Formulation and procedure for in situ stress back-analysis from borehole strain changes measured during nearby underground excavation

Estimation of in situ stresses based on back-analysis of measured stress changes and displacements has become an alternative to the direct stress measurement methods. In order to help users conduct own measurement and analysis, this paper presents in detail a field stress back-analysis approach directly from borehole strain changes measured during nearby underground excavation. Essential formulations in major steps and the procedure for the entire analysis process are provided to allow users to follow. The instrument for borehole strain change measurement can be the CSIR or CSIRO stress cells and other borehole strain cells that can measure strains on borehole walls. Strain changes corresponding to the stress changes at a borehole location are calculated in borehole environment. The stress changes due to nearby excavation can be calculated by an analytical model for a single circular opening and simulated by a numerical model for non-circular and multiple openings. These models are based on isotropic, homogeneous and linear elastic assumptions. The analysis of borehole strain changes is accomplished by multiple linear regression based on error minimization and an integrated process provides the best-fit solution directly to the in situ stresses. A statistical technique is adopted for screening outliers in the measurement data, checking measurement compatibility and evaluating the reliability of analysis results. An application example is included to demonstrate the practical application and the analysis procedure.