Numerical modelling of shotcrete for tunnelling

Abstract

Shotcrete is a special type of concrete which was invented at the beginning of the 20th century and is nowadays an important support element for tunnels constructed with the New Austrian Tunnelling Method (NATM). Immediately after tunnel excavation shotcrete is sprayed onto the tunnel walls at high pressure in order to provide temporary support. Unfortunately, in the past very little attention has been given to the development of sophisticated material laws for shotcrete, since its design and application was mainly based on experience. However, current design practice, such as that applied in the design of the new Crossrail tunnels, requires sophisticated modelling of shotcrete behaviour in numerical analysis of tunnel-soil interaction. Such models are not readily found in the literature, as they have been developed mainly for structural, rather than geotechnical applications. In this thesis, a constitutive model for the time-dependent behaviour of shotcrete has been developed within the framework of elasto-plasticity. Two independent yield surfaces control the behaviour of shotcrete in multiaxial loading conditions for both compression and tension. The model formulation is based on strain hardening/softening plasticity, where the expansion and contraction of the yield surfaces are governed by normalised plastic strains. Cracking of the shotcrete is considered within the smeared crack concept. Furthermore, the proposed material law includes the time-dependency of stiffness and strength behaviour. The reducing deformability of the shotcrete during cement hydration has been taken into account. The model has been extended to account for creep, shrinkage and hydration temperature induced deformations at early shotcrete ages. After a robust implementation into the Imperial College Finite Element Program (ICFEP), model calibration and validation have been performed for shotcrete experiments taken from the literature. The developed constitutive model has then been applied to the analysis of a typical tunnel construction in London Clay, modelling the early age material properties of the shotcrete tunnel lining in detail for various excavation schemes. Finally, it has been shown that the proposed constitutive model is capable of reproducing the complex behaviour of young shotcrete at early ages and can be applied successfully to boundary value problems in geotechnical engineering.