Building off the company’s 60-year experience in remote resources and energy projects to city-shaping infrastructure, McConnell Dowell has built thousands of quality assets and facilities for customers and communities.
Its expertise has grown steadily to span building, civil, electrical, fabrication, marine, mechanical, pipelines, rail, tunnel and underground construction.
With over 3000 employees and professional engineering and construction teams in Australia, New Zealand, the Pacific, and Asia, its customers benefit from its combined local knowledge and international experience.
Over the past 20 years, outfall pipeline construction in particular has evolved with the development of trenchless technology as a result of McConnell Dowell’s experience in New Zealand.
Engineering Manager NZ & Pacific Daniel Patten says this evolution has eliminated and greatly reduced a number of adverse effects, including: disturbance of the natural environment during the shore crossing construction, visual, emissions and noise impact during construction, and exposure to inclement weather and sea state.
“Elimination or reduction in disturbance of the natural environment then has the potential to ease the planning process, particularly assessment of effects, stakeholder engagement and ultimately obtaining resource consents – typically the most significant of client risks in the development of a treated wastewater discharge solution,” Patten says.
During these 20 years of evolution, McConnell Dowell has used a range of trenchless methods at numerous New Zealand projects to get where it is today.
Traditional methods
As recent as the early to mid-2000s outfall pipeline construction for McConnell Dowell has typically involved trenching or jetting techniques to bury large pipelines, particularly in foreshore area.
Examples include the Waimakariri District Council and Clandeboye Ocean Outfalls, both of which used staged temporary sheet piles cofferdams to complete shore crossings in tidal and active wave zones.
“The resulting methodologies were complex, requiring underwater flanged connections completed by divers to join the outfall pipeline sections with each stage of trenching and significant temporary works to enable crane and personnel access over water,” says Patten.
Adopting trenchless technology to reduce impact
Later in the decade the Christchurch City Council Ocean Outfall, through the adoption of pipe-jacking, eliminated the need for trenched pipeline construction through the foreshore area.
“While this was an improvement on previous approaches, tunnelling distance limitations and site constraints meant disturbance of the coastal dunes in Bromley could not be avoided,” Patten says.
In 2010, McConnell Dowell used its expertise in creative construction when it was contracted for the Rosedale WWTP Ocean Outfall project by the North Shore City Council in Mairangi Bay, Auckland.
The original scope of works included a section of open trench, a tunnel, and a marine outfall. Through an interactive tender process, the team developed an alternative for a single bored segment lined tunnel by Earth Pressure Balance machine and marine outfall connected by an innovative offshore riser. The alignment was also straightened resulting in a $7m cost saving to the client.
This collaborative approach led to robust, innovative design solutions, further exemplified by the 45 m deep, 6 m internal diameter cascade drop structure connecting the inlet work from the ponds and UV plant to the outfall pipeline. The cascade drop structure was the first of its kind in New Zealand and was developed in partnership with the University of Auckland.
“The project was successful for all parties with local residents and other key stakeholders very satisfied with the project execution – a key objective of the client,” says Patten.
Providing a step change
Fast forward to 2017, McConnell Dowell has found that Direct Pipe® provides a step change allowing shallow ground entry and potential for significant drive lengths further reducing impact.
Watercare engaged McConnell Dowell and its design partner, McMillen Jacobs in March 2017 to install a new wastewater outfall, upgrade the existing pump station and build a new ultraviolet disinfection facility to increase the outfall capacity for its Army Bay Wastewater Treatment Plant at Shakespear Regional Park on the Whangaparaoa Peninsula.
Direct Pipe®, a methodology new to New Zealand, was ideal for this project as it enabled pipelines to be installed over larger distances and more accurately than some traditional trenchless tunnelling techniques. In delivering the project, McConnell Dowell achieved a world-record for the longest Direct Pipe drive at 1929 m.
McConnell Dowell selected Herrenknecht’s Direct Pipe® as the most appropriate tunnelling methodology for this project because it was the most efficient and safest way to install the pipeline at Army Bay.
For this project, only an entrance and two inspection shafts (one temporary), (excluding the shallow dredged offshore retrieval pit) were required in the park to install the pipe – significantly reducing the overall site footprint.
“The ability to install long pipe strings in a single drive reduced the amount of time required for construction and the Direct Pipe method also significantly reduced the risk of frac-out (drill fluid loss) into the sensitive environment,” Patten says.
