Macdow's Tahuna triumph

McConnell Dowell constructs an ocean outfall in an often hostile environment and posts a four-from-four record in the Contractors’ Federation construction awards.   BY GAVIN RILEY

It’s getting to be a habit. For the fourth consecutive year McConnell Dowell Constructors has returned from the Contractors’ Federation annual conference with a prized construction award – a feat unprecedented in the competition’s 30-year history.

Two of those award-winning projects have been a design-and-build ocean-outfall pipelines on the South Island east coast – Clandeboye near Timaru in 2007 and now Tahuna for Dunedin City Council.

MacDow is building an impressive track record in this field. Before Clandeboye it constructed an ocean outfall at Waimakariri and is currently working on the Christchurch outfall and the Rosedale scheme on Auckland’s North Shore.

A key to MacDow’s continued top-level successes is its attention to detail. On all the company’s projects a planner sets out a comprehensive programme well in advance in which the project is laid out in sequence, with all critical activities noted and linked to each other to ensure the critical path is accurate. This includes resource management, alignment of material delivery, temporary works planning and design, development of safe work practices, and work methodologies. The programme is monitored on a weekly basis, with updates being produced which concentrate on the critical path.

The Tahuna project consists of a microtunnelled concrete pipe, with low-density polytheylene liner, and a high-density polyethylene outfall pipe. The 500-metre-long microtunnelled pipeline, which has an internal diameter of 1500mm, starts at a wastewater treatment plant and runs under a roadway, golf course and sand dunes, then under the sea for 250 metres. The 1400mm-internal-diameter HDPE pipeline connects to the microtunnelled pipeline and extends about 1000 metres into the Pacific Ocean, where it terminates with eight diffuser rosettes, each containing four duckbill outlet ports.

The $25.5 million contract was awarded in April 2006 and the new outfall began operating in January this year. It replaced a 100-year-old pipe which discharged city effluent just below low water, causing occasional contamination to Dunedin beaches.

MacDow also had to build, but not design, a junction chamber, sited near a future secondary treatment plant (stage two of the project due for completion in September 2011).

MacDow’s site team at Tahuna was a mix of New Zealand, English, Samoan, Indian, Canadian and French nationalities.

The marine crew had had experience on the Clandeboye and Waimakariri projects and was led by Kevin Egan, whose authority, energy and practical know-how proved outstanding. The tunnelling team was largely in-house trained, with some members being brought in from overseas to undertake such specialist roles as operating the German-made 50-tonne Herrenknecht tunnel boring machine (TBM) and the slurry plant.

The international mix at Tahuna extended to the materials. The offshore pipeline components and concrete jacking pipes were from Thailand, the sheet piles from Belgium, the clean-air system and slurry-treatment plant from England, and the pipeline welding equipment from Italy.

A feature of the TBM was a fully closed face-shielded machine utilising slurry-elevation techniques and transportation of the excavated material by slurry pipelines.

The jacking pit access shaft for the microtunnelling was excavated in the wastewater treatment plant grounds next to the golf course.

The jacking pipes were lowered into the jacking pit and pushed forward using an 800-tonne-rated jacking frame. The jacking loads were kept to a minimum by using a bentonite clay slurry lubricant system, with the lubricant injected into the surrounding ground through ports located on every pipe.

The lubricant system worked so well that the five inter-jack stations installed in the pipejack were not needed and the pipe-jack was completed on time and within construction tolerances.

A proven trestle system, with a crew who knew how to construct it quickly, was inherited from the Waimakariri project. The two temporary trestles, 290 and 245 metres long, were erected across St Kilda beach into the surf zone to enable the assembly and launching of the outfall pipe and to recover the TBM once it had been driven under and beyond one of the trestles to the desired location.

At this point and within a wet marine coffer dam, the microtunnelled concrete sewer pipe was connected to the high-density polyethylene (HDPE) pipeline. The 35-metre-long coffer dam, roughly three times the length of the TBM, was built to envelop the inshore and offshore ends of the HDPE and concrete pipelines respectively.

The last tunnel pipe was exposed and secured to the sides of the coffer dam to prevent any movement. The TBM was excavated out of the coffer dam, lifted to the surface, dismantled on a disassembly platform, and carried to shore in segments by an 80-tonne crawler crane.

“Basically we achieved a 40-tonne TBM pull through a 10-tonne effective rope lift by clever sheaving using a sliding gantry assembly – by clever design and a pulley-block system,” says Steve Proud, project manager for the latter stages of the Tahuna contract (Matt Dowler was his predecessor).

“It was a quite beautiful and innovative piece of engineering.”

At the mercy of the elements

The severity of the sea conditions ruled out typical float-and-sink or trenched outfall options and an innovative “bottom-pulled self-burial” method was used. While this minimised exposure of boats and the pipe to the elements, it meant increased concrete ballast was needed to provide security for the pipe even in an unburied condition.

The beaches at Dunedin’s south coast are exposed to southerly weather patterns which bring strong winds, high waves and currents. During the project McDow’s wave buoy sometimes recorded waves of more than five metres high and occasionally up to 13 metres.

