Measured short-term ground surface response to EPBM tunnelling in London Clay

Abstract

Earth-pressure-balance machines (EPBMs) were used for the construction of Crossrail tunnels in London, providing opportunities for field investigation of consequent ground response. Analysed results from an instrumented research site in Hyde Park with extensive surface and subsurface monitoring arrays are presented and discussed. The Crossrail tunnels at the site are 34·5 m below ground, deeper than those in most case histories of tunnelling in stiff clay in the UK. This paper characterises the tunnelling-induced ground response, both 'greenfield' and in the proximity of the existing Central Line tunnels, dealing with measurements at the ground surface. A companion paper covers the subsurface ground response. Vertical and horizontal ground surface displacements were obtained from manual precise levelling and micrometer stick measurements. Several key findings will benefit future tunnelling projects involving EPBMs. Volume loss values measured at the instrumented site were low, being less than 0·8% and 1·4% for the first and second tunnel drives respectively, higher values being associated with ground softening from the first tunnel construction. Smaller volume losses were recorded in the vicinity of the existing Central Line tunnels, compared with the greenfield location, suggesting that their presence inhibited the development of ground movements. Asymmetric settlement troughs developed due to either the nearby pre-existing tunnels or the construction of the first tunnel. Marginally smaller values of trough width parameter, Ky, were determined for these deeper tunnels compared with previous greenfield ground case histories. Resultant vectors of ground surface displacement were directed to well-defined pointsinks above the tunnel axis level. BackgroundIn major cities like London, constructing new tunnels inevitably influences nearby existing structures above and within the ground. Much has been learnt about the complex interactions between the ground and existing underground structures from new tunnel construction reported through case studies. With advances in tunnelling technologies and practices and construction materials, engineers and researchers continuously endeavour to update and improve their understanding of ground and structural response to tunnel construction. Recently a comprehensive research project 2 has been run by Imperial College London to investigate the effect of tunnelling by modern earth pressure balance tunnel boring machines (EPBMs) on existing tunnels (Standing et al., 2015). Field measurements at an instrumented site in Hyde Park formed an integral part of the overall research project.Short-term tunnelling-induced ground surface displacements are still frequently predicted using empirical approaches (Attewell, 1978). Vertical displacements are estimated using a Gaussian formulation where there are two unknowns: volume loss (VL) and the trough width parameter (K). Of these, volume loss is the more difficult to predict, it depends on various factors such as method of tunnelling, g...

Measured short-term subsurface ground displacements from EPBM tunnelling in London Clay

“…(e) In comparing the greenfield subsurface trough width parameters with other case histories relating to tunnelling in stiff clay, they were generally found to be lower than the range of the earlier field data and less than the relationship proposed by Mair et al (1993). As with the surface values reported by Wan et al (2017), this is attributed to the deeper level of the …”

“…Surface ground displacements were monitored on three instrumentation lines (X, Y and Z); the X-line (not shown in Fig. 1) was about 100 m away from the Y-line (see Wan et al (2017: figure 3; p. 422)). Of these, the Y-line was where most of the subsurface instruments were installed.…”

“…The detailed stratigraphy is shown and described by Wan et al (2017: figure 2) and is also shown in some of the key figures of this paper. The tunnels of the Central Line are within unit B2 of the LCF, while the deeper Crossrail tunnels have their crown in unit B2 and invert just within the more permeable A3ii unit (see King (1981) for a description of the units).…”

“…A typical London Basin stratigraphy is found at the Hyde Park site with made ground and Terrace Gravels overlying the descending sequence of the London Clay Formation (LCF), Lambeth Group, Thanet Sand and Chalk bedrock. The detailed stratigraphy is shown and described by Wan et al (2017: figure 2) and is also shown in some of the key figures of this paper. The tunnels of the Central Line are within unit B2 of the LCF, while the deeper Crossrail tunnels have their crown in unit B2 and invert just within the more permeable A3ii unit (see King (1981) for a description of the units).…”

Numerical investigation of the effects of tunnelling on existing tunnels

“…The adopted soil profile for the finite element analysis is presented schematically in Figure 1 and is based on the deepest of the boreholes sunk to install instrumentation at the greenfield site, HP6 (Wan et al, 2017). In the profile adopted there is a 6-m thick layer of superficial deposits (comprising made ground, alluvium and terrace gravels) overlying units B2, A3 and A2 of the London Clay (King, 1981) which have thicknesses of 30 m, 12.5 m and 11.4 m respectively.…”