Consideration of the Factors Impacting on the Design of the Laboratory Short Encapsulation Test for Cable Bolts

Fully grouted cable bolts were first introduced into the mining industry in the 1960s and they have played an increasingly important role in ground support particularly in both underground coal and hard rock mining. While there has much research and development in cable bolt design, the failure of cable bolting system especially slippage at the cable/grout interface is still a significant issue being reliant on the load transfer between surrounding rock mass and cable bolts. In order to better understand this mechanism and evaluate the influence of relevant parameters on the load bearing capacity of cable bolts, various methods have been devised over time including single embedment length test, double embedment length test, and the "split-pipe pull/push" test. The Laboratory Short Encapsulation Pull Test (LSEPT) is perhaps the most recent development in testing methods to assess the load transfer behaviour at the cable/grout and grout/rock interfaces. However, recent work has identified a number of issues with this method as outlined in this paper. Firstly, the diameter effect of the rock sample which is used to confine the grouted cable bolt is studied since the traditional LSEPT in the British Standard is only limited to a relatively small diameter of 142 mm being approximately only three times the diameter of some of the larger cable bolts. As well, the influence of bearing plate on anchorage performance due to similar dimensions of the hole size is considered, proposing an appropriate parameter dimension for the bearing plate. A modified version of the test method is proposed and is presented in this paper. This paper outlines the results of a study on some of the design parameters that formed the basis of the new modified test design.