The uMkhomazi Transfer Tunnel: Background, Geotechnical Conditions and Tunnel Design Considerations
Authors: R.L. Freese1, G.J. Keyter2
Conference: SANCOT Symposium 2026
Date: April 13-14, 2026
1 Knight Piésold, South Africa
2 SRK Consulting, South Africa
ABSTRACT
The uMkhomazi Water Project Phase 1 (uMWP-1) currently represents the largest civil tunnelling project in South Africa in three decades. Its centrepiece is the uMkhomazi Transfer Tunnel, a 31.2 km long, 3.5 m diameter tunnel designed to transfer water from the proposed Smithfield Dam to a new Water Treatment Works (WTW) which will supply potable water to the eThekwini metropolitan area in the KwaZulu-Natal Province (KZN), South Africa. The tunnel will be excavated through sedimentary and intrusive igneous formations using Tunnel Boring Machines (TBMs), following some initial drill and blast excavation of tunnel portals and access declines.
Feasibility study investigations indicated a mixed geological setting comprising horizontally bedded sedimentary sequences of shales, mudstones, siltstones, and sandstones, which are variably indurated and with significant variation in strength. These sedimentary strata have been extensively intruded by dolerite dykes and sills, while zones of fractured and weathered rock also occur. While rock mass conditions are generally favourable to TBM tunnelling, site-specific concerns such as mixed (hard-soft) face conditions, high water pressures, potentially gas-bearing rock, chemical aggressivity, weak/sheared shale horizons, rock durability and potential squeezing conditions will have to be considered during design.
The design phase underway now, includes comprehensive geotechnical investigations comprising rotary core drilling, geophysical (seismic and resistivity) surveys, wireline logging, packer testing, hydrofracture and overcoring in-situ stress testing. This is required to ensure optimal final tunnel alignment, ground support and final lining designs, excavation method selection, and waterproofing strategies, to mitigate technical and contractual risks. These investigations were developed to support the development of a Geotechnical Baseline Report (GBR) in line with ASCE and FIDIC Emerald Book Guidelines.
This paper presents the background and layout of the project, the scope of the geotechnical investigations, and expected key tunnel design considerations.
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