Tunnelling to the sea

The start of McConnell Dowell’s three-kilometre underground journey draws a crowd to Mairangi Bay.   BY GAVIN RILEY

Every now and again, marketed the right way, contracting can transform itself from being merely the subject of idle curiosity into a spectator activity. 

That’s what happened late last year when a proud North Shore City Council wanted the public to see at close quarters the construction beginnings of its $116 million Rosedale tunnel and marine outfall scheme. 

The project was the biggest the council had ever undertaken and was highly acceptable environmentally – but how to capture the public’s interest when, although an access shaft had been sunk, tunnel boring had not even begun?

Clever press advertisements did the trick, focusing on the tunnel-boring machine. Named Amelia Rose, after the three-year-old daughter of McConnell Dowell Constructors’ tunnel project manager Matt Dowler, the $10 million Canadian-made TBM would have to remain underground when its job was done because of the impossibility of retrieving it.

So...“Come face to face with the biggest mole you’ll ever see!” screamed the ads beneath a large photo of a dimly lit cavernous space at the foot of the shaft. “Once Amelia Rose goes underground – you’ll never see her again.”

Some 5000 people responded to the invitation to visit the project site, lured also by the promise of seeing excavation trains, a full-sized tunnel ring, a section of the large pipe to be used in the offshore section, and displays and videos.

McConnell Dowell is developing an impressive expertise within New Zealand for this type of project, designed to carry and discharge treated effluent much further offshore than hitherto. Rosedale is its fourth such project in recent years.

The company has enlisted the aid of Connell Wagner-DC on the design-construct contract and has assembled an experienced team of tunnellers consisting of New Zealanders, Australians, Britons and South Africans. The team is working 24 hours a day in shifts of 10-12 workers.

The 45-metre access shaft has been sunk at the Rosedale wastewater treatment plant site off Constellation Drive and in mid-January, after assembly at the shaft base, the TBM began carving a 2.8-metre-diameter tunnel on its three kilometre journey to Mairangi Bay.

It has been specially designed to handle the hard subterranean rock in the East Coast Bays area. At nine revolutions a minute its rotating head can grind through a metre of rock in 15 to 30 minutes. It is expected to progress as much as 30 metres a day, but the entire tunnelling journey is still expected to take about 10 months.

The TBM will excavate an estimated 30,000 tonnes of material, which will be spread over four hectares around the wastewater treatment plant to meet Auckland Regional Council’s standards for sediment control.

The TBM is no lightweight. It is nine metres long and with its 64 metres of back-up equipment weighs 150 tonnes. But as it crunches its way towards the coast, residents won’t feel or hear a thing as tunnelling is taking a downhill curving path at least 25 metres beneath roads, houses, businesses and parks. When the tunnel reaches a point some 600 metres offshore, a riser shaft will connect the tunnel to a pipeline buried two metres beneath the seabed.

As the TBM’s rotating head does its work, the cuttings come into a chamber and are extracted from the chamber by an Archimedes screw. The screw discharges directly onto a conveyor which carries the material back and tips it into muck wagons. A 13-tonne German-made locomotive hauls the wagons back through the tunnel along a purpose-built rail system. (Everything required in the tunnel goes in by rail so that there are not only locomotives and wagons but man-transport cars, segment cars and grout cars.)

After the TBM has cut a metre of spoil, it erects a metre of ring in six segments. As each segment is placed in position, rams push the tapered joints shut. The segments (18,000 in total) are connected by four temporary bolts and dowels to help position each segment perfectly. Tolerances on the segmental lining are very tight and very high quality concrete is involved.

When the ring is completed the TBM cuts again, pushing off the concrete lining. The ring is erected inside the tailskin in supported ground. It is extruded out the back of the machine and then the gap (called the annulus), between the bored hole and outside of the lining, is filled with grout.

Concurrent with the tunnelling has been some offshore operation. The dredge vessel Kamihia started work in November, using a dredging excavator to remove sandstone and silt from the seabed in preparation for connecting the tunnel and outfall pipes with the riser later this year about 600 metres offshore.

The Kamihia then began three months’ dredging work to prepare 2.1 kilometres of the seabed for the outfall pipe, which is 1600mm-diameter HDPE with concrete ballast and will lie about one metre beneath the sea floor. Some 73,000 cubic metres of dredging is required in total, about 7000 cubic metres of which is through East Coast Bays rock close to shore.

