TBM Tunnelling at the Stillwater Mine
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When considering the use of TBM as an alternative to conventional Drill & Blast tunnelling, it is of great importance to be able to give thorough and realistic performance predictions. Errors in prediction could be fatal for hard rock TBM projects, as these have a high geological risk and also demands high investment costs. This thesis is meant to provide field data to be used in the work of updating the NTNU prediction model. The effect of RPM on the rock breaking and TBM performance, as well as the effect of the rock mass fracturing have in particular been investigated. As a final purpose of this thesis laboratory testing has been performed to find the Drilling Rate Index and the Cutter Life Index of rock material. The field study of this thesis has been done at the Stillwater Mine in Montana, USA.The findings of the RPM trials conducted seem to support the notion that running the TBM at a lower RPM gives a higher basic penetration in millimeter per cutterhead revolution, and that the rock breaking process is more efficient. The effect on the rock breaking process may be explained by how the loading rate between cutter and rock surface is influenced by the speed the cutterhead rotates with. Through analyzing shift logs from the TBM in areas of high rock fracturing it has also been found that lowering the RPM will give a higher basic penetration. In this case it is however not certain if this is due to how the rock break off the surface, or if it is caused by the fact that the penetration rate in areas of high rock fracturing is limited by the muck removal capacity of the TBM conveyor. Through analysis of collected core sample data from probe drilling ahead of the TBM, it has been found that the rock mass fracturing has a strong correlation with the TBM penetration rate. It was also found that the rock mass fracturing highly influences the applicable thrust force of the TBM and thusly also has an indirect effect on the TBM performance. More work should be done at the Stillwater Mine regarding geological back mapping of the tunnel, in order to further assess the importance of the rock mass fracturing factor ks.A full laboratory testing of rock samples from the tunnel at the Stillwater Mine has been performed. Rock material in the form of both rock chips from the tunnel face and core samples from probe drilling have been tested. It was in the course of this testing observed that the Sievers' J test results are greatly different in rock chips compared with core samples. This is most likely caused by difference in mineralogy in the samples, and the fact that rock chips contain micro fractures from the rock breaking process. More work on assessing this difference should nonetheless be done.