A parametric study of irregularly shaped shotcrete shells

Abstract: The shape of a tunnel wall after the task of blasting and drilling is typically irregular. After the tunnel wall is sprayed with shotcrete, the resulting shotcrete surface will also be irregular and have a big variation in thickness due to uneven shotcrete coverage. Today, most design models assume that the shotcrete surface behaves as a concrete slab. Research has proved that this is not the case and this report investigates the behaviour of an irregular shotcrete lining and is a pre-study to further research. The existing literature on the subject is reviewed to gain an understanding of the important parameters that affect the shotcrete shell. In addition to the theoretical research, discussions have been held with different industry actors to gain an understanding of the practical problems and how shotcrete is presently done. The relevant parameters that influence the behaviour of a tunnel lining are discussed and the quantitative and qualitative importance of each parameter is estimated based on previous research. Some of the practical problems that arise when a shotcrete lining is constructed are discussed. These problems complicate the design in many ways, and makes shotcrete design very different from ordinary structural design. There is also a brief review of how shotcrete design is carried out in Sweden. The behaviour of a shotcrete rock support system subjected to several load combinations using different models that simulate irregularity and variation in thickness is analyzed. This is a further development of an existing concept and confirms some of these conclusions and poses additional questions. Finally, the results from the earlier chapter are compared and discussed. The results show that the influence of the irregularity is highly dependent on the shotcrete thickness and shape and that the placement of the rock bolts is very important. Several other aspects are discussed as well. Finally suggestions for further research are given, especially the need for a 3D-model where the interface strength is included.