Quality Inspection And Support Parameter Optimization Of Shotcrete Support In Dahongshan Copper Mine

Dahongshan Copper Mine is one of the most important copper mines in Yunnan Copper Industry Group. Today, the length of the underground tunnels excavated each year in this mine has been as high as40000m to60000m, with460000to720000m3in volume. Meanwhile, over70%of the tunnels excavated has to be supported, and the support fee has exeed dens million yuan per year. Further more, problems are still exist, on the way of both checking the quality of the shotcrete and determining the support parameters.In this paper, concrete samples have been made with the sand and cement that used in the mine. Labarotary tests with these samples are carried out to detenmine the relationship between the compressive strength and the ratio beteen the sand and cement of the concrete. Before the strength tests, some other tests have also been performed to find an effective way of determining the strength of the shotcrete in the supporting of underground tunnels, with non-destructive techniques such as Point load test, Rebound method, and Nail injection method. It was found from the tests that the injection method has some advantages such as higher accuracy and easier of conducting, and could be appled to examine the quality of the shotcrete in the tunnle excavation.Samples with the size of45×35×12cm have also been made in mine tunnels with directly the way of shotcrete, then cut them into the samples of10×10×10cm. Nail injection and compressive tests were carried out with the samples. The results show that the compressive strength of the shotcrete in the mine can hardly reach20MPa, the former demanded strength value, since it needs too much cement in the shotcrete. If reduce the compressive strength to15MPa, it would be realized easy by a cement-sand ratio of4to4.2. A formular was also obtained to calculate the strength by the way of nail injection.Samples have also made stochastic from mine shotcreting to check the practical shotcrete strength. It was found that the strebgths of the most samples are among5MPa to15MPa. Therefore, it is necessary to examine the real strength of the shotcrete.Software FLAC3D was then applied to simulate the supporting effectiveness and influences from some supporting parameters, such as the thichness and strength of the shotcrete. Analizing was perfomed on the displacements and plastic zone as well as stability conditions around the tunnels when the thickness and strength of the shotcrete have different values. The results show that the shotcrete with the strength of15MPa and a thickness of100mm is an optimum choce for the tunnels when the stability of the surrounding rocks are relatively poor.From the results both of tests and numerical simulation, it could be concluded that shotcrete having the strength of20MPa is difficult to be realized, while the strength of15MPa is possible and such a strength value is enough in pracatical mine supporting.