gaskets, one synthetic and one hydrophilic (see below). The tunnel boringmachines used for the tunnelling on this project were slurry machinesdesigned to be used in mix-shield mode, whereby the face would besupported by a combination of pressurized slurry and a balancing air bubble.This had two advantages for the tunnel lining segments. The first was thatthe bentonite in the area behind the cutterhead (plenum) of the machinehelped to reduce the jacking pressures on the segments and resulted in verylittle jacking damage. The other advantage was that the cutterhead torquewas greatly reduced enabling a better control of the roll of the rings, thusavoiding any shearing failure of the bolts in the previously constructed ring.

WATERPROOFING

Waterproofing is important in tunnel lining construction to prevent excessivewater flow into the tunnel. This is a particular problem if the tunnel isconstructed below the groundwater table where it can act like a drain.Waterproofing of segmental linings has traditionally been by the use ofcaulking (applying a sealing material to the inside of the lining at the joints),but these days is generally achieved by the use of preformed gaskets. Thereare two basic forms (BTS/ICE 2004, King 2006):

compression seals these are manufactured from man-made rubbers(ethylene-propylene-diene monomer (EPDM) or neoprene) and arefitted around individual precast concrete or SGI segments;

hydrophilic seals these are made from specially impregnated rubbersor specially formulated bentonite compounds that swell on contact withwater.

These waterproofing systems are not used for waterproofing the segmentsthemselves, but to prevent water from penetrating between adjacent seg -ments. The gaskets require a compression force to be applied to the liningas it is erected (compression seals more so than hydrophilic seals), whichcreates a line load on the segment that needs to be considered in the design.

TOLERANCES OF SEGMENTS

This needs careful consideration as they have practical implications for theconstructability of the ring and performance of the gaskets. Herrenknechtand Bppler (2003) recommend the following dimensions/tolerances:

segment width 0.6 mm; segment thickness 3.0 mm; segment length 0.8 mm; longitudinal joint evenness 0.5 mm; ring joint evenness 0.5 mm; cross-setting angle in longitudinal joints 0.04; angles of the longitudinal joint taper 0.01.

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