A state-of-the-art new Herrenknecht TBM Direct Pipe system was used for the tunnelling works. The TBM was an impressive 34 tonne 13 m long Herrenknecht AVN1000DP TBM, with a cutter head diameter of 1325 mm and dual pipe thrusters with a combined available thrust force of 1,250 t.
The Direct Pipe method was used to pipejack a 2 km long 1200 mm OD steel carrier pipe, with a 1100 mm diameter HDPE liner, 45 m deep from the new pump house to a transition point in the Hauraki Gulf.
Patten noted that, “this same technique has been repeated to similarly benefit the Snells Algies Ocean Outfall, which reset the Direct Pipe® distance record at 2,021 m and the Westland Milk Products Ocean Outfall in a high energy wave climate off the West Coast of New Zealand’s South Island”.
Building from the past for the future
With a strong footprint in New Zealand, McConnell Dowell was contracted as a specialist tunnelling subcontractor to deliver the Culvert E530 microtunnell pipejack for the Puhoi to Warkworth Motorway under State Highway 1.
Project Manager – Corban Reserve Stormwater Upgrade – Richard Atkin says the tunnelling operation was under the direct control of McConnell Dowell as the mine operator.
“The tunnel drive involved a 40 m long, 3 m diameter excavation, installing a total of 17 no. precast concrete pipes, weighing 13 tonne each,” he says.
“Tunnelling commenced by way of an open-face excavation utilising a semi-mechanised shield developed specifically for the project.”
McConnell Dowell undertook strong in-house temporary work designs, detailed construction planning and risk assessments, with the tunnel equipment developed and fabricated in house to provide a bespoke solution to a low cover pipejack.
Atkin says the scope of the project works included installing a 2.4 m diameter, reinforced concrete pipe, installed 40 m by pipe jacking and 70 m by open cut method.
“We also had to consider temporary launch and receiving pit for the pipejack, slope stability and settlement analysis, and permanent headwall and wingwall structures with associated aprons at the inlet and outlet,” Atkin says.
As part of McConnell Dowell’s construction method for the project, it equipped the lead pipe with a steerable shield to the required profile that controlled the amount of overcut to guide it back to the correct line and level from a fixed reference point.
In hard stable ground, the face was excavated then pipeline and shield were jacked forward.
“This sequence was repeated until the advance was complete, taking several cycles to complete a stroke and allow the next pipe to be installed,” Atkin says.
“The tunnel face was mostly self-standing and could be trimmed to the shield profile without significant overbreak or collapse.”
However, the project didn’t come without its challenges. Atkins says several obstructions were encountered including an unknown steel pipeline, redundant cables and service ducts containing data/communication cables, all of which could be exposed and tested by service specialists before being cut and removed by hand from within the front shield.
The open-face nature of excavation required there to be provision for face support within the shield.
Atkin says the face support system has been developed with due consideration to safety, effectiveness, and time taken to install.
“Most importantly, the system was to provide certainty that the working crew can quickly respond to support the face effectively in the event of ground disturbance or instability, poor or rapidly deteriorating ground conditions, surface settlement, or any other need to stop for a prolonged period,” he says.
With settlement markers surveying locations on the State Highway 1, spoil excavated from the face was fed directly into an apron fitted to the shield invert.
From this collection point the spoil was discharged onto a series of conveyors positioned under the sliding frame of the backhoe unit to remove the excavated spoil away from the face, and further transported in the tunnel via rail-mounted muck skips.
“The skips were initially manoeuvred by hand and then later by a pulling tug equipped with a secondary drop-pin braking system. They were also fitted with a drop-pin brakes at each end to prevent inadvertent runaway when parked,” Atkin says.
“When the skips were full, they were transported back to the jacking area for lifting of the shaft by crane. Spoil was tipped into a bunded stockpile via the quick-release doors while remaining rigged to the crane.”
McConnell Dowell used its expertise and experience to deliver not only the tunnel but the full scope of the project on time, within budget, and incident free.
Atkin says the culvert was constructed in accordance with the drawings and specification, achieving the alignment between points 110 m apart.
“This enabled the main contractor to construct the required connections from existing manholes within the area to tie into the main pipeline at the correct position, level and grade,” says Atkin.
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This article appeared in the October edition of Trenchless Australasia. Access the digital copy of the magazine here.