In-situ stress measurements and stress distribution characteristics in underground coal mines in China

Abstract

The theory, equipment and method of hydraulic fracturing stress measurement used in underground coal mine roadways in China are presented. A quality assurance system and a quality ranking scheme were developed for hydraulic fracturing stress measurements under specific conditions encountered in Chinese underground coal mines. During the last 8 years, a total of 204 hydraulic fracturing stress measurement campaigns have been carried out in 49 coal mines within 13 Chinese coal districts. The test sites which are located in sub-horizontal, inclined and steeply dipping coal measures range in depth from 69.2 m to 1 283 m. Each stress data record was assigned a quality between A and E according to a proposed data quality ranking scheme. A total of 97 stress data records of A and B quality were used to investigate the distribution characteristics of in-situ stresses within the coal districts. Generally, in-situ stresses increase with depth. In shallow coal mines, the rate of increase in horizontal stresses with depth is greater than the rate of increase of vertical stress. With increasing depth the rate of increase in horizontal stresses decreases. A considerable scatter in the in-situ stress test data may be due to distinct differences in both the strength and deformation moduli of strata located in varying geological environments and different coal districts. In addition, large folds and faults often result in a change in orientation of the maximum horizontal stress, and in distinct changes in stress values. Three types of stress field distribution have been noted with σ H > σ h > σ v found mainly in relatively shallow coal mines (< 400 m), the σ H > σ v > σ h type in moderately deep coal mines (400–600 m), and the σ v > σ H > σ h type predominantly in moderately deep to deep coal mines. The ratio of the maximum horizontal principal stresses to vertical stress is usually between 0.5 and 2.0 in the coal districts.