3D Numerical Modeling of Rock Mass Failure in an Uplift Test of a Rock Anchor with Focus on the Role of Rock Joints

Abstract

This paper aims to numerically study the failure mechanisms of the rock mass during the uplift test of a rock anchor installed in limestone. The rock anchor consisted of a smooth tendon, with short threaded sections at both ends. At the distal end of this tendon, a washer and a nut were screwed together to form a plate-anchor. The annulus between the rock anchor and the borehole wall was filled with cementitious grout. The rock mass had three distinctive joint sets. One of them was vertical, i.e., parallel to the anchor axis, while the other two were inclined with a dip angle ≤ 40°. The uplift test was designed so that failure would occur in the rock mass. Geotechnical monitoring data collected from the field was used to calibrate a numerical model with the 3DEC (Three-Dimensional Distinct Element Code) software. This software was chosen because it can simulate the behavior of rock joints. The calibrated numerical model was able to replicate quite accurately the pull force over anchor displacement during testing until the ultimate load and the shallow rock cone failure observed in the field. The model indicates that rock joints start to open at 60% of the ultimate pull load and the key input parameters controlling the ultimate load capacity of the anchor are the tensile strengths of the inclined joint sets.