Greenfield tunnelling in sands: the effects of soil density and relative depth

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.

Centrifuge modelling of tunnelling induced ground displacements: pressure and displacement control tunnels

“…To investigate settlement trough shape characteristics, modified Gaussian curves (Vorster et al, 2005) were fitted to the settlement data using the approach of Marshall et al (2012). Marshall et al (2012) indicated that the conventional Gaussian curve does not always provide a good fit to settlement data for tunnels in sand, which was substantiated by several other studies (Vorster et al, 2005;Marshall et al, 2012;Zhou et al, 2014;Franza and Marshall, 2018;Franza et al, 2019a). The goodness of fit (based on the coefficient of determination R 2 ) of the modified Gaussian curve to vertical displacements is shown in Figure S1 of the supplemental data.…”

“…This over-prediction is mainly due to the fact that the numerical analyses were not able to accurately replicate the localised zone of vertical settlement close to the tunnel crown, with tunnel boundary displacements therefore propagating through the soil towards the surface more in the numerical analyses than in the centrifuge tests. As suggested by Franza et al (2019a), for dense sand, the arching mechanism has an influential impact, and the soil tends to dilate below the developed arch. However, in the numerical simulations, soil dilation was not accurately replicated, which affected the ability of the numerical models to replicate soil displacements.…”

“…Note that for an undrained case (zero volumetric strain), V l,s,surf = V l,t . As discussed in Marshall et al (2012) and Franza et al (2019a), for dry sand (or drained soil conditions), soil volume loss will differ from tunnel volume loss due to the dilative/contractive nature of the soil, which depends on the relative density of the soil, magnitude of the shear strain, and the confining pressures during shear. At a given soil depth, if the value of V l,s,surf is lower than V l,t , the soil below that level has undergone an overall dilative response.…”

Centrifuge Study on the Influence of Tunnel Excavation on Piles in Sand

“…Centrifuge testing has been widely accepted as a tool to investigate pile shaft shearing mechanisms (Bruno, 1999;Nicola and Randolph, 1999;Klotz and Coop, 2001; White and Lehane, 2004). In recent years, tunnel-pile-structure-interaction (TPSI) problems have been investigated using geotechnical centrifuge testing (Loganathan et al, 2000;Jacobsz, 2003;Lee and Chiang, 2007;Marshall and Mair, 2011;Franza et al, 2019). In these centrifuge tests, individual piles or a group of piles connected to a rigid pile cap have been used to investigate tunnel-pile interaction mechanisms during tunnelling, neglecting the effect of a connected structure, which may impact the load transfer between piles during tunnel volume loss, therefore changing an individual pile's resistance or load distribution.…”

Predicting the Ground Movement Above a Tunnel in Chittagong Coastal Area of Bangladesh Under Seismic Loading

“…Most of tunneling analysis(Franza et al, 2018, Pescara et al, 2011, Franza and Marshall 2015 has been carried out based on plain strain consideration which has not included longitudinal settlement profile of tunnel. A few researches have been expressed in longitudinal settlement above a tunnel based on field observations Attewell and Woodman (1982).…”