Geomechanics of hydrothermal veins: Insights from laboratory direct shear and numerical triaxial experiments

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Rudderham, Gisele
Geomechanics , Rock mechanics , Triaxial Compressive Strength , TCS , UCS , Uniaxial compressive strength , DS , Direct Shear Strength , RS2 , Numerical modelling , Numerical calibration
Geotechnical rockmass strength characterization is a task regularly undertaken by rock engineering practitioners for underground and open pit excavations. As the demand for mineral resources drives extraction into deeper, more challenging and complex geological environments, the effect of intrablock structure is commonly overlooked in conventional field and laboratory characterization practice though it may be particularly important to rockmass behaviour in high stress environments. This research focuses on the characterization of the shear strength of intact hydrothermal veins through laboratory direct shear and triaxial numerical experiments. Direct shear testing of basaltic and dacitic rocks with calcite veins produced recommendations for direct shear sample selection and preparation, and a greater understanding of the challenges with testing intact specimens in direct shear was gained. Laboratory uniaxial compressive strength and triaxial compressive strength results of mafic rocks with and without sulfide veins from the El Teniente mine were used to calibrate numerical input parameters for triaxial simulations of veined core specimens. This investigation, whereby the vein orientation and vein thickness of veined specimens was systematically varied, was used to evaluate the effect of sample geometry on strength and elastic model output properties, with the ultimate purpose of being used to represent intact joints in excavation scale models. Vein orientation played the most significant role on peak strength with the highest reduction in strength, compared to the homogeneous wallrock, occurring in samples with veins oriented at 20° < α < 35°. To incorporate the results of the laboratory scale tests of intact veins into excavation scale numerical models, the practitioner must decide which method will be utilized for the excavation scale model. To include veins as discrete structures in the excavation scale model, the shear strength of the fractured vein in the laboratory scale model should be calculated. For this method, a range of vein orientations must be tested at a variety of confining pressures. Alternatively, the intact rock strength can be modified to account for the presence of the veins and in this case, the strength of the laboratory scale specimens as a whole should be utilized.
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