710-mm Diameter Trenchless Stormwater Pipe

HDD in New Zealand

new zealand flagThe Memorial Park traffic tunnel project in Buckle Street, Wellington, New Zealand, is in full swing to be completed by ANZAC day 2015, a day that commemorates the country’s servicemen and women who have served and have died serving. This date also marks the 100-year anniversary of the Gallipoli campaign.

The project of “national importance” involves undergrounding the traffic on Buckle Street and building a memorial park in front of the National War Memorial where the Unknown Soldier was laid to rest in 2004.

The project is twofold and achieves another step toward improving traffic flow to the airport through the Wellington CBD. Lowering Buckle Street by 9 m created a low point in the cut-and-cover tunnel thereby creating a stormwater collection issue. A gravity pipeline was required to connect the tunnel’s low point to the existing stormwater network at the top of Cambridge Terrace (next to the Basin Reserve). This was no easy task.

To maintain a gradient, the pipe needed to pass within centimeters under a brick arch sewer constructed 100 years ago and no one really knew its foundation construction details. The pipeline would also need to pass under SH1 at depth and then beneath an historic building before exiting in Cambridge Terrace. The ground conditions are variable and range from rock under Buckle Street to marine sediments in Cambridge Terrace. When the first European settlers arrived in Wellington around 1840, there was a shallow lagoon on the site that the Basin Reserve is now located, which was linked to the harbor by a stream following the route of what is today Kent and Cambridge Terrace. The colonists planned to turn the lagoon into an inland anchorage for the city. However, when New Zealand’s most powerful earthquake on record struck in 1855, the area was raised by 2 m and the lagoon instead became a swamp. The ground conditions at this end of the drill shot were a real concern to the success of the pipeline.

The accuracy expectations for the placement of the pipeline by the Memorial Park construction alliance were of the highest standard and bordering on unrealistic given the changeable ground conditions.

Universal Underground Ltd (UUL) was awarded the project in July 2013 and was chosen for its depth of experience, which would provide the best chance of achieving the highest quality outcome.

UUL used its largest horizontal drill rig for the project, a Universal HDD UNI 200×240 (100-ton pull back). This drill has a proven track record and has the current record for installation of the longest on-shore HDD pipeline in New Zealand, a 1,036-m long pipe with a total pipe weight of 200 tons.

The pilot hole was drilled with the drill rig set up in the traffic trench. This was necessary to avoid drill rod sinkage in the softer ground closer to the Basin Reserve. A Para-Track 2 location system was used and required running a wire along the ground surface to induce a current into the ground for the drill head to follow. Although the Para-Track 2 is one of the most advanced underground location systems available, it struggled to provide the required information at a critical section along the deepest point under SH1. Contributing to the difficulties was the presence of man-made electro-magnetic interference: A cell tower 30 m away, high-voltage underground power cables and more than 10,000 large magnetically charged moving vehicles crossing the drill path daily. A DCI Eclipse transmitter was run parallel with the Para-Track 2 and when the field from the aboveground wire was turned off, additional information could be obtained to make comparisons. Grade information was never compromised because the drill head was hardwired back to the on-board drill computer with a wire connection down the inside of every drill rod. The 200-m long pilot hole was drilled without compromising the grade or the line; only reaching its maximum allowable tolerance once.

The drill rig was now re-located to Cambridge Terrace and a back reamer was connected to the drill string. In order to reduce fluid consumption, UUL’s mobile solids control fluid recycle plant was introduced to the system.

The recycler reduces fluid consumption, fluid waste and drilling additives. This saves money and reduces impact on the environment.

As the hole opening proceeded, the tunnel was upsized to 850 mm in diameter. The tunnel could not be reamed the full length because the ground conditions at Cambridge Terrace were so weak that the tunnel would not hold open. A carefully balanced fluid system was maintained for the duration of the hole opening.

In tandem with the tunnel opening operation, the 710-mm diameter, SDR 11 polyethylene pipe was delivered and welded into two 100-m long pipe strings. Once welded and tested, they were positioned ready to be pulled into the tunnel. The front end of one string was fitted with a steel towing cone and connected to the drill string.

The pipe was installed into the tunnel over a 12-hour period. This included stopping for four hours to weld the two pipes together and reaming the final 30 m of tunnel in the collapsing ground.

On completion of the pipe installation, a cement plug was pumped around the pipe at each end to control groundwater tracking along the outside of the pipe. The pipeline was pressure tested, CCTV undertaken and handed over to the alliance.

This was a technically difficult project with an expectation of high accuracy in immensely variable ground conditions and tight working space. Due to the fast-track construction, hundreds of workers and dozens of subcontractors compete for space to undertake their component of the work, requiring a high level of time management and communication.

A team approach by all involved contributed to the ultimate success of the stormwater outlet pipe from the Memorial Park Tunnel in down town Wellington.
Neil Vanner is contracts engineer at Universal Underground Ltd.
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