Mechanical Properties of Intact Rock and Fractures in Welded Tuff from Newberry Volcano

Abstract

In this paper we present the results of a testing program to characterize the rock mechanical properties of welded tuff from Newberry Volcano. The rock samples used in this work are four drill cores from the GEO-N2, GEO-N1, and Oxy-72 wells on the western flank of Newberry Volcano. Multistage triaxial compression tests were performed to determine Young’s modulus, Poisson’s ratio, and failure envelop. In addition, multistage triaxial shear tests were performed to determine the mechanical properties and shear strength of the fractures developed in triaxial compression tests. Joint roughness coefficient (JRC) and Joint Wall Compressive Strength (JCS) were obtained through back-analysis of the shear tests. It was found that the JCS of tested joints are larger than the intact rock Uniaxial Compressive Strength. The joint surfaces were characterized by a laser profilometer to correlate the surface roughness profile to the JRC from back-analysis of experimental data. Joint normal stiffness and shear stiffness were estimated and it was observed that a higher confining pressure results in higher joint shear stiffness. The stiffness is gradually reduced as the contact surfaces become smoother with additional shear displacement.