In order to study the dynamic behavior of rock support elements, a dropweight test apparatus located at CANMETMMSL research laboratory was used. The test structure, used in the present study, allows for a maximal drop height of 1.5m and a drop weight of 1.5 ton. It is instrumented with high speed cameras and load cells to precisely monitor the response of the rock bolt.A first series of tests on grouted cone bolts revealed two energy absorption mechanisms; the steel plastic deformation and the sliding of the bolt in the resin. These mechanisms are always present, but their proportions vary significantly from one test to the other. Then, tests with various combinations of drop weights and heights showed the influence of those parameters on the preferred energy absorption mechanism and on the maximum amount of energy the bolt can absorb. Also, from the experimental data collected, the energy balance of the system was calculated to assess the validity of the testing apparatus.
An increasing problem faced in underground mines results from seismic events called rockbursts. Considering the dynamic nature of these phenomena it is essential to understand the dynamic behavior of reinforcement systems in order to properly design the ground support and ensure safety. The work presented in this paper is part of a research collaboration between McGill University and CANMET Mining and Mineral Sciences Laboratories (MMSL) in Ottawa. This paper starts by briefly presenting the test structure and its instrumentation along with the advantages and disadvantages of the testing apparatus. Then, the effect of different parameters on the energy absorption capacities of cone bolts will be analyzed based on experimental results. Finally, an experimental energy balance will be performed to assess the validity of the test and of the experimental measurements.
Figure 1 shows the drop weight installation used for all the tests along with some details about its instrumentation. Each rock bolt to be tested is grouted in a sample tube that can be easily connected to the test structure by a pin. The drop weight is lifted and dropped by a remote controlled magnet. During its fall, the mass is guided by two rails installed on each column to avoid introducing significant bending on the rock bolt.
In this study, only modified cone bolts were tested. The bolts have a diameter of 17mm and a length of 2.2 m. One end is threaded over 140mm and the other end is the cone. The cone profile has a maximum diameter of 26.5mm and a length of 55 mm. A domed plate is installed at the threaded end, held by a spherical seat and a nut. Grease can also be added on the bolt to debond it, before grouting it in the sample tube using Dupont Fasloc Resin (so-called Coneloc).
