Boreability assessments for the Lesotho Highlands Water Project

The Lesotho Highlands Water Project (LHWP) is a scheme in southern Africa in which the use of TBMs may well play an important role. Phase 1A of the project requires the construction of the 45 km long Transfer Tunnel and the 37 km long Delivery Tunnel. These tunnels have drive lengths of between 5 and 10 km. With headings of this length the estimation of likely TBM advance rates is of considerable importance to both the owner and the Contractor. The advance rate of a TBM is dependant on a number of factors, such as : the size of the tunnel the characteristics of the rock mass the level of support that has to be installed the timing of the installation of the support the boreability of the rock material This paper describes the methods used to assess the boreability of the rocks to be encountered in these two tunnels. The assessments were done following small scale laboratory tests, and in the case of the Transfer Tunnel, large scale tests as well. The small scale laboratory tests were done in Norway and the USA, and the large scale tests, in the USA.