March 2015, Vol. 70, No. 3
Features
Honoring The Fallen (1)
The War Memorial Park traffic tunnel project on Buckle Street in Wellington, New Zealand is scheduled to be completed by ANZAC Day in 2015, marking the 100-year anniversary of the Gallipoli campaign commemorating all New Zealanders killed in war and honoring returned servicemen and women.
The project is of national importance as it the involves underground traffic on Buckle Street how it affects construction of the Pukeahu National War Memorial Park in front of the National War Memorial where the Unknown Warrior was laid to rest in 2004.
The project is two-fold and achieves another step towards improving traffic flow to the airport through the Wellington CBD. Lowering Buckle Street by nine meters (29.6 feet) created a low point in the cut and cover tunnel, which created 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 proved to be no easy task. To maintain a gradient, the pipe needed to pass within centimeters under a brick arch sewer constructed 100 years ago. Adding to the problem was that foundation’s construction details were unknown.
The pipeline would also need to pass under SH1 at depth and then beneath an historic building before exiting in Cambridge Terrace. In addition, ground conditions are diverse 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 where 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 two meters (6.5 feet) and the lagoon became a swamp.
The ground conditions at this end of the drill shot were a real concern to the success of the pipeline. The expectations for the accurate placement of the pipe by the Memorial Park construction alliance and the changeable ground conditions were an additional concern.
Universal Underground Ltd. (UUL) was awarded the project in July 2013. The company was selected for their in-depth experience that would provide the best chance of achieving the highest quality outcome.
UUL used their largest horizontal directional drill, 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 horizontal directional drilled pipeline in New Zealand – a 1,036 meter (3,398 foot) 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 keep the drill rod from sinking in the softer ground closer to the Basin Reserve. A ParaTrack 2 locating system was used that requires running a wire along the ground surface to induce a current into the ground for the drill head to follow. Although the ParaTrack 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 ParaTrack 2’s difficulties was the presence of manmade electro-magnetic interference. A cell tower 30 meters (98 feet) 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 ParaTrack 2, and when the field from the above ground 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 meter (656 foot) long pilot hole was drilled without compromising the grade or the line; only reaching its maximum allowable tolerance once.
The HDD rig was re-located in the night to Cambridge Terrace and a back cutter (reamer) was connected to the drill string. 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 up-sized to 850 millimeters (34 inches) 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 millimeters (28 inch) diameter SDR 11 polyethylene pipe was delivered and welded into two 100 meter (328 foot) long pipe strings. Once welded and tested, they were positioned 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 meters (98.5 feet) 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 ground water tracking along the outside of the pipe. The pipeline was pressure tested, CCTV was performed and handed over to the alliance.
This was a technically difficult project with an expectation of high accuracy in immensely variable ground conditions and a 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, which requires a high level of time management and communication. A team approach by all involved contributed to the ultimate success of the storm water outlet pipe from the Memorial Park Tunnel in down town Wellington.
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