| In recent years, with the increase of the mining depth, the rock burst situation in Hegang mining area becomes more serious, and the rock burst accidents happened several times, which affect the safety production of coalmines seriously. The high stress field caused by the high in situ stress in the deep area and the mining superposition stress is the fundamental cause for the rock burst hazard. In this paper, Hegang mining area is the research object, and the “point- area” combined analysis method was used to study the distribution of the in situ stress field in Hegang mining area. First, the geology theory was used to analyze the evolution laws of geological structure in Hegang mining area and the distribution characteristics of the tectonic stress field for the rock burst coalmines were obtained by analyzing the geological structures in the coalmines of Hegang mining area. On the basis of the analysis, the geological structures of Hegang mining area were analyzed to obtain the distribution laws of the tectonic stress field in Hegang mining area. Second, the in situ stress values of the measuring points were obtained to verify the tectonic stress field analysis results and determine the distribution laws of the in situ stress field. The paper used the method combined with plastic zone test, rock mass structure test and the hollow inclusion strain cell overcoring method to measure in situ stress. The depth of the test holes were determined by the results of the plastic zone tests and rock mass structure tests so that the strain cells could be installed in the elastic area where the surrounding rock was integral. This method improved the success rate and the accuracy of in situ stress measurements. According to the geological materials of Hegang mining area, eight representative exploration lines were selected to established the 2-D numerical models, and the factors such as surface elevation, regional density and the in situ changes along with the depth were fully considered in the numerical simulation. Finally the stress distributions of the exploration lines were obtained and the contour maps of the maximum horizontal principal stress for the three principal mining levels were obtained by using the linear interpolation method. The rock burst characteristics of Hegang mining area were obtained through the investigation and analysis of the rock burst situation in the area. And according to the geological prototype of the typical rock burst work face, the numerical model was established to analyze the influence of the direction of in situ stress field, the maximum and the minimum horizontal principal stresses on the energy distribution of the area in front of the work face, and the laws of the in situ stress influence on the rock burst were obtained. According to the energy guideline and the least energy principle, the criterion for the rock burst zone assessment of Hegang mining area was established. Based on the analysis results of the distribution laws of the in situ stress field, Hegang mining area was divided into several danger zones with good application effects. The research results of this paper can be summarized as follows:1. Characteristics of the present tectonic stress field in Hegang mining areaCoal strata in Hegang mining area has experienced 5 stages of tectonic activities, the late Himalayan tectonic activity affects significantly the characteristics of the recent geological structures in the mining area. And in this stage there are short axis folds in NNW direction, tensional or tenso-shear faults in NEE direction, compressional faults in NNW or NNE direction and the strike-slip faults in NE direction formed. These geological structures cut other and control the present tectonic stress field in Hegang mining area. The direction of the current tectonic stress in Hegang mining area is NEE–SWW, which is consistent with the direction of the late Himalayan tectonic stress field. The tectonic stress field of Nanshan coalmine is affected by the mid Himalayan tectonic activity, with the performance of the NW-SE direction.2. Deep in-situ stress measurements in Hegang mining areaThrough the plastic zone tests and rock mass structure tests, the depth of the test holes were determined so that the hollow inclusion strain cells could be installed in the elastic area where the surrounding rock was integral. When the tools for the in situ stress measurement were improved, the disk for orientation is replaced of a cylinder with a cone head, the success rate of in situ stress measurements is improved. The paper used the method combined with plastic zone test, rock mass structure test and the hollow inclusion strain cell overcoring method to measure in situ stress in Hegang mining area, and 24 stress values were obtained. The results show that,(1) in-situ stress field in Hegang mining area is a typical tectonic stress field type, and the maximum principal stress is in the horizontal direction;(2) the direction of the in-situ stress in Hegang mining area is NEE or nearly East-West, which is consistent with the analysis results of the tectonic stress field.(3) the point stress distribution in Hegang mining area is obviously regional, and the difference between the maximum and minimum values for the maximum horizontal principal stress is 10~20MPa, which is affected by the regional structure. By fitting analysis, the vertical stress is highly correlated with the depth, and the correlation is positive. The average unit volume weight of overlying strata in Hegang mining area is 0.029 kN/m3, which is slightly larger than the international statistical value.