Marching on with microtunnelling
New Zealand civil engineering contractor March Cato is past the halfway point of a project to install almost 1 km of new water main for water utility Watercare in the Wairau Valley, north of Auckland.
Taking place two years after the Eastern Rising Main Replacement project was completed by March Cato in July 2015, the Wairau Pump Station: Eastern Rising Main Replacement (Wairau Road Section) is a New Zealand first, involving the installation of approximately 750 m of HOBAS glass reinforced plastic (GRP) pressure pipe by microtunnelling at a depth of 5.5 m, under Wairau Road.
Other works include the construction of six manholes, with one housing an air release valve and another housing a scour valve on completion, and the installation of an additional 140 m of pipe by open cut methods.
The new main is required to replace the existing wastewater rising main installed in the 1960s; it had been identified for replacement due to its age and susceptibility to breaks. This upgrade, among a number of others on the Auckland-wide wastewater network, is a key component of Watercare’s long-term strategy to improve Auckland’s wastewater infrastructure, prepare for the forecasted population growth and improve service levels.
March Cato Project Engineer Dan Casey says the installation has been made challenging by its location in the commercial and industrial area of Wairau Valley, in addition to the alignment running under one of Auckland’s regional arterials, Wairau Road, carrying approximately 11,000 to 12,000 commuters every day.
“Selection of the trenchless installation option provided the best solution to accommodate the necessary jacking and reception shafts within the carriageway, while minimising commuter disruption at the same time,” says Mr Casey.
“Pre-design geotechnical investigations confirmed the published geology of the site being Waitemata Group soils and undifferentiated alluvium of Holocene age, predominantly mud, sand, silts and gravel.”
“The alignment through the Wairau Valley section occurs within ‘recent’ alluvium, comprising a variety of sediments: typically clays to silty sands with some organic layers, fibrous peat and woody materials.
“Herrenknecht’s AVN 800XC, a closed faced slurry microtunnel boring machine (MTBM) capable of installing 1,000 mm to 1,300 mm diameter pipe, was selected based on the ground conditions, as well as the design pipe diameter and drive lengths. The original design was for the installation of a polyethylene pressure pipe within a jacked concrete carrier pipe.
“Investigations into alternative options, in particular the installation of a single pipe from a practical perspective with time and cost savings, resulted in Watercare opting for a microtunnelled HOBAS GRP jacking pressure pipe for the new pipeline.”
Mr Casey says March Cato identified the challenges early in the project’s development.
“Along with the usual challenges of any construction and microtunnelling project, the key risks identified from the outset included working in the carriageway, particularly in a location with very high traffic flows, and confining the aboveground setups within the given construction footprint,” he says.
“Other challenges faced included unforeseen ground conditions, striking an unidentified underground service, and the effects of construction on neighbouring businesses and property owners. However, opting for a trenchless method over open cut has significantly reduced the potential for major disruption to road users.
“Establishing the bi-directional jacking shaft as the central site for the slurry separation plant, March Cato has been able to contain its construction footprints at the reception shafts located in the carriageway within the available footprints.
“Pre-construction consultation carried out by Watercare with the local businesses, road users and wider community has delivered a positive outcome and interest in the project.”
The project started in July last year and is planned to be completed by October 2018.
This article was featured in the Spring edition of Trenchless International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
For more information visit the March Cato website.
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