## 1 Introduction  
Underground space engineering has entered a high-speed construction period with the development of deep excavation technology, but more and more underground engineering (such as mining/tunnel engineering) is prone to structural instability and result in disasters (such as rock burst/coal bump) due to the complex crustal stress conditions and lithological structures in the construction process [1, 2]. The development and the expansion process of the surrounding rock internal fissures under the complicated loading conditions of the underground space are closely related to its structural instability disasters. The stability monitoring has become the focus of engineering construction at home and abroad [3; 4; 5]. The gestation and occurrence of rock instability disasters in deep rock engineering is fundamentally the evolution process of internal primary crack closure, new crack generation and expansion of the large-scale rock under loading. As a concomitant phenomenon of releasing strain energy to the outside world in the form of elastic wave during the process of deformation or fracture of loaded rock material, the acoustic emission (AE) signal is able to monitor the failure process of sample timely, dynamically and continuously and to conduct three-dimensional localization of fracture event. Therefore, it is an effective tool for studying the material internal damage degree, fracture evolution and macroscopic cracks generation and expansion [6; 7; 8]. [PERSON] et al. [9] used AE signals to analyze the development of the fracture during the force rise phase and the fatigue damage characteristics of the static load phase, and monitored the development of the crack before the damage becomes visible. [PERSON] et al. [10] used AE technology to timely record the process of crack generation, expansion, penetration and failure under uniaxial compression. The results were found that the AE energy and localization can timely reflect the whole expansion, penetration and failure process of the crack samples, which leads to better understanding the damage mechanism of rock mass in underground engineering. The fracture process of rock roadway by AE ringing count, energy, main frequency and other parameters were analyzed by [PERSON] et al. [11], which showed that the AE signal was closely related to the expansion stability and the crack size of rock crack. [PERSON] et al. [12] monitored the AE signals of different kinds of microscopic particles during the cracking process of the samples and found that AE signal was closely related to the micro-structure of the particles during the crack propagation and expansion process. These above studies have shown that the occurrence of rock dynamic disasters is the result of the evolution of the internal fissures of the rocks, and has nonlinear characteristics in time; the changes of the parameters of the AE signals are not only related to the mechanical state of the rocks, but also to loading form, deformation field evolution, local crack expansion, macroscopic crack generation, etc. [13; 14; 15; 16]. How to better invert and characterize the whole process of propagation, failure and instability of the crack of the loaded rock by the characteristics of the AE parameters and the variation, and warn about the occurrence of dynamic disasters in advance during the critical instability stage, were always the research direction of the rock AE application research. Based on these, in this paper, intact and holey rock samples were simulated different types of underground space structure, and the graded loading was simulated the increasing stress conditions of the rock mass induced by the overburden rock layer and underground mining. The characteristics of AE signals were tested during the process of load-damage of two kinds of rock samples, and their differences were also found. Meanwhile, combining with the results of the AE localization, the damage evolution process of the sample and the special behavior of AE signals in the sub-stability phase were also analyzed.