1、翻译部分英文原文 Stress evolution with time and space during mining of a coal seamAbstract:Mining of the upper protective coal seam is widely practiced in China for coal mine safety, but relief gas may present a new risk of blasting. To control the relief gas effectively, a strain-soften model was builtby F
2、LAC30 software to investigate the stress evolution during the process of mining the upperprotective coal seam. The results show that the abutment stress changes rapidly within 10 min front of the coal face, and the maximum abutment stress is approximately twice the original when the coal seam is min
3、ed 20-30 m. The abutment stress should break the rock mass and cause the gas to flow easily. In the stable mining period, the change trends of the x-stress and z-stress are different, and these should also pre-break the rock mass. The stress distributions of the rock mass at different distances unde
4、r the protective coal seam are different, especially near the coal face, which should greatly affect the gas flow when the space of the protective and protected coal seams change over a large range. The relief angle also changes over a large range, increasing to a maximum approximately 30 m behind t
5、he coal face, and it decreases gradually when it is far away from the protective coal seam. The results are helpful for designing the coal face of protected coal seams and borebole layouts to control the relief gas.1. IntroductionChina is a counriy rich in coal resources but poor in oil and gas. For
6、 many years, coal has supplied more than 70 of Chinas energy. In 2010, coal production reached approximately 3.2 billion tons, and it currently supports the rapid growth of Chinas GDP. However, more than 95 of Chinas coal mines are underground, and 17.6% of key state-owned coal mines are coal and ga
7、s outburst mines, which seriously threaten coal mine safety. In 2008, for example, 25 accidents caused 120 deaths. To effectively control coal and gas outburst accidents, the State Administration of Work Safety unveiled the Provisions for the prevention of coal and gas outburst in August 2009, which
8、 stresses regional gas control mainly by mining protective coal seams and by methane pre-drainage to eliminate the risk of outburst from a large region. Additionally, the mining of protective coal seams should be preferentially selected if possible. After the protective coal seam is mined, the stres
9、s of the protected coal seam decreases and the permeability increases, which causes the methane to be released continuously, and the outburst danger is eliminated. However, during the process of mining the protective coal seam, the relieved gas from the current and adjacent seams may flow into the c
10、oal face, causing the methane to overrun. The relieved gas flow and the protected range are determined by the stress distribution. The stress distribution of mining fields and roadways has been extensively studied for many years, but most of these studies only address some static states . Because mi
11、ning is a dynamic process, the stress field evolves dynamically with time and space, which causes the gas flow and the protected range to change dynami-cally. Therefore, the stress distribution during the process of mining a protective coal seam must be studied from a dynamic viewpoint while conside
12、ring the stress evolution with time and space for gas relief control and coal face arrangement. However, this question is not addressed in the current literature, so it is necessary to study the issue thoroughly. The geological condition of a coal mine is non-uniform and complex, which makes it a ch
13、allenge to study the stress distribu-tion and evolution with time and space with only field orlaboratory experiments. Professional geotechnical numerical ana-lysis software could be a good choice for this situation. 2. Establishment of a numerical modelFLAC30 is one of the most important numerical s
14、oftware tools in current rock mechanics calculations. This tool is particularly suitable for solving nonlinear large deformation problems in geotechnical mechanical engineering and is widely used in the field of mining engineering, among other fields 8,9. The software contains eleven kinds of materi
15、al constitutive models, as follows: one empty element model, three elastic material models and seven plastic models. Mining a coal seam will cause rock mass deformation and will decrease the rock strength; thus, the rock mass is assumed to be an elastic-plastic material, and it presents strain-softe
16、ning characteristics after it is damaged. Therefore, the strain-softening model was chosen. The coal strength parameters are shown in Table 1 according to the typical values of the Hancheng Mining Co. Ltd.The strain is both elastic and plastic. The stress initially changes linearly with the elastic strain, but a strain-softened character will appear after the rock mass is damaged beyond the yield point 8. Thus, the friction angl
