Pattern Recognition of Signals for the Fault-Slip Type of Rock Burst in Coal Mines

Hindawi Publishing Corporation Shock and Vibration Volume 2015, Article ID 806969, 8 pages http://dx.doi.org/10.1155/2015/806969 Research Article Pattern Recognition of Signals for the Fault-Slip Type of Rock Burst in Coal Mines X. S. Liu,1 J. Tan,1,2 Y. L. Tan,1 and S. C. Hu1 1 State Key Laboratory of Mining Disaster Prevention and Control Co-Founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China 2 College of Engineering, Western New England University, Springfield, MA 01119, USA Correspondence should be addressed to J. Tan; Received 25 May 2015; Revised 16 July 2015; Accepted 21 July 2015 Academic Editor: Farhang Daneshmand Copyright © 2015 X. S. Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The fault-slip type of rock burst is a major threat to the safety of coal mining, and effectively recognizing its signals patterns is the foundation for the early warning and prevention. At first, a mechanical model of the fault-slip was established and the mechanism of the rock burst induced by the fault-slip was revealed. Then, the patterns of the electromagnetic radiation, acoustic emission (AE), and microseismic signals in the fault-slip type of rock burst were proposed, in that before the rock burst occurs, the electromagnetic radiation intensity near the sliding surface increases rapidly, the AE energy rises exponentially, and the energy released by microseismic events experiences at least one peak and is close to the next peak. At last, in situ investigations were performed at number 1412 coal face in the Huafeng Mine, China. Results showed that the signals patterns proposed are in good agreement with the process of the fault-slip type of rock burst. The pattern recognition can provide a basis for the early warning and the implementation of relief measures of the fault-slip type of rock burst. 1. Introduction The fault-slip type of rock burst is one of the main types of rock burst in coal mines, and it is a great threat to the mining safety due to its devastation and the large amount of coal extruded [1, 2]. Effectively recognizing the signals patterns of the fault-slip type of rock burst not only is the prerequisite for its early warning, but also plays a great role in the implementation of relief measures [3–7]. Many researchers have made contributions to the monitoring and early warning of rock burst and proposed many kinds of monitoring approaches (e.g., electromagnetic radiation, AE, microseismic signals, and stress) [8–16]. The electromagnetic radiation monitoring is an approach that obtains the stress magnitude and variations of coal and rock by monitoring the intensity and pulse of electromagnetic radiation [8]. Song et al. and Liu et al. investigated the approach to estimate the rock burst danger from the intensity and pulse of electromagnetic radiation and proposed an early warning criterion [9, 10]. AE is accompanied by the damage of microcracks in rocks [11]. The damage evolution of rocks can be obtained by monitoring the AE energy and frequency. Tan et al. and Ji et al. proposed the patterns of AE signals based on two particular geological conditions, respectively [12, 13]. Microseismic monitoring is an approach to determine the failure degree of rocks by analyzing the energies, frequencies, and wavelengths of microseismic events gathered by demodulators. Jiang et al. investigated the features of microseismic signals during the process of rock burst and proposed some early warning criterions [14]. All these approaches play important roles for the early warning of rock burst and lay the foundation for its prevention. In particular, they can accurately forecast the strain type of rock burst induced by the mining abutment pressure, which improves the mining safety effectively. Although the fault-slip type of rock burst is greatly influenced by the mining abutment pressure, it is different from the strain type of rock burst. In fact, the root of the fault-slip type of rock burst is the relative slipping of fault walls [17–19]. Therefore, the existing recognition approaches 2 Shock and Vibration q1 C1 Upper wall L2 C2 Coal 𝜃 H C3 C1 q2 Fault sliding surface FN q2 L1 Lower wall Goaf Lower wall Coal face advancing Upper wall Figure 2: The cross section of a normal fault and abutment distribution: C1 is the preexisting tectonic pressure curve; C2 is the mining abutment pressure curve; and C3 is the superimposed pressure curve. FT q1 Figure 1: Mechanical model of the fault-slip. of signals patterns for rock burst cannot apply to the faultslip type of rock burst [20, 21]. For this reason, this paper first established a mechanical model of the fault-slip and analyzed the mechanism of the rock burst induced by the faultslip. We then proposed the recognition approaches of the electromagnetic radiation, AE, and microseismic signals for the fault-slip type of rock burst. Finally, in situ investigations were presented to demonstrate the validity of the approaches. 2. Pattern Recognition of Signals 2.1. Mechanical Rationale. Influenced by the geological changes and tectonic activities, there are many geological formations in coal-bearing strata. Usually, large build-up of elastic strain energy occurs near the geological formations. The forces in coal and rock are in an equilibrium state without mining activities. Taking a single normal fault as an example, the mechanical model of the fault-slip is shown in Figure 1. It shows the forces on the two fault walls around the fault sliding surface. Uninfluenced by mining activities, the fault walls are in stable state and do not move. According to the force balance of the lower wall, the shear force, 𝐹𝑇 , and the normal force, 𝐹𝑁, at the sliding surface can be expressed, respectively, as 𝐹𝑇 = 𝑞1 sin 𝜃 (𝐿 1 − 𝐿 2 ) − 𝑞2 𝐻 cos 𝜃 (1) 𝐹𝑁 = 𝑞1 cos 𝜃 (𝐿 1 − 𝐿 2 ) + 𝑞2 𝐻 sin 𝜃, (2) where 𝑞1 is vertical stress loaded on the top and bottom of the fault, 𝑞2 is the horizontal stress loaded on the sides of the fault, 𝐿 1 is the length of the downside of the lower wall, 𝐿 2 is the length of the upside of the lower wall, and 𝐻 is the height of the lower wall. Considering the geometrical relationship 𝐿1 − 𝐿2 (3) , 𝐻 then the shear stress, 𝜎, and the normal stress, 𝜏𝑇 , at the sliding surface were calculated, respectively, as tan 𝜃 = 𝜎 = 𝑞1 sin2 𝜃 + 𝑞2 sin2 𝜃 (4) 𝜏𝑇 = 𝑞1 sin2 𝜃 tan 𝜃 − 𝑞2 sin 𝜃 cos 𝜃. (5) The shear strength of the sliding surface can be obtained as [22] 𝜏 = 𝑐 + 𝑞1 sin2 𝜃 tan 𝜑 + 𝑞2 sin2 𝜃 tan 𝜑, (6) where 𝑐 is the cohesion and 𝜑 is the friction angle. If the shear stress at the sliding surface is larger than its shear strength, the fault walls will slip relatively, and vice versa. Thus, the slipping criterion of the fault walls is (...truncated)