characteristic impedance possess slightly higher coefficient of determination for estimating dissipated energy in rock specimens. Further investigations showed that the characteristic impedance of rock specimen remains unchanged during elastic deformation of rock specimen. However, a decrease in characteristic impedance before breakage was observed in some specimens. The forementioned decrease in characteristic impedance due to lower density and wave velocity measurements can be attributed to the presence of newly formed cracks and wave interactions (reflection, refraction, and diffraction) at interfaces of forementioned cracks. Accordingly, characteristic impedance can be applied in monitoring engineering support structures in regions prone to cyclic dynamic loading. 1. INTRODUCTION Dynamic stress, which refers to force applied through impact, vibration, blasting, pulse, hydraulic, seismic, and rolling mechanisms at very short time intervals, results in deformation and breakage of rocks when the bearing capacity is exceeded. On the other hand, cyclic loading refers to repeated application of stress to rockmass. Early investigations on cyclic loading studied fatigue strength and fatigue life, which refers to the number of loading cycles before rock fracture/breakage (see Mann 1966; Attewell and Farmer, 1973; Vutukuri et al 1978; Singh 1989). More recently, several experimental designs have been employed to improve knowledge on rock response to cyclic dynamic loading. These can be grouped based on loading wave amplitude comprising constant, step-increasing and random wave amplitudes (see Heap et al 2009; Yang et al 2015; He et al 2016; Jia et al 2018; Yang et al 2018; Kumar et al 2018; Liu et al 2018; Li et al., 2019; Peng et al 2020), as well as loading type comprising compressive, shear and tensile cyclic loading (see Coviello et al 2005; Bagde and Petroš 2005; Chen et al 2013; Dai et al 2013; Song et al 2013; Vaneghi et al., 2018). In addition, effects of environmental conditions comprising temperature, freeze-thaw, and pressure on rock response to cyclic dynamic loading have been investigated (see Chen et al 2004; Zhao et al 2017; Xie et al 2018; Liu et al 2018; Sun and Zhang 2019).