A major milestone for the Lesotho Highlands Water Project in Africa was reached in June 2002 when the 30.5km long Mohale tunnel broke through.
The 4.9-5.4m o.d. tunnel was excavated through basalt using two refurbished shielded TBMs – a modified NFM/Mitsubishi-Boretec machine and a modified Wirth TBM – and is lined with a non-bolted hexagonal precast concrete segmental lining.
The tunnel is part of Phase 1B of the multi-phased, multi-billion Rand investment by the Republic of South Africa to secure water supplies to Johannesburg, Pretoria and the country’s Witwatersrand-Vereeniging industrial heartland. It will link the reservoir of the Mohale Dam (also constructed under Phase 1B) to the Phase 1A Katse Dam reservoir to join the gravity fed supply to South Africa.
The Wirth machine started in March 1999 from the Mohale reservoir end portal while the NFM/Mitsubishi-Boretec TBM was launched in July 1999 from the Katse portal. The Wirth TBM was withdrawn in February 2002 after completing 14.6km. The TBM from Katse eventually broke through on 22 June 2002 after a drive of 16km.
The TBMs achieved progress rates of up to 235m/week. Average progress rates however fell short of the approximate 30m/day program average with final breakthrough 18 months later than scheduled. Harder than anticipated rock conditions and excessive water, followed by increased mechanical and equipment breakdowns are blamed by the contractor. Extensions have been agreed, but negotiations continue.
Mohale Tunnel Contractors (Hochtief/Impregilo/Concor) was awarded the US$200M (approx) Mohale Tunnel contract in December 1998 after successfully bidding for all three main elements of Phase 1B. The total US$320M contract (in 1998 prices) included construction of the 145m high x 620m wide rockfill dam to create the Mohale reservoir, and the 5.6km long Matsoku diversion tunnel and 180m wide x 20m high weir to divert high-volume flow of the Matsoku River into the Katse reservoir.
The 30km long Mohale tunnel was designed for the Lesotho Highland Development Authority by the engineering partnership of Lahmeyer International, Mott MacDonald, and Consult 4.
The Mohale Tunnel will be the last element of Phase 1B to come on line in January 2004. The Mohale Dam was completed in June 2002 and the Matsoku weir has been delivering flow into Katse reservoir since October 2001.
