Distribution law of underground in-situ stress field in Xishan coal mine field

Xishan mining field is an important part of coal mining area in Shanxi Province.In order to make up the blank of in-situ stress measurement and to study the distribution law and variation characteristics of in-situ stress field,using a small-bore hydrofracturing in-situ stress measurement device,some geostress measurements were carried out at 59 sites in 9 coal mines in Xishan,the variation characteristics of the parameters,such as the type of in-situ stress field,the magnitude of in-situ stress,the direction of horizontal principal stress and the ratio of lateral pressure,which characterize the distribution law of in-situ stress field,were analyzed,the relationship between the horizontal principal stress and lateral pressure ratio of rock mass with different lithology to the sedimentary strata of coal measures and burial depth were also analyzed in Xishan mining area,to study the distribution law of in-situ stress field,and to explore the influence of lithology on the distribution law of in-situ stress field.The result shows that the coal mines are at a moderate depth in Xishan,the underground stress field is mainly tectonic stress field,the horizontal stress is absolutely dominant,the magnitude of in-situ stress is mainly medium level,and the low stress level is next,the in-situ stress increases with the buried depth,but the discrete type is also very significant,the buried depth not only determines the type of in-situ stress field,but also influences the magnitude of in-situ stress.The direction of the maximum horizontal principal stress are NW and NE,mainly N5°W~N90°W and N6°E~N87°E,each accounting for about 50%.The horizontal principal stress around the normal fault zone increases obviously,and the direction of the horizontal principal stress is reversed,which is the main influence factor of the abnormal and discrete ground stress value.The maximum lateral pressure ratio is 0.80-4.23,which is related to the type of in-situ stress field,less than 1 is the gravity stress field,and more than 1 is the tectonic stress field,and more than 2 horizontal stress is completely dominant.Average lateral pressure ratio is 0.61-3.19,the fitting curve of it and burial depth is similar to the curve proposed by Hoek-Brown,but because of the measured data of sedimentary rock strata in specific Xishan coal mining areas,there are obvious differences in values.The values of horizontal principal stress to different lithology rock mass all increase with the increase of burial depth,but the gradient of stress increase is different,the two types of lateral pressure ratio to different lithology rock mass all tend to be fixed with the increase of mining depth,but the curvatures of them show obvious difference,lithology is only one of the factors that affect the distribution of in-situ stress field in Xishan mining area,and it is difficult to assess the distribution characteristics of tectonic stress field accurately based on single factor.