It was a textbook case study of the advantages of Trenchless Technology: A project to upgrade a sewer in the sleepy town of Te Awamutu, New Zealand, called for the installation of a sewer main within an easement through private property and beneath the foundations of an existing commercial building.
Adding to the challenges, the new main also had to connect into an existing manhole outside a busy burger franchise in the carriageway of the southbound lane of a national state highway.
For this leg of the project, the tender document stipulated the use of horizontal directional drilling (HDD). Subcontractor Smythe Contractors Ltd was selected to complete the HDD shot, but once the team arrived on site to review the pipeline alignment, a recent change to the surrounding environment re-wrote the project’s parameters entirely.
Changing circumstances
During the project’s tender phase, Smythe had proposed to position its drilling equipment at the busy intersection of State Highway 3 (SH3) and Rickit Road. The drill shot was approximately 80 m in length and was to connect into a new manhole, adjacent to SH3, on Jackson Street.
Provision was thus made to drill to Jackson Street, where a vacant section with an old garage was to be used as a pipe-string storage site to support the drill shot.
It was originally planned to use the vacant section as a lay-down and storage site for fusion services and the pipeline. However, once the contract was in place, it was discovered upon arrival to Jackson Street that the old garage had been demolished. In place of the garage was a housing construction site and a new concrete block wall. These changes to the surrounding work site meant a traditional polyethylene pipe-string would not be installable.
Given these new circumstances, it was decided that the way forward was the HDD installation of an Iplex-supplied segmented restraint jointed pipe system. With previous experience installing Iplex’s Restrain㢠PVC Gravity Sewer Pipe in smaller diameters, Smythe opted for a technique known as the Cartridge Method – Pit to Pit, selecting 3 m length DN300 pipes for the installation.
Cartridge Method Pit to Pit
The installation pit was established within Jackson Street using trench shields to ensure the construction team would remain safe at an invert depth of 3.3 m. Once the pre-reamed borehole had been completed, the pipe was connected to the back reamer swivel and the first pipe was in place for an early pull-back to begin the next day. But Mother Nature had other ideas.
The following morning, a weather bomb hit the North Island, filling Te Awamutu’s old and leaking sewers beyond capacity. The team persisted despite this weather and, at dawn, the construction team carried on with the planned pull-back.
This decision was possible thanks to the jointing of Restrain PVC pipe being able to be completed in wet conditions.
The four-man construction team worked in pairs. One pair prepared the pre-lubricated 54 kg pipes on the roadside, bagging the pre-lubed pipe ends with plastic rubbish bags to keep the mud out of the threaded joints while lowering them into the installation pit. The other pair received the pipes and began jointing pipes simultaneously within the trench shields. Once the pipe joiners established a methodology while working together, the pull-back cycle times decreased, eventually recording five minutes cycles from pipe-pull begin to pipe-pull end.
With the last pipe pulled into position by lunchtime, the project – one that was faced with environment changes and unfavourable weather – was a well-earned success for Smythe Contractors Ltd and Iplex’s Restrain product line.
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