Laboratory block model tests simulating rock anchoring in rock mass

Abstract

Rock anchors are used to stabilise large scale infrastructures. Failure of rock anchors could threaten the stability and lead to severe economic and social consequences. The design of rock anchors is based on four principal failure modes, which are breakage of the steel tendon, tendon-grout interface failure, grout-rock interface failure and rock mass failure around the anchor. The current dimensioning methods are uncertain due to limited knowledge of the grout-rock bond strength and the rock mass failure. The dimensioning method against rock mass failure is based upon the weight of the overlying rock mass in an inverted cone and the shear strength of the rock mass along the inverted cone, which results in conservative design of rock anchors. A small laboratory model was developed to evaluate the rock mass failure of a rock anchor at maximum pullout load. The model contained small concrete blocks set in a staggered pattern representing the rock mass. The loading of the model showed that the load was transferred to the sides by rotation and lifting of the blocks, which created a small load bearing arch in each layer during loading. This resulted in the model being able to carry much higher loads than what was calculated with the dimensioning methods. Back calculation of the results from the physical tests in UDEC showed the same arching effect in each layer during loading. The testing has improved the knowledge on the concept of how the load transfer from a rock anchor to the rock mass.