Influence of large syncline on in situ stress field: a case study of the Kaiping Coalfield, China
Kaiping coalfield is located in Tangshan, northern China, and the area has been mined for 130 years. The main structure is the Kaiping syncline with an axis that generally strikes north-east to east-west. Since 1998, we have measured the in situ stress using overcoring and obtained 30 sets of data in eight coal mines with depths from −260 to −1238 m. Based on the measured data, we established the trends in maximum horizontal stress, vertical stress, minimum horizontal stress, and their ratios, and the ratio of mean horizontal stress to vertical stress and depth. The relationship between magnitude and orientation of maximum horizontal stress and their relationships to the Kaiping syncline are discussed. The results show that (1) the stress field of the Kaiping coalfield is complex and non-uniform. The orientation and magnitude of principal stress from stress measurement diverge widely from the regional stress field and the stress regime, with 77 % strike-slip faulting and the remainder thrust faulting. (2) Both maximum (σH) and minimum (σh) horizontal stresses have significant differences at a similar depth; however, vertical stress σv increases linearly with depth with a gradient of 0.025 MPa/m. The ratio (k) of the mean horizontal stress to vertical stress is 160/h + 1.04 < k < 300/h + 1.14. This is higher at shallow depth and gradually decreases as depth increases. Up to 90 % of σH/σh falls between 1.17 and 2.39 with an average of 1.96, and it is not closely related to depth. The σH/σv ranges from 1.06 to 2.43 with a bandwidth of 1 and decreases gradually with increasing depth. (3) The in situ stress field in the Kaiping coalfield is the result of the synthetic action of different levels of geological structures where both the orientation and magnitude of in situ stress are influenced by the Kaiping syncline. The σH near the axis of the Kaiping syncline is remarkably high and decreases 8 km from the axis. The orientation of σH near the axis of the Kaiping syncline is almost perpendicular to the axis of the Kaiping syncline, while far from the axis the orientation gradually changes to east-west.