The large pipelay barge Sea Tow 80 also arrived on site in November and began installing casing for 1.5 metre-diameteriser pipe. The barge, which can operate only where the waves are up to half a metre, will erect a machine to drill a two metre-diameter casing on top of the hole down the tunnel alignment.

The Sea Tow 80 installed the riser late last year and began its outfall pipe-laying operations in January. The laying of the pipe-strings, five lengths of 420 metres, is due to be completed in May.

The submarine pipe is being made in Thailand in 12-metre lengths and manufacturer KWH is welding it into the pipe-strings at Kaiaua in the Firth of Thames, where the pipestrings are stored until being towed to the project site as required.

The pipe-laying operation is relatively simple. The pipes are pulled over the top of the barge, concrete weights are installed, the pipes are winched off the back of the barge using a stinger, and are then tensioned when laid in the trench using controlled flooding.

One end of the pipe is pulled down slightly, flooding is begun at that end, the weight of water drags that end into the trench, and the flexible pipe-string just carries on dropping like a big “S”. The pipe-strings are connected by spools, which divers bolt into place on the seabed.

Ballast blocks for the pipe are being made by a precast supplier and delivered as required. The supplier casts seven half bracelets per bed and the blocks are vertically split around the pipe. They are clamped together by temporary steel bolts, but permanent fibreglass bolts are then inserted to provide permanence. 

Working offshore has its hazards, in the form of marine traffic and changing weather.  A marine exclusion zone, or no-go area, has been established round the outfall site – but of course there is no fence, so notices have had to be sent to mariners and placed in local publications. The barge is lit up at night and a night-watch is posted. For operations such as towing of the pipe-strings, escort vessels are used as a safety measure.

The contractors also pay close attention to the weather – and not just on a day-to-day basis. They have a seven-day look-ahead which embraces not just weather but wind direction and velocity, waves, swells and daily tides. They also receive a two-week forecasting for the Tasman Sea. All this information helps the team to plan its operations, some of which require calm conditions.

McConnell Dowell is taking a partnering approach on the Rosedale tunnel and marine outfall project not only with its design consultant, Connell Wagner-DC, but with client North Shore City Council and the council’s project manager, Maunsell /AECOM.

The payoff will be an effluent-treatment system fit for a city that is already New Zealand’s fourth largest (220,000) and growing.

The current effluent flow is discharged at about 0.7 cubic metres per second and the system could handle a volume of 1-1.5 cubic metres per second. But the new scheme, which will have a 100-year design life, will be able to handle peak flows of 6 cubic metres per second – and discharge them a healthy 2.7 kilometres offshore. 

Te Kaha leads in a tale of two TBMs

What’s in a name? One may be called Amelia Rose and the other
Te Kaha (“The Strength”), but apart from that there’s little to choose between the two Canadian tunnel boring machines McConnell Dowell is currently operating in the Auckland area. 

Since the middle of last year Te Kaha has been boring a three-kilometre-long tunnel at depths of between 25 and 60 metres beneath Orakei and Hobson Bay to replace a 90-year-old sewer pipe. Since late last year Amelia Rose has also been on a three kilometre journey, a few kilometres to the north, from the Rosedale water treatment plant to Mairangi Bay.

Te Kaha, a 270-tonne monster 75 metres long, is creating a sealed and lined tunnel with an internal diameter of 3.70 metres in the $118 million project, which is being carried out by a joint venture between McConnell Dowell and Fletcher Construction for client Watercare. The project, including demolition of the old sewer, is due to be completed in mid-2010.

Both TBMs were assembled at the base of their access shafts. They are what are known as earth-pressure-balance machines – they create a pressure at the boring face to counteract the forces exerted by earth and water underground.
It is the first time the technology has been seen in New Zealand and it is being used because it is eminently suitable for the East Coast Bays-formation material and variable ground conditions on both projects. In the case of Project Hobson that means largely weak silty-sandstones with two areas of soft marine sediment.

Te Kaha, like Amelia Rose, does all its tunnelling work in one pass. As it moves forward it drills through the rock and at the tail end of the machine the tunnel is lined with precast concrete segments and the gap between the segments and the rock is grouted and “ready to go”. There is no need to prepare the tunnel further for use.

When Amelia Rose began tunnelling downhill last November, Te Kaha had completed 65 percent of its uphill journey. It had been placing an average of 20 segmented tunnel rings a day – a rate of progress the Amelia Rose team is hoping to match. 

But Te Kaha will still end up the winner. While Amelia Rose must remain beneath Mairangi Bay at journey’s end because it cannot be retrieved, Te Kaha will be taken apart underground and removed.