A modified initial in-situ Stress Inversion Method based onFLAC3D with an engineering application

To improve the accuracy of an initial in-situstress field determined by inversion, we describe a modified initial in-situ stress inversion method that uses partialleast-squares regression based on FLAC3D. First, eachstress component is regressed to improve the fitting accuracyof locally abnormal stress regions, and then the relationshipbetween element stress and unbalanced nodeforce is analyzed according to the computational principlesof FLAC3D. The initial in-situ stresses obtained fromthese regression calculations are added to a numericalmodel, and the unbalanced node forces are recalculated.An external force equal to the recalculated unbalancednode force is then exerted on the node in the direction opposingthe original unbalanced node force to satisfy theequilibrium condition. For the in-situ stresses of elementsthat do not satisfy the strength conditions, they are modified by assuming the average stress is constant and reducingthe partial stress to satisfy the equilibrium andstrength conditions, which also resolves the unreasonabledistribution of the boundary nodal forces and results ingood regression estimates. A three-dimensional hypersurfacespline interpolation method is developed to calculatethe in-situ stress tensor at arbitrary coordinates. Finally,we apply this method to an underground engineeringproject, and the results are shown to agree well withthose obtained from field monitoring. Therefore, it is concludedthat this modified in-situ stress inversion methodcould effectively improve the fitting accuracy of locally abnormalstress regions.