Prediction of instability and ground movements during tunnel construction in non-homogenous conditions

Abstract

Advances in tunneling technology, notably in the widespread use of closed-face tunneling boring machines, have reduced risks and greatly increased the productivity of underground construction projects. For shallow tunnels in 'soft ground', instabilities can create collapse mechanisms that propagate to the ground surface, while the prediction and control of tunnel-induced ground deformations remain a key challenge. These problems are especially significant for 'mixed face' conditions, where the tunnel boring machine encounters soils of contrasting mechanical and/or hydraulic properties. This thesis uses finite element simulations to investigate the control of face stability and ground deformations for shallow tunnels in soft ground. The primary control parameters of interest are the face pressures imposed by earth pressure balance (EPB) or slurry shield machines, and the grouting pressures used to control deformations around the tail void at the rear of the tunnel shield. Stability is evaluated by methods of (c-phi) strength reduction, for a range of ground conditions including i) undrained failure in low permeability clays with continuous and discontinuous (mixed face) strength profiles; and ii) drained shearing in high permeability sands above and below the water table, where the closed-face conditions at the tunnel face provide an impermeable membrane. The numerical results are synthesized into a series of generalized design charts for immediate application. Numerical simulations using 3D finite element models are also used to investigate the factors influencing ground movements for closed-face tunneling operations using a relatively simple elastic-perfectly plastic model of soil behavior. These analyses show that maximum surface deformations above the tunnel can be estimated by simple addition of constituent sources associated with the face pressure, shield shape (conicity), machine weight (buoyancy) and grouting pressure. The research proposes a simplified method for predicting the surface settlements based on analytical parameterization of the numerical results, enabling direct design of parameters for controlling ground deformations associated with tunneling for mixed-face conditions in clays. The proposed methodology for predicting ground settlement has been evaluated using data from three well-documented case studies: 1) Crossrail C300 beneath Hyde Park in London, where tunnels were bored through units of stiff clay; 2) Downtown Line DTL3 (C933) in Singapore where tunnels traversed an interface between stiff Old Alluvium and overlying soft marine clays; and 3) northern sections of the MTRA Blue Line in Bangkok, where tunnels were built below the interface between soft and stiff clays. The comparisons between computed and measured data from these projects show that the proposed generic design method achieves comparable agreement to site specific numerical simulations and empirical methods such as ANN. Further studies are now needed to extend the model for mixed face conditions with contrasting permeability.