Development of a methodology for in-situ dynamic testing of ground support

Abstract: The increasing mining depth leads to higher stress magnitudes, resulting in increased seismic activity and more seismically-induced damage. The effectiveness of the ground support system under dynamic loading conditions has therefore become of prime interest to the mining companies in order to provide safe mining conditions with a minimum of production disturbances caused by unstable infrastructure. The problems of mining-induced seismicity have necessitated the use of ground support systems which are capable of withstanding strong dynamic loads. Although there are large amounts of measurement data from the site-installed seismic systems, they cannot be used directly to design and select the appropriate support systems due to lack of control over the location and nature of the seismic source and the effect of the rock mass on the seismic waves. Large-scale tests using explosives as the seismic source have therefore become a useful method to evaluate the performance of rock support systems for seismic conditions. A series of seven large scale dynamic tests of rock support was conducted in the Kiirunavaara mine. Explosives were detonated in boreholes in the pillar between two cross-cuts in order to generate a dynamic load on the rock support system installed on the cross-cut wall. This was done with the aim to develop a testing methodology for in-situ testing of ground support. Furthermore, the response of the installed support system to strong dynamic loading was also evaluated. The tests included ground motion measurements, fracture investigation, ground and support motion imaging, as well as the deformation measurements. The results of the measurements in Tests 1 to 7 are presented and the methodology used to design the tests is discussed. The results indicated that the relation between the burden distance and the used amount of explosive material and number of blastholes has a vital role in either reducing or involving the effect of detonation gases in test results. The large amount of data recorded during these tests will be useful for the calibration of more advanced numerical models. The energy absorption by the Swellex Mn24, 100 mm fibre reinforced shotcrete (40 kg/m3 steel fibre) and 75 mm x 75 mm weld mesh with 5.5 mm diameter was estimated and compared to that obtained from the large scale in-situ tests and laboratory tests conducted in different countries. The comprehensive ground motion data provided for the whole test wall was used to estimate the kinetic energy transmitted to the fractured zone where the support system was installed.