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3 years ago in Geotechnical Engineering , RockMechanics By Gayatri
Will multiple lithologies affect the value of rock mass strength?
We're modeling a slope in a ~200m thick sandstone unit. However, geophysics shows it lies unconformably on a mix of older shales and volcanics in the river valley (~20m deep). Even though we're excavating only in the sandstone, could the contrast in stiffness and the potential for weak interfacial planes at the contact reduce the effective rock mass strength we use in our design?
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By Nisha Ali Answered 1 year ago
Absolutely, it can, and this is a critical site-specific detail often overlooked in preliminary models. I have seen slopes fail along such lithological contacts even when the upper unit is competent. The strength contrast creates a natural plane for stress concentration and potential sliding or toppling. You must characterize that interface: is it a weathered zone, a clay seam, or a sharp contact? I would recommend incorporating this as a discrete weakness in your numerical model and possibly conducting direct shear tests on core samples that include the actual contact zone to get representative shear strength parameters.
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