Prediction and prevention of rockburst

Rock burst is a special dynamic engineering geological phenomenon induced by underground mining, which is due to the destruction of the original rock stress state by mining activities, resulting in the high concentration of surrounding rock stress and the sharp deformation of coal seam and surrounding rock. When the deformation rate exceeds the maximum possible plastic deformation rate of coal seam and surrounding rock due to the increase of pressure per unit area, the elastic energy accumulated in coal seam and surrounding rock is suddenly released, leading to large displacement and destruction of coal seam and surrounding rock. With the occurrence of dynamic engineering geological phenomena such as vibration (mine earthquake), shock wave and cracking sound, this dynamic engineering geological phenomenon appears in both metal mines and non-metal mines, especially in coal mines. Therefore, the following coal mine as the object, to discuss this issue.

For coal mines, since 1930s, coal mines such as Fushun, Kailuan, Zaozhuang, Beipiao, Mentougou and Nantong in China have all experienced rockburst. This is a kind of engineering geological disaster phenomenon with the increase of underground mining depth in coal mines. Moreover, with the continuous increase of mining depth, the frequency of rock burst is increasing, the disaster intensity is becoming increasingly fierce and the harm degree is becoming more and more serious. Rock burst occurred in Mentougou Mine 1947 of Jingxi Coal Mine. According to statistics, from September of 1976 to the end of 1980, the average number of occurrences per month increased from 53 to 498, among which the average number of mine earthquakes above 2.2 on the Richter scale increased from 24.2 to 83.9. This shows that the problem is very serious and must be paid attention to as soon as possible.

1. Location and depth of rockburst

Rock burst will occur in both mining face and tunneling face. For example, underground mining in Fushun coal mine mostly occurs in the working face, and Tianchi coal mine mostly occurs in the heading face. This phenomenon occurs in the roof and floor of coal seam. In the heading face, it also occurs on both sides of the roadway, but most of them occur in the coal seam.

Rock burst has a great relationship with mining depth. For example, when the mining depth of Zaozhuang Bayi Mine is 140m, the rock burst is not obvious. The mining depth reaches 185 meters, and a small amount of rock burst occurs when driving coal roadway; When the mining depth reaches 370 meters, the rock burst increases obviously; However, when the mining depth reaches 500 meters, the rockburst is very intense. A large number of facts show that there is a critical depth of rockburst. The above Zaozhuang coal mine is185m, Fushun coal mine is 280m, Tianchi coal mine is 240m, Mentougou coal mine is 240m, Kailuan coal mine and Tangshan coal mine are 500m, Xinzhou kiln mine in Datong coal mine is 236-270m, and Yanshitai mine in Nantong coal mine only appears when the mining depth is-160m. Obviously, the occurrence of rockburst is related to the level of geostress.

2. Rock burst prediction

The occurrence of rock burst is regular, and its formation process is mainly manifested in the sudden release of stress accumulation and accumulated energy, which is mainly controlled by the force of coal (rock mass) and the law of deformation and failure. The regularity of rockburst can help us understand this problem. Rock burst has two characteristics: first, the roof and floor are bent, accompanied by explosion; The other is roadway spalling, coal explosion and rockburst. Usually, it appears in the same mining layer in a mining area in a similar way. For example, when the mining of Xinzhouyao Mine in Datong Coal Mine entered the second level, rock burst occurred when mining 9 #, 10 #, 1 1 # coal, and its manifestations were basically the same, all of which occurred in the form of roof and floor crushing, accompanied by blasting. The author believes that this phenomenon is related to the characteristics of in-situ stress in this area. According to the author's actual measurement, the horizontal stress component measured by in-situ stress measurement in Yungang Mine of Datong Coal Mine is about twice as large as the vertical stress component. Obviously, the failure should first occur on the roof and floor, and then induce the rupture of both sides, which is in line with reality. It occurs in the form of energy accumulation and release. Since the 1960s, many scholars who study rockburst believe that coal seam-surrounding rock, namely roof rock-coal seam-floor rock, should be regarded as a unified bearing system. If the mechanical balance of the bearing system is destroyed, it will suddenly release a lot of elastic energy. If the elastic energy is much greater than the elastic energy of the system itself, rockburst will occur. The more brittle the bearing system is, the less energy is consumed by deformation, the greater and faster the elastic energy is released, and the greater the possibility of rockburst. At present, the ratio KE of released elastic energy to consumed energy and the energy ratio KW of total deformation are used as indicators to predict the possibility of rockburst. It is generally believed that there is a danger of rockburst when KE≥6 and KW≥0.7, and preventive measures must be taken in production.

KE and KW are determined according to the stress-strain curve characteristics of coal and surrounding rock. (figure 13- 1). The elastic energy released by loading and unloading of coal or surrounding rock and the elastic energy stored by the system are S4 and S3 respectively. In order to obtain the values of KE and KW, stress-strain curves are usually made by cyclic loading under uniaxial compression, and the deformation curves of uniaxial compression stress level corresponding to about 80% failure stress are determined, as shown in figure 13- 1, where △OAH is releasable elastic energy S4 and △ABH is dissipative deformation energy S3.

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Figure 13- 1 Characteristics of stress-strain curve of materials

The author thinks that the above criterion is only one of the possible conditions of rockburst, but not a sufficient condition. Another situation is that the coal seam, roof and floor in the supporting system are damaged, that is:

(1) If rockburst is caused by insufficient material strength, the criterion is

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Where: σc is the uniaxial compressive failure strength of the material; σt is the maximum tangential force around roadway or stope; η is the stability coefficient, and when η is1,it will be destroyed.

(2) When the bearing system is a rock mass with a plate-like crack structure, its failure criteria are as follows

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Where: p is the effective force acting on the plate crack; Pcr is the critical load of plate fracture;

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When η1,there is a failure condition, and the energy criterion KE, KW plus the failure criterion is a sufficient condition for predicting rockburst. Therefore, it is necessary to obtain rock mechanical properties parameters e, μ, σc and geostress data when predicting rockburst.

3. Prevention and control of rockburst

According to the above analysis, rockburst actually has two conditions: in-situ stress and mechanical properties of coal and surrounding rock. In order to eliminate the first condition, on the one hand, the possibility of large tangential stress around the roadway should be eliminated as far as possible from the layout and section selection of the roadway; On the other hand, appropriate rock mass transformation measures are taken to reduce the stress difference σt-σr between coal and surrounding rock. In order to realize the second condition, appropriate rock mass transformation measures should be taken to reduce the stiffness or improve the strength of coal and surrounding rock materials. In order to reduce the stiffness of materials, water injection technology can be used to soften the materials in the system or high pressure water cracking method can be used to reduce the stiffness of the system; In order to improve the strength of the material, grouting or prestressed anchor cable can be used for reinforcement. Which treatment technology should be adopted depends on the possibility of construction technology and economic comparison. If the brittleness of the material is high, rockburst may occur after excavation, and there is no time for supporting and anchoring. Obviously, the supporting and anchoring scheme is not feasible, so pretreatment measures to reduce the stiffness of the system must be taken to prevent rockburst.