(4) the ratio of maximum horizontal principal stress to the vertical stress in the Hegang mining area is 1.3~2.2. The ratio of minimum horizontal principal stress to vertical stress is 0.6~1.6.3. Distribution laws of the in situ stress field at Hegang mining area using numerical simulationAccording to the geological material of Hegang mining area, we selected 8 representative exploration line and use the numerical simulation method to study the distribution laws of the in situ stress field at Hegang coal mine.(1) The faults have a great influence on the maximum horizontal principal stress. The factors such as the pinch out, dip and the combined distribution of the faults and the combined distribution of the strata, have the interference effect on the distribution characteristics of the maximum horizontal principal stress. Stress concentration appears at the fault pinch-out area. The bigger the fault dip angle is, the bigger the stress relief area is. The maximum horizontal principal stress at the area surrounded by the faults is smaller than the outside area. When one pinch-out of a fault is near to another one, the stress superposition appears at the area between the two pinch-outs and the horizontal principal stress between the two pinch-outs is far bigger than the stress at the surrounding area.(2) Under the influence of the faults, the distribution of the vertical stress at the Hegang coalmine has its obvious characteristics. The vertical stress increases along with the depth. Great changes of the vertical stress occur at the pinch-out of the fault, where the stress concentration appears. The distribution difference of the vertical stress between the hanging wall and footwall of a fault, generally the vertical stress in the footwall is bigger than the stress in the hanging wall.(3) We have used the linear interpolation analysis method to draw the contour maps of the maximum horizontal principal stress for the three mining levels of-330,-450 and-530 in Hegang mining area. The maximum horizontal principal stress of Hegang mining area is obviously regional, and the stress changes appear in some areas as a result of the faults. The maximum horizontal principal stress in the east and west area is bigger than the stress in the middle area, and the stress reduction zones appear at the middle area.4. Analysis of the rock bust characteristics in Hegang mining areaThe rock burst characteristics in Hegang mining area were obtained through the analysis of the rock burst situation at the roadways and work faces in Hegang mining area.(1) The roadway rock burst: the rock burst area is in the range of 50 m from the excavation face, the energy release is about 2.0×105J~1.07×108J, and the rock bursts cause the decrease of the roadway section, the damage of the support, the loss of the equipments and casualties.(2) The work face rock burst: the rock burst area is at the work face and the up-roadways( the air return laneway), and the rock burst area at the air return laneway is in the range of 110 m from the up outlet of the work face. The energy release is about 3.57×104J~9.12×109J, and the damage of the rock burst in the work face area is more serious than the rock burst during the roadway excavation.(3) The reasons of the rock burst could be divided into three categories: the in situ stress, physical and mechanical properties of the coal and rock mass, and the production technology. The physical and mechanical properties of the coal and rock mass determine its rock burst tendency, and the in situ stress and the production technology provide the condition of the rock burst. The joint effect of the three factors results in the rock burst.5. Influence of in situ stress in Hegang mining area on the rock burstAccording to the geological prototype of the typical rock burst work face, the numerical model was established to analyze the influence of the direction of in situ stress field, the maximum and the minimum horizontal principal stresses on the rock burst. The results show that,(1) with the increasing of the angle between the direction of the maximum horizontal principal stress and the mining direction, the peak energy of the area in front of the work face is transferred into the inner of the coal, and the energy concentration area become bigger. When the angle is about 70°~90°, the peak energy is the biggest;(2) when the ratio K1 of the maximum horizontal principal stress to the vertical stress increases, the peak energy of the area in front of the work face increases, and the energy concentration area become bigger;(3) there is a positive correlation between the energy storage and the ratio K2 of the minimum horizontal principal stress to the vertical stress, however the changes of K2 have little influence on the energy distribution of the area in front of the work face.6. Rock burst zone assessment for Hegang mining areaAccording to the energy guideline and the least energy principle, the criterion for the rock burst zone assessment of Hegang mining area was established. The criterion for the coal of weak rock burst tendency is as following.smin eminU ?U ?U Safety zone,min emin smin emaxU ?U ?U ?U ?U Threatened zone,smin emaxU ?U ?U Danger zone.And the criterion for the coal of strong rock burst tendency is as following.smin eminU ?U ?U Threatened zone,smin eminU ?U ?U Danger zone.Us is the total storage energy of the rock mass before its dynamical failure, Umin is the minimum energy for the dynamical failure of the rock mass, Uemin is the minimum energy arriving at the free face when the rock burst happens, Uemax is the warning energy of the rock burst monitoring in the coalmine. According to the criterions for the rock burst zone assessment, Hegang mining area was divided into several zones, and 74.2% of the areas where the rock burst has happened are in the rock burst danger zones. Therefore the method for the rock burst zone assessment is rational, and it shows that in situ stress is an important factor of the rock burst in Hegang mining area. |
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