ABSTRACT: This study investigated the precision damage of the rock under step loading. The method of spatial acoustic emission (AE) evolution based on the variation of P-wave velocity at stages was proposed. During the step loading test, Acoustic emission signal and P-wave velocity were measured by eight attached sensors with accurate coordinate and a pair of ultrasonic sensors respectively. The experimental results showed there was a good correlation among AE location points, P-wave velocity and stress when the stress reached 90 MPa (74.4% of peak strength), which could be together as multiple precursor indicators of rock fracture. Furthermore, the spatial location patterns of AE events based on variations of P-wave velocity can accurately identify the spatial damage area of the rock. It demonstrates that such an approach is significant to study the microscopic damage of rock and evaluate the failure character of the actual rock mass.
1. INTRODUCTION
The actual stress state of rock mass in deep underground engineering is relatively complicated, and many underground structures frequently experience complex loading. The study on rock failure mechanism and crack propagation rule has always been concentrated by researchers, especially in underground mining engineering (Xu et al. 2009; Bagde and Petros, 2005; Xiao et al. 2010). However, the present research has lagged behind the demand of actual engineering development due to the intrinsic microstructure of rock mass. Therefore, it is significant to study the precision damage law of the rock subjected to step loading using a variety of monitoring methods since it would help to accurately understand the damage and fracture mechanism of practical rock mass(Momeni et al. 2015; Liu and He,2011; Song et al. 2016). Acoustic emission (AE) is a concomitant phenomenon that can be real-time reflection of the crack initiation, expansion and destruction of the crack on the rock (Yang et al.2014; Zhang et al.2015; Nasseri et al.2014). Primarily, acoustic emission is widely used for three-dimensional spatial evolution to furtherly investigate and reveal damage laws and mechanical characteristics under complex external load. In recent years, some theoretical and experimental work on this field had been extensively carried out. Xu et al. (2009) analyzed AE space-time evolution rule of the damage under cyclic loading, which demonstrated that most events occurred in the core zone forming at the static loading process. Acoustic emission experiment and numerical simulation of rock fracture under high-fluid source were used together to understand rock fracture and fault instability (Lei et al.2015). The expected failure mechanisms were related to the AE event rate of sample deformation and the percentage of isotropic and deviatoric components of the seismic moment tensors (Aker et al.2014). Stoeckhert et al. (2015) studied fracture initiation and propagation processes in a highly anisotropic rock slate under uniaxial as well as triaxial loading via AE event localization. Xu et al. (2012) investigated temporal-spatial evolution process of microcracks during the similar pillar material experiment, which indicated that the pillar and roof of the specimen belonged to the danger zone of macroscopic failure.
