Non-destructive testing of rock bolts for estimating total bolt length

•An evaluation of the potential of existing NDT methods for estimating the total length of rock bolts is presented.•Existing dynamic testing methods used for load detection need further development to provide total length estimation•The numerical model shows that higher natural frequencies change with the total length change•The model shows that a short impulse generated by the NDT device is important when higher frequencies need excited.

This paper explores the use of dynamic testing of rock bolts, currently used for collar load estimation as a means for indirect measurement of the effective installed bolt length, in order to determine potentially broken bolts. Both experimental and numerical studies were undertaken. A number of rock bolts of different total length were installed in a concrete block. These bolts were assessed using non-destructive testing techniques commonly used to assess prestress load. Initially a system based on the application of an impulse load (GRANIT) was used, followed by a periodic (swept sine) loading using a magneto-strictive shaker. A dynamic model, developed previously, was used to assist in interpretation of the results obtained from the experiments. Analysis of the data obtained with the two testing methods is undertaken and the results compared with those from the dynamic model. The results suggest that both methods give similar responses but also that both excitation methods need to be altered to allow effective fixed/total lengths to be estimated. The numerical model used in this study confirms the findings from the experiments and gives a frequency range which, if excited can provide data which can be related to effective total bolt length. This length could be potentially different from the original design due to either incorrect installation or bolt breakage.