Time dependent deformations during tunnelling and stability of tunnel faces in fine-grained soils under groundwater

Abstract

The measures required for driving a tunnel below the groundwater table depend on the permeability of the soil. In coarse-grained, highly permeable soils additional measures, for example compressed-air support combined with a reduction of the permeability of the soil, e.g. induced by grouting, are necessary. Compared to this, it is possible to do without such measures in fine-grained, cohesive soils because of the increased short-term stability of the tunnel face under undrained conditions. In this publication the results of 3-dimensional finite-element calculations are presented to show the influence of the permeability of the soil and also the rate of the tunnel driving on the deformations around the tunnel as well as on the ground surface. The calculated deformations can furthermore be considered as an indicator for the time dependent stability of the tunnel face due to a higher redistribution of stresses and by that an enlargement of the plasticized zone. Usually the stability of the tunnel face is reduced by the presence of water because of the flow of water towards the tunnel. In low permeable soils undrained conditions prevail immediately after an excavation step. In this case relatively high stability-ratios may occur. The stability of the tunnel face will be reduced with increasing time until reaching the lower boundary of possible values, possibly leading to failure. If calculations are done under the assumption of drained conditions, the real stability of the tunnel face during construction may substantially exceed that of the calculated one. On the other hand, if calculations are done for undrained conditions, the effective stability may lie on the unsafe side [10]. There is therefore a big demand to optimize the method of investigating deformations around the tunnel, so as to ensure a safe tunnel excavation on the one hand and to guarantee a cost-effective process on the other. In this paper the tunnelling process is modelled by a step-by-step excavation under atmospheric conditions. The soil is described by a material model which distinguishes between primary and unload-reload stress paths and also accounts for stressdependent stiffness parameters. The failure criterion is described by the Mohr-Coulomb criterion that considers cohesion, friction angle and angle of dilatancy.

Study on Instability Mechanism and Support Scheme of the Tunnel Face in Carbonaceous Phyllite Stratum under High Geo‐Stress

“…e comprehensive FEM study on the face stability of shield tunnels was carried out by Zhang et al [47,48], who used stepwise-reduced support pressure to achieve collapse failure at the tunnel face and took the critical support pressure to judge whether the face had been damaged. Hö e et al [49,50] studied the stability of the tunnel face through the deformation development law of the surrounding rock at the face and determined whether the face is unstable according to the deformation value of the face. ese studies indirectly described the instability of the face through a certain quantitative index but could not reverse the essential phenomenon that the rock mass at the face fails and breaks away from the matrix due to excessive deformation.…”

Spatial estimation of geotechnical parameters for numerical tunneling simulations and TBM performance models

“…(13). The term ''simulation cycle'' refers to the swapping process by using the same simulation annealing parameters.…”

“…For example, we may refer to the recent paper [13] investigating the effect of permeability on the deformations of shallow tunnels; also to the paper [12] that investigates the effect of the nature of constitutive model of the soil on the ground deformation behavior around a tunnel. However, past studies [6,7,11,21] have shown that inherent variability of soil or rock masses viewed at the scale of the core or the scale of elements needed for numerical simulations (mesoscale) can produce ground deformation behaviors that are qualitatively different from those predicted by conventional deterministic analyses with averaging of mechanical properties over large volumes such as this of an entire geological unit (macroscale).…”

The construction stability of large section tunnel considering the deterioration of clay mechanical properties

“…Theories and methods of stability analysis of large-section clay tunnels are also based on the stability of ordinary tunnels (Höfle et al, 2008;Lü et al, 2020;Song et al, 2021;Xue et al, 2023b). The tunnel stability problem is derived from the difficulties encountered in the construction of underground projects (Huang et al, 2019;Liu et al, 2020;Kumar and Sahoo, 2021;Zhang et al, 2022b).…”