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Numerical investigation on strength upscaling and its application to a back analysis of an open pit slope failure Guajardo, Cristian Andres


Strength scale effect refers to the decreasing of rock strength when specimen size increases. The drop of strength is specific of the rock type and is related to the presence of natural defects. Scale effect has been widely studied in laboratory test and numerical simulations and there is consensus on the importance of upscaled rock strength for excavation design. However, due to lack of data at scale of rock block, is not uncommon that non-upscaled laboratory properties are applied directly for geotechnical assessment. Besides, literature is scarce on practical applications of scaled rock block strength. In this thesis, numerical upscaling of rock strength is performed and used to back analyze a major instability. The study case corresponds to a highly defected and fractured leached rock that participated in a major slope failure of an open pit mine. First, geological and geotechnical characterization of the defected rock is presented. Then, rock strength is numerically upscaled using synthetic rock numerical samples. Finally, the upscaled rock strength is applied to estimate rock mass strength as input for a bidimensional slope failure back analysis. Synthetic rock experiments were performed in ELFEN FDEM code, on bidimensional samples with diameters between 5 centimeters to 1 meter. A discrete defect network was built in Fracman software based on core logging data. Uniaxial, biaxial and indirect tensile test were performed. The FDEM code was able to simulate realistically cracking patterns and stress-strain curves. The scale effect of the unconfined strength was verified while friction angle showed to be size invariant. The back analysis of slope failure demonstrated that the confined strength was overestimated, likely due to the lack of constraint that the third dimension impose. The bidimensional back analysis of the slope instability was performed in ELFEN FDEM code and RS2 continuum code. A discrete fracture network of faults was included in ELFEN analysis. Assessments applying upscaled and non-upscaled properties were compared. There was small difference between the two cases due to the larger influence of the joints regarding the upscaled rock strength. However, the case based on upscaled properties reproduced the failure more accurately in both, FDEM and continuum code.

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