Rapid field test for shear strength characterization of dacite at Cerro Rico de Potosi (Bolivia): tilt test with a Schmidt hammer and compass-clinometer

In competent rock masses submitted to low shear stress (e.g., close to the surface), stability is controlled by the geometry of blocks and strength of the discontinuity planes that separate the blocks. The basic friction angle is a relevant parameter to estimate shear strength of discontinuities. The most employed technique to determine the basic friction angle is the tilt test, which can be applied in laboratory settings or in situ, utilizing two blocks of rock that are tilted until the top block slides. No studies on the determination of basic friction angle in field settings were found for specific volcanic rocks such as andesites, dacites and rhyolites, which are typical of the acid and intermediate volcanism of Andean mountain range volcanoes. Rhyodacitic rocks of the Potosí mine, in Bolivia, were selected for basic friction angle characterization in situ. Cerro Rico is 4782 m high, and has been declared a humanity heritage site along with the city of Potosí in 1987, being one of the most important silver mines in the world. Cerro Rico is constituted by an intrusion of dacitic volcanic rocks with silver and tin mineralization. The results of the study agree with basic friction values of intrusive rocks with similar texture. It is highlighted that this simple field methodology is useful and effective to obtain preliminary values of shear strength for discontinuities. The obtained values of basic friction for rhyodacite add to the database of geomechanical rock characterization.