DESIGN AND CONSTRUCTION OF A SEGMENTAL LINING FOR A MACHINE-BORED TUNNEL - DELIVERY TUNNEL NORTH, LESOTHO-HIGHLANDS-WATER-PROJECT

The 21-km-long Delivery Tunnel North for the Lesotho Highlands Water P roject is split into true main tunnel drives of 8 km and 11 km lengths . The tunnel is located in the sedimentary rocks of the upper Karoo se ries with cover of up to 350 m. The rock units comprise claystones, si ltstones and sandstones. In the poorer geology, early support of the t unnel periphery is essential. The method of tunnel construction involv es excavation by a TBM and the simultaneous erection of a precast conc rete segmental lining that provides both rock support and a final tunn el lining. The segmental lining is designed and constructed as part of an integrated excavation/support/lining system. There are three types of segments, differentiated by the amount of steel reinforcement. The distribution of the different segment types along the tunnel, relativ e to the geological conditions and depth of cover will be optimized by monitoring of the performance of the lining during the construction p eriod.