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Evaluating TBM Design and Performance, 30 Years Apart: The Lesotho Highlands Water Tunnel, Phase 1 and Phase 2

Two massive tunnel projects 30 years apart: The Lesotho Highlands Water Project (LHWP) is a multi-phased project that has taken place over decades to provide water to the Gauteng region of South Africa and to generate hydroelectricity for Lesotho. Phase I tunneling began in 1992 and utilized multiple Main Beam TBMs to bore long tunnels in basalt and dolerite ground conditions. Phase II, now under construction, consists of 38 km of transfer tunnel to be excavated by both TBM and drill & blast.

This paper details the design for the TBMs to be used on the Phase II tunnel, while comparing the modern-day machines to the history-making Main Beams used in the 1990s. It draws some conclusions and lessons learned about boring in volcanic rock at depth utilizing Main Beam vs. Double Shield TBMs.

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