Not all of the damage done to Christchurch, New Zealand, by a string of bad earthquakes since 2010, is visible from the surface. A large portion of any city’s infrastructure lies underground, and has either been destroyed or damaged from the earthquakes. This damage includes most waste water systems in the city and the surrounding foothills.
In instances where sewer pipes completely failed, emergency measures during the natural disasters included temporary pumping, haulage or diversion until either the lines were restored or individual septic systems or collection tanks could be installed. Years after the earthquakes, there are still hundreds of miles of gravity-fed, clay-pipe sewer laterals requiring rehabilitation or replacement.
A trenchless solution
The Stronger Christchurch Infrastructure Rebuild Team (SCIRT) was created to oversee the reconstruction of Christchurch. SCIRT is a consortium of government agencies, known as “÷owner participants’ and of non-owner engineering, construction and maintenance providers.
One of the SCIRT’s fundamental tasks was to investigate and select techniques and materials that would not only repair infrastructure, but would also be able to stand the test of time and survive
future earthquakes.
In many places, trenching out the old lines was not an option. The durable, long-lasting materials SCIRT has chosen for replacing affected sewage laterals included polyvinylchloride (PVC) or polyethylene (PE) pipe.
These materials could be installed without the use of open-trenching through pipe bursting replacement technique.
Pipe bursting has proven to be not only one of the quickest trenchless installation techniques but one of the least intrusive to surrounding landscape and properties, as well as bearing a lower cost than other methods.
General Manager of Ditch Witch New Zealand John Grant said about 30 HammerHeadå¨ PortaBurstå¨ units were currently being used for pipe bursting projects in New Zealand.
PortaBurst units are 30-tonne pipe bursting systems used for replacing 50-150 mm lateral pipes.
As its name implies, the units are compact. Their modular design for easy disassembly, assembly and transport gives them even greater portability. Yet, the compact units have the power to easily fracture, press aside and pull 50-100 mm diameter PVC pipe through 45 m runs of existing vitrified clay pipe at rates of up to 3.7 m per min – without requiring excavation of the old pipe.
“In a lot of cases our contractors can’t use traditional excavating machinery to get to the pipe.
“They have to hand dig or use hydro-excavators, and they have to avoid disruptions to other services using the same trench,” Mr Grant said.
Mr Grant added that while initially his clients were just using the PortaBurst product for replacing pipe infrastructure underneath footpaths, driveways and around other utilities, they began selecting it for jobs that didn’t even require a trenchless method.
Pipe bursting became their first choice,” he said.
No longer a pipe dream
The feature of the pipe bursting system that struck Mr Grant first was how straightforward it was.
“Training does not take much time, it’s mostly done right on a job, learning how to dig the pits and shore them up properly, threading cable and doing the pulls,” he said.
For some of Ditch Witch’s customers who had never engaged with trenchless techniques during pipe rehabilitation works before, the uptake was easier than expected, Mr Grant said.
“At first the pull is done slower than in typical production, but customers catch on fast and are able to work on their own after that,” he said.
Mr Grant said he had one customer who reported he was astounded by what the product allowed him to do.
“Just doing crazy things, pulling around beams and retaining walls, he had heard that the PortaBurst could be used to make 45 degree bends, but he couldn’t believe it until he actually did it,” he said.
Typical runs on a Christchurch pipe burst are about 25 m, though under-road laterals may be 10-20 m long. They are almost all completed within a standard working shift.
Straight-forward bursting process
During a pipe replacement the PortaBurst contractor digs a pit on each end of a planned run down to the existing pipe. Much of the work is in pre-worked ground with silty sand matter, so pits are hydrovacced. The hydrovaccing makes a neater hole and is easier to restore.
SCIRT selects the replacement pipe options that contractors can choose from, some contractors have used threaded PVC, whilst others prefer to work with single lengths cut from reel stock or with fused lengths of straight pipe.
As the burst head progresses through the existing pipe, it fragments it, pushing the pieces aside whilst pulling the replacement in behind it. When it exits the other side, the contractor removes the head, documents his replacement work with CCTV, trims the pipe and reconnects it.
Ground conditions will return to normal around the pipe, with very little evidence of disruption.
THE LONG ROAD AHEAD
There is a tedious process residents go through to get approval for a sewer line replacement, insurance companies first have to approve the scope of repairs and the technique that will be used to fix them. Contractors are sometimes held up, waiting to start a job until the work is approved.
But there’s no end in sight for the work yet to be done. There may be anywhere from five to ten years of work to be undertaken. Not much work has been done in the foothills around Christchurch yet. There is ground seepage, and subsidence. And the earthquakes haven’t stopped.
Several years after the 7.1 magnitude earthquake in September 2010 and the deadly 6.3 quake in February 2011, the residents of Christchurch still live with consistent earthquakes yearly. More are inevitable due to New Zealand being located on the infamous “÷Ring of Fire’.
Nicknamed the “÷Shaky Isles’, New Zealand has consequently become advanced in post-earthquake rehabilitation techniques. Now, pipe bursting with the HammerHead PortaBurst system is one of them.
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