Mairangi Bay Gravity Sewer Upgrade

‍Location:

‍Mairangi Bay, Auckland

Project Overview:

‍The wastewater network in the Mairangi Bay catchment is subject to wet weather overflows. CDS was engaged to undertake the preliminary design and construction methodology for a new pipeline to divert flow from the overloaded network.

Challenges:

The steep terrain in the Mairangi Bay catchment required multiple route options to be assessed for constructability. Due to the topography and deep shafts making pipe lead-in trenches unpractical, uPVC restrain pipe was designed for much of the new pipe network.

To ensure that data relevant to the HDD process was captured, CDS defined the scope of the geotechnical investigation, oversaw the field work and accounted for the results in the design and methodology process.

The new pipe alignment depth and high ground water table required a method to prevent ground water drawdown. CDS proposed an approach to control flow of ground water into excavations and drilled boreholes during construction.

With seven separate pipe installations across the project, CDS worked to keep construction outside of private properties across six of the seven locations. With deep manholes this required accurate design of rig setback locations and pilot hole lead-in alignments.  

Large elevation differentials and long installation lengths raise the likelihood of hydro-fracture. The highest risk locations were identified, and a hydro-fracture analysis was undertaken at these locations. Control measures were recommended to reduce this risk.

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