Complete stress determination for a railway cavern in Hong Kong by hydraulic tests on pre-existing fractures

A complete stress determination has been performed by hydraulic tests on pre-existing fractures conducted in two inclined boreholes for a railway cavern in Hong Kong, China. A total of ten complete tests including normal stress and fracture plane orientation determination have been successfully carried out in the zone where the cavern is located. The test depth in two holes varies from 101 to 120 m beneath the ground surface. After a brief introduction to the test system, procedures, and results in this paper, the inversion method for in situ stress field, neglecting both the vertical and horizontal stress gradient, is reported. Moreover, the genetic algorithm is applied to search for the optimal solution, which minimizes the misfit function, and the misfit associated with this solution is used to evaluate the 90 % confidence level domain with the Monte Carlo Technique. The results of this study are useful for a better understanding of the stress level of the planned railway cavern and also helpful for the safety design of the cavern.