Behaviour and stability of shallow underground constructions

Sammanfattning: The growing needs of urbanization, like more transport possibilities, sewers, maintenance tunnels, the need of constructing shallow tunnels increase. By a better understanding of the mechanical properties and state of stress in the shallow parts of the bedrock it may be possible to determine which factors that are of importance at a specific construction site, and thereby increase the cost efficiency of pre-investigations and the tunnel construction. The objective of this thesis is to identify and describe the important factors that influence the behaviour and the stability of shallow seated tunnels. This knowledge will make it possible to determine which data that needs to be collected and the accuracy needed to be able to make reliable analyses. Shallow tunnels, in this report, are tunnels that have an overburden of less than 0.5 times the tunnel diameter. This thesis consists of a literature review that contains studies of the mechanics and design of shallow tunnels. Furthermore it contains conceptual numerical analysis as well as numerical analysis of a real case. The conceptual analyses included factors regarding rock mass strength, virgin state of stress, the location of the tunnel in the rock mass and geological structures. The results show that the most important factor concerning the stability is large geological structures. And moreover it is the dip angle of the structure that is most significant when assessing the stability; steeper dip angles are generally more favourable. Other factors that can be considered as more important are the virgin state of stress and rock mass strength (especially GSI). And since failure was primarily in the form of tensile yield, the tensile strength is another important strength parameter. As important as the tensile peak strength, is the post failure properties of the tensile strength. From this, a check list was made where the significance level that the different factors have the behaviour and stability of a certain case. The real case analysed in this thesis is a section of Arlandabanan, Shuttle station 2. It is a shallow tunnel that is seated underneath Terminal 5 of Arlanda airport. Other factors that were present in the real case was a clay filled structure, and weaker layers in the mica schist, which made up the rock mass. The comparison between measured deformation and results from the numerical analysis was partly contradictory. Probable reasons are overestimation of the significance of mica schist orientation and the clay filled structure as well as the conclusions of the virgin stress drawn from the stress measurements. It also shows that steep structures may be vulnerable combined with surface loads and low horizontal stresses.