Grout and bentonite flow around a TBM: Computational modeling and simulation-based assessment of influence on surface settlements

Adequate consideration of the various interactions between the Tunnel Boring Machine (TBM) and the surrounding underground is a pre-requisite for reliable prognoses in shield supported tunneling based upon numerical analysis. In addition to face support and the grouting of the annular gap the contact conditions along the shield skin between the moving TBM and the surrounding, deforming soil constitute the most relevant component of TBM–soil interactions in mechanized tunneling. This paper is concerned with the analysis of the interface conditions between the shield skin and the soil and its adequate numerical representation in the context of a process-oriented numerical simulation model for mechanized tunneling. The situation around the shield skin is influenced by the design of the Tunnel Boring Machine, the deformational behavior of the surrounding underground and by a possible inflow of process liquids into the steering gap. A novel simulation method is proposed which allows to model the viscous flow of the process liquids into the steering gap and its interactions with the face support, the tail void grouting, the deforming soil and the moving TBM. The proposed numerical model for the TBM–soil interaction is part of a recently developed three-dimensional, process-oriented finite element model for shield tunneling (Nagel et al., 2010). It allows to investigate the effects of the inflow of process liquids into the steering gap during TBM advance considering realistic machine-related and geological conditions. It is, in particular, capable to compute the pressure distribution within the developing liquid film in association with the face support and grouting conditions and to predict its influence on the surface settlements and the overall TBM–soil interaction affecting, e.g. the hydraulic jack forces or shield deformations.

Research highlights
► A process-oriented computational model for shield tunneling has been extended by a novel contact algorithm which allows for the consideration of an inflow of pressurized process liquids into the steering gap and its impact on surface settlements.
► Assuming viscous behavior of the process liquids, a finite difference scheme used for the spatial discretization of the governing equations for the longitudinal fluid flow is coupled with a surface-based contact algorithm for frictional contact between the TBM and the surrounding soil.
► The presented numerical simulations, have shown, that the flow of the process liquids within the steering gap affects the steering gap widths, the position of the TBM within the steering gap and the jacking forces as well as the shape and depth of the settlement trough.
► Neglecting the influence of the flow of process liquids within the steering gap may lead to overpredictions of the tunneling induced subsidence.