ABSTRACT

New construction is usually of circular crosssection in contrast to much of the older Victorian 'egg-shape' cross-section. The circular section lends itself to easier construction; the egg-shape, with smaller radius at invert, encourages higher gravity flow velocities, and better cleansing, at low volume (dry weather) flow rates. More modern techniques of lining are usually by reinforced concrete segments for the primary lining, with a cast-in-situ concrete permanent lining. Older sewers were lined in high quality durable brick, typically of Accrington character, and a mortar having lower durability. Sometimes for new sewer tunnels a patent 'one-pass' type of primary lining (for example, Charcon or C.V. Buchan p.c. concrete segments with hydrophilic sealing glands), which allows a secondary lining to be dispensed with, might be used. Grouted-up Lytag will often be used for void infilling at the extrados. These linings will not have the longterm integrity of a two-stage lining, particularly if the soil and groundwater conditions become aggressive during the lifetime of the lining. Lining segments of trapezoidal shape are particularly suitable for adjustment of line and level, and for use when tunnelling round curves. As one example, this type of lining, with a Hydrotite gasket, was used at Southport on North West Water's Coastal Water's Interceptor Project (modified IChemE Green Book project) in association with a Lovat M131 TBM used in both open-face (conveyor belt and compressed air) and EPB (with screw) modes. The 15mm taper on the trapezoidal liner assisted the building of 600m radius curves. The rings themselves had dowels on the circle joints and bolts on the cross joints, with caulking grooves cast on the segments. In another example, the C.V. Buchan trapezoidal segments used in the Barking Reach combined cycle gas turbine power station cooling water tunnels were sealed with a C E Heinke EPDM sealing gasket.