Analyses of shotcrete stress states due to varying lining thickness and irregular rock surfaces

Abstract: Shotcrete is sprayed concrete applied pneumatically under high pressure and was invented in the beginning of the 1900's. Soon after, attempts were made to use shotcrete to support rock tunnels. This new technique decreased the construction time and when steel fibres were introduced in the shotcrete during the 1970's, the heavy labour work of placing ordinary mesh reinforcement could be reduced. Since then, shotcrete has been the primary support method for tunnels, especially in hard rock where thin layers of shotcrete can be used as the only reinforcement.Tunnels are normally excavated using the drill and blast method which creates a highly irregular rock surface on which shotcrete is applied. While spraying, the actual applied thickness is hard to determine and the final shotcrete lining will therefore have a varying thickness. Depending on in situ conditions, unreinforced or fibre reinforced shotcrete in combination with rock bolts can be used as rock support. The structural behaviour as well as the loads acting on the shotcrete lining depends on the interaction between the shotcrete, rock and rock bolts. There are several parameters influencing this interaction, e.g. bond strength, the stiffness of the rock and thickness of the shotcrete. All of these parameters are difficult to predict accurately which makes the structural design of the lining to a complex problem.This thesis present the first part of a research project with the long-term goal to improve the understanding of the structural behaviour of the shotcrete lining. To achieve this, numerical modelling have been used to study the build up of stresses and cracking of shotcrete when subjected to restrained loading caused by e.g. temperature differences and drying shrinkage. The response in the lining when subjected to a gravity load from a block has also been studied. A numerical model for the analysis of shotcrete stresses is presented in which time-dependent material behaviour. Furthermore, the model is capable of describing the non-linear deformation behaviour of both plain and fibre reinforced shotcrete and uses presented in situ variations in thickness to more accurately account for the effects of expected variations in thickness. The thesis discuss and demonstrate the effect of important loads that acts on the shotcrete lining and how the irregular geometry of the rock surface in combination with the varying thickness of the shotcrete affect the development of stresses in the lining. It is also discussed how a full or partial bond failure affect the structural capacity of a shotcrete lining.