Design of Anchors in Horizontally Jointed Rocks

Abstract

A series of ten field pullout tests was performed on vertical cementgrouted rock anchors. The rock was an unweathered moderately horizontally jointed granite. In five of the tests the anchor bars were instrumented with strain gages. These testing results, together with rock surface vertical deflection measurements and visual inspection of a 2‐dimensional model of an anchor grouted in similar rock, indicate that as the pullout load increases, the rock bulges upward to a distance roughly equal to the anchor depth, with simultaneous formation of tensional cracks along the joints and separation of the blocks along horizontal joints. Based on these observations and other testing results, this design method is proposed for rocks with horizontal and vertical joints based on the idealization of the discontinuous rock mass in an advantageous way, so that the pullout capacities can be predicted from empirical values of certain variables. More specifically, the pullout capacity of a certain anchor in a certain discontinuous rock can be estimated based on empirical values from past experience of the probable total anchor deflection at failure.