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Contribution by J. N. ShirlawThe authors are to be congratulated on the very detailed measurements they have made of surface and subsurface movements during the construction of two parallel, earth pressure balance (EPB)-driven tunnels (Standing & Selemetas, 2013). The extent and detail of the measurements are significantly greater than normal for such projects, and are potentially of great value in developing our understanding of the interaction between pressurised tunnel-boring machines (TBMs) and the ground.In their conclusions, the authors state thatThe pressurised face of the EPBM and tail-skin grouting led to outward displacements and increases in pore water pressure in the near vicinity of the TBM, extending about one tunnel diameter from the extrados.The passage of the shield resulted in movements that were generally away from the tunnel, and positive excess pore pressures over the crown of the tunnel. This is clear in the results presented by the authors. However, there is an inconsistency between those movements/pore pressures and the face and grouting pressures reported by the authors. As shown in Table 3, the reported face and grouting pressures were 44% to 59% of the total overburden pressure, and 57% to 75% of the horizontal in-situ stress. If the pressures applied were significantly lower than the in-situ stresses in the ground, how could those pressures have caused local ground deformations that were generally away from the tunnel, and positive excess pore pressures just over the tunnel crown?

Earth-pressure-balance machines (EPBMs) are frequently used in preference to more conventional non-pressurised tunnel-boring machines (TBMs) to minimise ground movements in the urban environment. The greenfield ground response to EPBM tunnelling in London Clay from a field monitoring research study is presented and discussed. Vertical and horizontal displacements and pore pressure changes were measured with an extensive array of surface and subsurface instruments. Surface settlement troughs observed above the tunnels were small (w max , 10 mm) and can be modelled using the commonly adopted inverse Gaussian curve, but a complex kinematic mechanism took place within the subsurface ground. In the near vicinity of the pressurised closed-face tunnel-boring machine an 'expanding' displacement field was observed, in contrast to the 'contracting' field often observed and associated with open-face tunnelling. This expanding response is dependent on TBM variables, such as face pressure, and should be contemplated when predicting ground movements and assessing subsurface structures such as piles and existing tunnels.

The installation and working test performance of four full-scale instrumented driven piles and their subsequent response to twin tunnels constructed below the pile bases are described. One pair was designed to be largely friction piles and the other pair end-bearing.Their locations relative to the new tunnels were carefully chosen to optimise the understanding of pile response at varying offsets from the centre-lines. The site conditions and the greenfield response to EPBM tunnelling at the site were described in a companion paper that reported an expanding displacement field around the tunnels compared with contracting fields usually observed. Field monitoring results indicate that, during construction, zones of influence exist around tunnels (similar to those proposed by Kaalberg et al., 1999 andJacobsz et al., 2001), where the ground and piles are subjected to different degrees and senses of relative vertical displacement. Redistributions of load along the pile lengths occur as the tunnel boring machines approach, pass beneath and continue beyond the pile bases and lateral pile deflections and bending moments are also induced. Based on the results from this field study, implications for the capacity of existing piles (and design of new piles) subjected to tunnellinginduced movements are assessed for cases of expanding and contracting displacement fields.

The installation and working test performance of four full-scale instrumented driven piles and their subsequent response to twin tunnels constructed below the pile bases are described. One pair was designed to be largely friction piles and the other pair end-bearing. Their locations relative to the new tunnels were carefully chosen to optimise understanding of pile responses at varying offsets from the centre-lines. The site conditions and the greenfield response to earth pressure balance machine tunnelling at the site were described in a companion paper that reported an expanding displacement field around the tunnels rather than the contracting fields usually observed. The field monitoring results indicated that, during construction, zones of influence existed around tunnels, where the ground and piles were subjected to different degrees and senses of relative vertical displacement. Redistributions of load along the pile lengths occurred as the tunnel boring machines approached, passed beneath and continued beyond the pile bases; lateral pile deflections and bending moments were also induced. Based on the results from this field study, implications for the capacity of existing piles (and design of new piles) subjected to tunnelling-induced movements are assessed for cases of expanding and contracting displacement fields.