The replacement of the existing services tunnel underneath Durban harbour, with a new R210-million tunnel, for the water and sanitation department of the eThekwini municipality, has almost been completed.
This is the first soft ground tunnel constructed in the country using the mixed shield tunnel-boring machine (TBM), to support the tunnel face while boring. The tunnel is being constructed with precast concrete segments and is placed 30 m below sea level and 9 m below the sea-bed. The tunnel has a 4,4-m internal diameter and a 13,8-m circumference. It is constructed with 1,2-m-long and 250-mm-thick precast concrete seg- ments produced at the Point road on-site pre-cast facility.
The contract for the new tunnel and the decommissioning of existing subsea services was awarded to a joint venture (JV) with construction firm Concor Holdings and German-based partner Hochtief Construction. The JV is trading as Durban Harbour Tunnel Contractors (DHTC) or the DHTC JV. Supervision of construction, on behalf of the eThekwini munici- pality, is being con-ducted by consulting engineer firm Goba.
DHTC J V engineer Franz Scholtz tells Engineering News that all major developments are complete and that the stripping of the elements from the existing old tunnel is in progress. The 20-month project, which started in 2005, and was expected to be completed earlier this year, will only be completed in June 2007 owing to unforeseen circumstances. However, he adds, “The tunnel came out on the south side within 10 mm of target, which is an incredible achievement on this world-class tunnelling pro-ject.”
Construction access to the tunnel was through a 15-m deep shaft, constructed by means of the diaphragm wall method by South African construction group and specialist subcontractor Franki Africa. Scholtz says, “The construction of the temporary shafts using the diaphragm method, excavation of the three shafts under dry and wet conditions, and the construction of 3-m-thick concrete base plugs underwater, are, individually and collectively, milestones in this project.”
Scholtz points out that other milestones in the project were the smooth transition to using new services in the tunnel, installing the TBM in the reception shaft and the working relationship and effort of the personnel in the separation, compressed-air and water-cooling plants.
Subcontractors on the pro- ject included engineering company Herrenknecht for the supply and maintenance of the TBM, bulk-handling materials company Flexicon installed the pipework, stainless-steel fabricator Benfab supplied and installed the structural steel, and Electrical Projects was responsible for all the electrical and mechanical support. Scholtz says there were many small local suppliers and subcontractors that contributed signifi-cantly to the project. The concrete aggregates used for the tunnel were locally sourced, while all specialised sockets, seals and hydrophilic strips were imported.
A new technology employed on this project was the two component grouting systems for the backfilling of the cavity left around the lining of the tunnel. Twelve international and local tunnelling engineers and a labour force of about 140 people were employed during the construction of the tunnel. The JV assured the municipality that by the completion of the project, one South African company would be skilled in replacement tunnelling methods and activities. “It needs to be stated that the experienced international tunnel-ling crews were extremely complimentary about the South African crew’s ability to quickly learn and apply their skills successfully,” he adds.
Scholtz says, “On comple- tion of the tunnelling works, temporary and permanent access structures and buildings, the JV will be responsible for completing ancillary works, including topsoiling, landscaping, stormwater control, erosion control and minor road works.”