Abstract
Long term ground movements above a tunnel may continue to increase with time after tunnelling in clayey soils as the tunnelling-induced excess pore water pressures dissipate, whilst the changing earth pressure acting on the tunnel leads to further tunnel deformation during consolidation. Furthermore the tunnel itself introduces new drainage conditions; that is, depending on the drainage condition of the tunnel lining, the effective stresses around the tunnel change with time, inducing further soil consolidation. A seepage rate from low permeability clayey soil is often very small and the groundwater seeping into the tunnel can evaporate quickly. Although a tunnel may look impermeable because the surface looks dry, it is possible that the tunnel drainage conditions are actually permeable. This paper summarises the investigation of soil-tunnel consolidation interaction, particularly focusing on ground surface movements and tunnel lining deformation in the interest of engineering concerns. Analysis results show that tunnel lining permittivity relative to the permeability of the surrounding ground plays an important role on both long-term ground movements as well as tunnel lining behaviour. The findings published in literature are reviewed step by step starting from a single tunnel, twin tunnels to complex cross passage structures. The mechanisms of tunnelling-induced soil consolidation for these structures are identified and, where applicable, possible engineering methodologies to assess the magnitude of long-term ground surface settlements and tunnel lining loads are proposed.
Introduction
In clayey soils, ground movements above a tunnel can continue to build up following construction whilst the tunnel develops a further deformation as the soil consolidates (e.g. Bowers, Hiller, & New, 1996; Harris, 2002; Laver, Soga, Wright, & Jefferis, 2013; Nyren, 1998; O’Reilly, Mair, & Alderman, 1991; Peck, 1969; Shirlaw, 1995). The increasing consolidation settlement may induce surface building damages, for example, as reported by Harris (2002), above the Jubilee Line Extension tunnels in London, whilst long-term tunnel deformation may lead to cracks, water infiltration and differential displacements at risk of derailment for tunnel safety (Shen, Wu, Cui, & Yin, 2014).
Closure and recommendations for future study
Tunnels are often used for decades or even longer. This paper describes various mechanisms of long-term ground movements and tunnel behaviour after tunnelling in clayey soil based on a series of 3D finite element analyses conducted and interpretation of limited field case studies. A single tunnel generates a radial seepage flow around the tunnel during soil consolidation. Twin tunnels and cross passages alter the development of the effective stress and pore pressure around the tunnel with time from the radial seepage condition and therefore complex tunnel-soil interaction occurs. In this paper, the long term behaviour of a single tunnel, twin tunnels and cross passages are discussed with particular emphasis on surface ground movements and tunnel lining behaviour.
