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?

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As part of the construction of the Singapore Mass Rapid Transit, two tunnels had to be driven from a shaft to Raffles Place Station. Each drive was approximately 170 m long and was mostly in the Singapore ‘Boulder Bed’. This formation underlies much of the Central Business District of Singapore, and is thought to be a colluvial deposit comprising sandstone boulders in a stiff clay matrixThe first 60 m of the upper tunnel had a mixed face with an old coral reef above axis and the Boulder Bed below it. The corals and sands of the reef were highly permeable and had to be injected with chemical grout before tunnelling commenced.The Raffles Place area is in the commercial heart of Singapore and the tunnels passed close to two high-rise buildings. One of these buildings had an underground car park within 5 m of the upper tunnel. Shield drives in the Boulder Bed had recorded high surface settlements, and there was concern about the underground car park which was known to be sensitive to settlement. It was proposed to drive the tunnels using the New Austrian Tunnelling Method, and one of the justifications for choosing the technique was that the resulting soil movements could be minimized. In order to check this claim surface settlement arrays, inclinometers and extensometers were installed to monitor movements caused by the tunnelling.Records of ground movements measured are presented, and compared with the movements recorded above shield driven tunnels in the same deposit. Methods of grouting and testing used in the coral and sands are also reported.

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