Rock Behaviour near the Tunnel Face

For tunnels it is not only important, how the tunnel and the surrounding rock behave during the life time of the tunnel. It is also very meaningful to understand how the tunnel is behaving, while it is under construction. The basis for this thesis is to understand, how the tunnel behaviour, concerning deformation and stability, is for different rock conditions during the excavation process and what its influence is on the tunnel in final state. Additionally a case study is made of a praxis-example in Switzerland, in which it is pointed out, how difficult circumstances can be during the excavation and what measurements had to be done to cope with all the problems of excavating the tunnel. This example emphasises once more, how important a good understanding of rock behaviour during the excavation is. The report is built up in three more or less different sections: Chapters 2 to 4, chapters 5 and 6, and chapter 7. Chapters 2 to 4 form the first part, that is functioning as a basis for the calculations and analysis made in chapters 5 to 7. An analytical analysis about stress-deformation relations together with basic rock mechanic principles is worked out in chapter 2. Due to the complexity of the rock many rock classifications have been made to qualify different rock classes with their corresponding behaviour. One of these rock classes is used to obtain basic rock parameters, with which calculations are performed. This is, together with an description of different types a specific rock behaviour, outlined in chapter 3. An analytical approach for face stability problems is presented and further worked out in chapter 4. Chapters 5 to 6 are the main part with the calculations and the analysis, in which chapter 5 deals about the aspects, concerning the tunnel ring, and chapter 6 deals about the aspects, concerning the tunnel face. Basic rock calculation is given, about how the rock responds to the excavation of a tunnel. This calculating formed the basic for further calculations and research. The influence of different tunnel constructing aspects have been computed and compared with the basic rock-behaviour. In this way a good impression is obtained about how different constructing aspects, such as lining -constructed far away and close to the face-, pre-tunnelling, sequential excavation and rock improvement are performing in respect to stability and deformation of both the tunnel face and the tunnel lining. Chapter 7 is the case study of the Adler tunnel in Switzerland. Rock mechanical problems are analysed and described. It is also recapitulated, what measurements have been carried out to deal with the risen problems and how successful they were. The result of all these calculations for behaviour of the tunnel ring is, that lining gives an extensive reduction of the radial inward movement. The performance of the lining is at its most in the weakest rock and when the lining is constructed close to the face. The addition of a pre-tunnelled construction improves the performance in weaker rock. The deformation is further reduced. This is however not the case in good rock. In those cases pre-tunnelling should not be used, as it is a waste of time and money. An extra aspect of these performances is, that by reducing the lining the pressure on the lining is increased. The lining has to be dimensioned on these higher pressures. The most important aspect for the tunnel face is stability. Pre-tunnelling is not an option to stabilise the tunnel face. Grouting may introduce inadmissible tension stresses in the grouted area, which nullify the desired stabilisation. A good option for making the face more stabile is to excavate in sequences, because decreasing the diameter means a high reduction of the risk for face collapse. A stabile tunnel face is very much depending on the diameter of the tunnel, there where the depth of the tunnel determines the amount of plasticity around the tunnel face. Readers who are interested in typical geo-hydrological aspects of rock tunnelling, shield tunnelling, such as EPB and Slurry Shields, with counter pressures on the tunnel face, typical lining aspects, such as differences between different linings or the improvement of the lining strength in time, or time dependent behaviour of the rock are kindly requested to read other papers or reports. Those aspects, although very interesting and highly important, are not a part of this report.Hydraulic EngineeringCivil Engineering and Geoscience