New utility tunnel for Africa’s busiest harbour

Durban Harbour is the busiest container port and harbour in Africa, and approximately 5000 ships visit it each year. As part of the approximately £80 million harbour expansion, a new 4.5m diameter, 500m long tunnel to carry services under the harbour, will replace an existing immersed tube tunnel when the harbour is widened and deepened in 2006.

Dredging works for the harbour expansion will cut through the existing tunnel. This means that the new, longer and deeper tunnel needs to be in place and fully fitted-out before the harbour expansion can begin – resulting in a tight design and construction schedule.

Mott MacDonald was part of the team that won the design competition for the type of tunnel that should be constructed, and was awarded the contract to develop the tunnel for Durban’s eThekwini Municipality Water and Sanitation. The competition involved both the new utility tunnel and a 'people mover' tunnel, which would ferry people across the harbour to help in promoting tourism. However, this second tunnel was omitted from the construction contract.

Working with South African engineers, Goba, our design role involved providing technical assistance on the various tunnelling options as well as advice on the people mover options and soft ground tunnelling. South Africa has strong experience and knowledge in hard ground tunnelling, but not as much practice in soft ground tunnelling. The ground conditions encountered included river deposits and sandy clay which suited the use of a slurry tunnelling machine, which had not previously been used in southern Africa.

Construction work on the tunnel commenced in mid 2005. Of particular note was the steep 20%gradients on each side of the crossing, and the use of a purpose-built vehicle to deliver the tunnel segments. Mott MacDonald had a permanent presence on the site during construction. Although the start-up of the tunnelling works proved difficult, the works were completed only slightly behind schedule in August 2006.