Abstract
Tunnel construction is vital for the development of urban infrastructure systems throughout the world. An understanding of tunnelling-induced displacements is needed to evaluate the impact of tunnel construction on existing structures. Recent research has provided insight into the complex mechanisms that control tunnelling-induced ground movements in sands; however, the combined influence of relative tunnel depth and soil density has not been described. This paper presents data from a series of 15 plane-strain centrifuge tests in dry sand. The cover-to-diameter ratio, C/D, of the tunnels ranges between 1·3 and 6·3, thereby including relatively shallow and deep tunnels. The sand relative density varies between 30 and 90%, corresponding to loose and dense soils. The effects of C/D, soil density and volume loss on vertical and horizontal soil movements, shear strains and ground reaction curves are discussed. Analysis of surface and subsurface settlement trough characteristics shows that the mechanisms are non-linear and the effects of soil relative density and volume loss on deformation patterns are highly dependent on C/D. The role of soil arching in the definition of the displacement mechanisms and a discussion of the implications of the results to the assessment of damage to existing structures are also provided.
Highlights
Current needs for infrastructure development in urban areas require the construction of new tunnels
In contrast to the trapdoor results, it was shown that the greater the cover-to-diameter ratio (C/D) and/or the looser the soil, the higher the tunnel volume loss corresponding to the minimum tunnel pressure
(c) The new data for loose and medium dense sand showed a similar pattern to previously published data for dense sand (Marshall et al, 2012) in that settlement trough width increases with C/D, whereas it decreases with an increase in tunnel volume loss
Summary
Current needs for infrastructure development in urban areas require the construction of new tunnels. Because the excavation of new tunnels results in ground movements that affect existing surface and buried structural systems, engineers need to estimate the magnitude and distribution of greenfield displacements. Ground movements due to greenfield tunnelling in undrained clay are generally well understood (Mair et al, 1993; Mair & Taylor, 1997; Grant & Taylor, 2000). Mair & Taylor (1997), who reported a typical range of surface settlement trough width parameter of 0·25–0·45, but values as high as 1·0 were provided). The causes for this variability are not well known; this is one of the motivations behind the research presented here
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