Shear Strength Properties Of The Bor Clastites

The subject of this work is addressing geotechnical aspects of the latest gold ore discovery, Čukaru Peki, in the Bor volcanic province in east Serbia sitting some 400-600 m underground. A twin tube decline has been excavated along the axis of a north-plunging anticline mostly in clastic rock, colloquially called the Bor clastites, with irregularly alternating coarse-grained conglomerates and sandstones, locally tectonically deteriorated, interlayered with marl and siltstone. In the course of excavation, rock properties were reassessed. A set of laboratory and in-situ tests targeting various index properties were utilized for this, including: Uniaxial compressive strength UCS; Point load index (PLI); Indirect tensile strength (Brazilian test); Joint compressive strength JCS; Joint roughness coefficient JRC; Basic friction angle φ_b; and GSI value estimation. The objective was to recommend reliable Mohr-Coulomb and Hoek-Brown shear strength parameters of the rock mass and along discontinuities, as well as Barton-Bandis parameters along common and worse case (smooth/slickenside) discontinuities. The RMR values, regularly logged along the target section of the decline, were made available as additional indicator for correlating with shear strength properties, while the actual wedge and block failures were used to back-analyse the parameters and correlate with laboratory and in-situ findings. The worse-case analysis for altered conglomerates with smooth joints resulted in c=0.4 MPa and φ=27° , and UCS=40 MPa; GSI= 20, m_i=7 for the rock mass, and c=0 MPa; φ=30° along smooth or slickenside discontinuities, i.e. JRC=2; JCS=12 MPa; φ_r=25°.