In-situ stress solution theory for borehole overcoring technique incorporating time-dependent effect of stress relief and its applications

In-situ stress is a key parameter for underground mine design and rock stability analysis. The borehole overcoring technique is widely used for in-situ stress measurement, but the rheological recovery deformation of rocks after stress relief introduces errors. To improve accuracy, this study proposes an in-situ stress solution theory that incorporates time-dependent stress relief effects. Triaxial stepwise loading-unloading rheological tests on granite and siltstone established quantitative relationships between instantaneous elastic recovery and viscoelastic recovery under different stress levels, confirming their impact on measurement accuracy. By integrating a dual-class elastic deformation recovery model, an improved in-situ stress solution theory was derived. Additionally, accounting for the nonlinear characteristics of rock masses, a determination method for time-dependent nonlinear mechanical parameters was proposed. Based on the CSIRO hollow inclusion strain cell, time-dependent strain correction equations and long-term confining pressure calibration equations were formulated. Finally, the proposed theory was successfully applied at one iron mine (736 m depth) in Xinjiang, China, and one coal mine (510 m depth) in Ningxia, China. Compared to classical theory, the calculated mean stress values showed accuracy improvements of 6.0% and 9.4%, respectively, validating the applicability and reliability of the proposed theory.