System behaviour in weak ground: comparison of yielding elements

摘要

When constructing long base tunnels passing various geological formations and tectonic faults, advance through weak ground under high overburden frequently occurs. Loads unsustainable by a stiff support concept, support damage and very costly re-profiling works are a frequent consequence.Various approaches for a ductile support system appropriate for such geotechnical conditions have been proposed, developed and applied. The ductility of a shotcrete lining is usually ensured by integrating yielding elements. In order to prevent overloading of the young shotcrete, the elements have to fulfil the requirement of low initial stiffness, preventing mobilization of excessive thrust in the initial deformation stages. On the other hand, the capacity of the shotcrete lining should be maximally utilized, after it has developed sufficient strength. No objective comparison between currently available yielding element types has been conducted. In order to examine the system behaviour with different yielding element types, a numerical parameter study in FLAC3D has been conducted. The separate mobilization of cohesion and friction has been captured by the strain softening constitutive law. The parameter calibration has been performed on the basis of direct shear and triaxial laboratory tests.The rheological behaviour of shotcrete is simulated by using Norton's power lawcreep model and continuous time and strain-history-dependent updating of shotcrete properties via FISH routines. The numerical calculation procedure thus switches hence and forth between mechanical calculation (initial equilibrium after round excavation) and viscous calculation, modelling the creep and shrinkage processes in the young shotcrete. The comparison is conducted with regard to displacement development and shotcrete utilization.