Offshore work was possible during less than one-third of the project period. MacDow had anticipated phases of up to five days of good weather, but windows of only two to four days proved to be more common. Adverse weather particularly affected the TBM recovery and installation of the onshore-offshore transition because each operation required more than five days of good weather.

The seabed was regularly on the move, particularly round the end of the trestle and launchway where it rose 2.5 metres during one of the pipe-launching pulls, causing difficulties in pulling the HDPE pipestring.

Steve Proud says that although there were no accidents or lost-time injuries in the water, the weather and sea conditions proved very challenging, especially for the divers, who did much of the permanent offshore work. 

“For example, our marine coffer dam, though well designed, had to be ‘abandoned’ to relieve stresses, due to the tremendous wave forces, by lifting the offshore and inshore gates whenever the significant wave height exceeded two metres.

“It’s never easy for the guys to empty it of sand and kelp over two days, only to watch it fill up again. This happened more than 30 times.

“Nature also conspired to remove large chunks of the coastline occasionally and to dump the constituent bricks, rubble and other debris against the pipeline. Much of this had to be removed to make and torque connections between the pipestrings. Patience was indeed a virtue.”

There was another, unexpected setback. Sheet piling for the marine coffer dam to remove the TBM and complete the connection from the microtunnelled pipe to the outfall pipe encountered a hard clay layer where a dense sand/gravel layer had been expected. This extended piling operations for more than two months and added time to the excavation because the clay stratum then had to be excavated, in part, from within the coffer dam using clay cutters in order to complete the transition section.

Launching the pipe

The 1000-metre-long pipestring was pre-assembled into four approximately 250-metre-long sections on John Wilson Ocean Drive nearby and moved onto the launchway along a specially constructed road across the dunes. Crane and launchway trestles were built to the end of the surf zone.

The crawler crane and associated vibro hydraulic hammers were used to construct the trestle. Ballast blocks were attached to the HDPE pipeline and connected with 48mm rope so that installation loads were not transmitted through the pipe.

Concrete bases were then incrementally laid onto load skates and pulled out along the launchway using a 100-tonne winch. The pipe was transported on trolleys down John Wilson Ocean Drive and the access ramp, sliding on sand.

The pipe was then turned 85 degrees over the shore-work platform using excavators, crane and guide works, and pulled down over the base blocks with the winch. Once the pipe was in place, ballast caps were installed and bolted down along with the installation of the head and tail frame.

The winch was fitted with three kilometres of synthetic rope. A 45-metre-long steel ramp was constructed to transition the pipe from launchway to seabed, pivoting on the end of the launchway and supported by a lift frame to which it was secured when lifted out of the waves.

Offshore an anchor set was installed by the divers (contracted from New Zealand Diving & Salvage) and pulled into the seabed by the winch. A 12-tonne buoy was used to hold a pulley sheave off the seabed, a rope was taken under the launchway out to the anchor and sheave, back to the pulley mounted on the front of the pipestring, and back out and terminated on the buoy for the first string. The winch then pulled the pipestring out to a buoy at a marked position on the seabed.

Subsequent strings had the pull rope terminated at the end of the previous pipestring in a mating frame, which ensured the next pipestring was pulled into the correct position. Divers then recovered the frames and bolted together the pipe joints.

The transition piece consisted of an HDPE dogleg, secured within the coffer dam by an innovative frame connecting and stabilising it temporarily through four 600mm-diameter piles.

Once it was installed and connected between the buried microtunnel-bored concrete and the ballasted HDPE pipe (a task spread over four weather windows), about 170 cubic metres of concrete were pumped 250 metres along the trestle and into place around it. The concrete was designed to be a thrust block against any differential forces between the microtunnel and the HDPE pipeline.

After completion of an air relief line (to relieve a potential air bubble in the high point of the HDPE pipe), a “pup” piece was installed along with saddle blocks between the pipeline and the transition piece. The pipeline, which has a 50-year design life, was then ready for testing and commissioning.

Keeping it clean

MacDow’s key environmental consideration was to minimise the company’s footprint both while the project was in progress and afterwards. Biodegradable oils were used in machinery, noise was kept to a minimum, a high level of site cleanliness was maintained, and care was taken daily to ensure there were no incidents of pollutions, spills or other transgressions. The beach was kept open at all times, except when the trestle was being installed and during pipe-pulling, by providing a reinforced walkway under the trestle.

The use of microtunnelling minimised environmental disturbance, and tunnel spoil was screened and used on the adjacent golf course to re-form greens and as fairway contouring fill. This use of spoil was a commedable alliance-style initiative that avoided 200-250 truckloads having to travel through the city to the municipal dump.

Steve Proud says a key factor in MacDow’s award-winning success at Tahuna was the “open and honest” relationship it enjoyed with Brian Turner, project manager of the Dunedin wastewater treatment plant scheme, and his engineers, BecaCH2M. 

He adds that the commission testing went very well and the pipeline was “open for business” the next day.

“When that happened, we celebrated success and partied hard. The pipeline was discharging ahead of the consent deadline date and we worked with and alongside other contractors before and during the commissioning to ensure this.

“Testing and analysis from beach samples taken since commissioning show a dramatic improvement in water quality. MacDow is very pleased to have been instrumental in making a real and lasting difference.”