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Dora the Bora, the colourful tunnel boring machine, leads the way as McConnell Dowell achieves an Australasian best during a challenging and complex ocean outfall project on Christchurch’s coast. BY GAVIN RILEY
This pipeline is designed to take Christchurch’s treated wastewater three kilometres offshore and is the biggest construction project the city council has ever undertaken. It has been described by ex-mayor Gary Moore as “an amazing engineering feat”, and McConnell Dowell general manager Roger McRae says it is more challenging and complex than Clandeboye. The pipeline is costing $61 million. However, the building of a pump station (a separate contract, let to Works Infrastructure), plus consents, design and other works, has brought the total project cost to $87 million. McConnell Dowell is tasked with supplying and installing: The land part of the pipeline is being constructed in three sections by micro-tunnelling/pipe-jacking techniques. The 870 metre-long first tunnel, driven west from South New Brighton Park under the Avon-Heathcote estuary, was completed at the end of August. “That’s an Australian and New Zealand pipe-jacking record,” says Roger McRae. The first tunnel connects with the pump station, which is being constructed on the edge of the oxidation ponds. The 600 metre-long second tunnel, starting again at South New Brighton Park, is being driven east under Jellicoe Street to the sand dunes on the beach side of Marine Parade. It is due to be completed in early November. A quick turnaround will see the 830 metre-long third and final tunnel continue east under the beach and surf to about 700 metres offshore. There it will be connected to the marine section of the pipeline, which will carry the treated wastewater three kilometres out to sea. Spearheading the onshore-pipeline operation is a new Macdow star – a $7 million tunnel boring machine (TBM) built by Herrenknecht in Germany. Called Dora the Bora (the name chosen by a local schoolboy in an informal competition), the TBM is 13.3 metres long, has a 2.19 metre diameter, and is painted in the red-and-black colours of the Canterbury rugby team. Dora bores a tunnel slightly larger than the pipe, while a powerful set of hydraulic jacks is used to progressively push each section of pipe behind the machine. Operated remotely from a control panel in a cubicle at McConnell Dowell’s South New Brighton Park site, the TBM features a “full face” cutter head which is “pressure balanced” to ensure minimal impact on the surrounding ground as the machine moves forward. Over the last four to five weeks of the first tunnel drive, Dora advanced at an average of 120 metres a week, with a maximum distance of 21 metres achieved in a single 10-hour shift. The 10-hour shifts have now been replaced by 12-hour shifts, involving three crews working to a roster of seven shifts on and four off to provide a 24/7 operation. “This is to keep the machine working continuously, to keep the momentum going with quick changeovers,” tunnel manager Gwyn Jones says. “Not only is this increasing productivity, hopefully it will help us set some more records.” Material excavated by the TBM is carried to the surface in bentonite slurry, where a separation plant removes the material, allowing the slurry to be recycled. The waste, totalling about 8800 cubic metres, is being stockpiled on a site near the oxidation ponds. It consists largely of sand but also contains shell fragments and, surprisingly, some ancient timber. To facilitate the launch and retrieval of the TBM at depths of up to 12 metres below ground level, three temporary shafts have been constructed using sheet-piling, steel frames, and a concrete base slab. The biggest of these is at the jacking shaft at the South New Brighton Park site, which is 12 metres deep and 14 metres in diameter. For launch and retrieval, the TBM is dismantled into four cans. The 25-tonne front can is lowered and lifted by a 100-tonne crane, while the remaining cans, two of 11 tonnes and one of 3.4 tonnes, can be handled by a gantry crane which straddles the shaft. When Dora has completed the third tunnel under Pegasus Bay at year’s end, McConnell Dowell will undertake the tricky task of retrieving the TBM from the seabed. The machine will be sealed from the inside and the tunnel flooded. A 280 tonne Liebherr excavator mounted on a barge will excavate around the TBM, with close-in excavation carried out by a team of divers. Buoyancy tanks attached to the TBM will give it an approximate underwater weight of just 20 tonnes. With its four cans supported by a lifting beam, the machine will be raised to the surface intact. “Recovery of the machine from the seabed is going to be technically and logistically challenging, because it’s dense sand and we’re going to dig it out – and we’re going to do it off a barge,” Roger McRae says. “That hasn’t been done in New Zealand before, but we have the ability to engineer our way through it.” While McConnell Dowell’s team of 25-30 at South New Brighton Park has been carrying out the tunnelling and land-pipeline installation, a second team of about 20, led by marine works manager Keith Griffin, has been preparing the HDPE marine pipe at Lyttelton in seven 360 metre-long pipe-strings. Each string, welded together from 12 metre-long sections on site, weighs more than 1000 tonnes and is fitted with 15.6 tonne ballast blocks at six metre centres. On completion, the string is launched on bogies along a rail track into Lyttelton Harbour and floated across to Diamond Harbour where it is stored on the sea floor until required. Three or four strings are expected to be banked up by early November when the first string is due be towed by up to six tugs on the 10-hour, 18 kilometre journey to the project site off South New Brighton beach. The marine section of the outfall pipeline will be installed in a seabed trench eight metres wide at base and varying in depth from 4.5 to six metres. This work will be performed using a fleet of marine plant that includes the barge-mounted 280 tonne Liebherr excavator, a second support/equipment barge with a 65 tonne crawler crane, bottom-dumping hopper barges, tug boats, and a host of support vessels. Divers will have the difficult job of joining the seven pipe-strings in zero visibility and “torquing” the 36 M36 bolts on each joint. Under its project director, Ian Campbell, McConnell Dowell has assembled a mini-United Nations workforce for what is its fourth ocean-outfall project in three years (Waimakariri, the award-winning Clandeboye and the in-progress Dunedin are the others). Team members are from England, Scotland, Wales, Ireland, Germany, Switzerland, Zimbabwe, China, Argentina, Chile, Fiji, Pakistan, Sri Lanka, Botswana…and New Zealand. Supporting the skills of this ensemble are a number of subcontractors. The larger ones are: Polyethylene Pipe Systems (PPS), HDPE pipe welding; a PPS-Franck (Germany) joint venture, HDPE pipe manufacture and supply; Heron Construction, dredging; Bay Underwater Services (NZ), diving; CP&P Thailand, concrete pipe manufacture and supply; Verticon, crane hire; Smith Crane & Construction, precast concrete ballast block manufacture and supply. URS (NZ) is the main consulting engineer, and the marine section engineers for Christchurch City Council are from OECL Consultants. A partnering charter has been established between McConnell Dowell, Christchurch City Council and URS to ensure what Roger McRae describes as alignment of objectives and effective working relationships and communication on the project. “We score ourselves against a set of criteria and discuss any areas where there are relationship difficulties,” he says. “It’s an informal commitment between the parties and we work together in the best interest of the project as a whole. Good communications and working relationships are important to us on all jobs and are essential on projects of this nature.”
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Fresh from winning a Contractors’ Federation construction award for the Clandeboye ocean outfall near Timaru, McConnell Dowell Constructors has set an Australasian pipe-jacking record during the early stages of another outfall project off the South Island’s east coast.