Dynamic testing of reinforcement systems

The Rock Mechanics Group at the Western Australian School of Mines (WASM) in Kalgoorlie commenced development, design and construction of a unique dynamic testing facility in 2002. The facility is designed to quantify the force-displacement responses of reinforcement systems subjected to dynamic loading. Full scale reinforcement systems are tested with a double embedment configuration to simulate the load transfer action of in situ reinforcement. The facility uses a momentum transfer mechanism. The specimen is dropped on to a reaction surface to rapidly decelerate the reinforcement anchor length while the momentum of a large mass loads the collar of the reinforcement. This test method is quite different from previous testing methods. Evaluation of the results from a two year program of commissioning and processing of data were promising. The facility and the method of data analysis are described in detail by Player et al. (2004) and Thompson et al. (2004). The facility was recognized as the most advanced mining dynamic test facility for reinforcement systems at the time by Brown (2004). A systematic program of testing of different reinforcement systems has been undertaken since 2004. Potentiometers, load cells, accelerometers and high speed video are used to monitor the tests. Software has been developed to analyse the data. The results from the analyses are presented in the form of displacement, velocity, acceleration and force variations with time for all components involved in a test and a force-displacement response and energy absorption capacity for the reinforcement system. The results from the testing program have resulted in the compilation of a database of force-displacement responses to different dynamic loading conditions. The results from a number of the reinforcement systems tested are presented and discussed. Assessments are made in regard to the likely effectiveness of these reinforcement systems when subjected to dynamic loading resulting from the violent failure of overstressed rock. 1.0 DYNAMIC TESTING OF REINFORCEMENT SYSTEMS The WASM Dynamic Test Facility was purpose built to test reinforcement systems, support elements, and their combinations under dynamic loading conditions. The results from the tests are used to assess dynamic force-displacement responses and the energy consumed by the sample during the test. Elements such as cone bolts, cable bolt strand and threadbar were encapsulated with cement grout contained within thick wall steel pipes. Other systems that are sensitive to the mechanical installation (e.g. jumbo installed resin bolts) or the borehole conditions (e.g. friction stabilisers) were driven by a jumbo into simulated bore holes with a rough internal layer. SAIMM, SANIRE and ISRM 6 th International Symposium on Ground Support in Mining and Civil Engineering Construction John Player, Alan Thompson and Ernesto Villaescusa Page 598 1.1 Dynamic loading mechanism A potential may exist for dynamic ejection of failed material when a seismic wave encounters an excavation, see Figure 1. The potential depends on the energy in the wave (e.g. radiated energy and seismic moment), seismic source parametres (e.g. stress drop, corner frequency, and source radius), and site characteristics of the excavation (e.g. degree of fracturing, stored strain energy, and rock properties). The installed ground support (dynamic or non-dynamic capable) only acts to control the rock behaviour after failure; it does not prevent dynamical induced failure from occurring. Figure 1 : Seismic Loading A block of rock could be ejected or fragmented from the surrounding rockmass into the excavation following the encounter of a seismic wave with a susceptible excavation. This detachment process is unlikely to be instantaneous, but rather very quick. It will be related to the seismic wave velocity, amplitude and frequency and / or fracture growth velocity within the rockmass. The non-instantaneous process is suggested because the excitation source is a wave that has velocity, frequency and wave length. Instantaneous and very quick are defined in the context of the data acquisition system at the WASM Dynamic Test Facility. Instantaneous implies faster than the data acquisition rate, i.e. faster than 0.04 milliseconds, and very quick is defined as any time slower than the data acquisition rate. 1.2 Dynamic test facility Figure 2 is a photo of the constructed facility with its primary components identified. The standard reinforcement test performed at the WASM Dynamic Test Facility uses an impact velocity of 6m/s and 2000kg of simulated ejected rock. A simulated discontinuity to allow double embedment testing was typically located 1.0m from the collar. This provides a nominal input of 36kJ of kinetic energy that must be absorbed by the reinforcement system. Drive Ground support scheme