Client North Shore City
Project Value $6 million
Description : This project involved the construction of a new trunk sewer tunnel as part of the North Shore City’s strategic wastewater undertaking, Project Care, to reduce wet weather sewer overflows to the city’s beaches and to accommodate future development and population growth. Temporary works comprised about 65 per cent of the total value of the project.
We assisted North Shore City to find a new pipeline route that did not compromise the public amenity of Torbay beach or cultural impacts (e.g. through beach reclamation).
Our proposal presented the Council with a way of achieving its objective, by following the route of the existing tunnel, and therefore avoiding further consent applications. In addition, we designed a temporary flow diversion, which matched the capacity of the existing sewer pipe in the tunnel, in the form of a one-kilometre DN 300 SDR17 PE pipe installed by directional drilling along residential roads (plus one 70m section fixed to the surface of a public walkway). A compact, 155kW pumping station was constructed on a small coastal reserve.
Work was carried out from both ends of the tunnel, achieving an average production of four metres a day for each tunnel crew to lay rail, install tunnel support, remove spoil and withdraw pipes. A hydraulic winch was used to pull the new pipe into place along rails, using a double purchase pulley arrangement to achieve a 12-tonne pulling force.
Work undertaken included:
• Replacement and upgrade of an operational DN480 trunk sewer running through a 40-year-old, 750-metre long, unlined rock tunnel below a residential area of North Shore City.
• Design and construction of a temporary pumping station capable of pumping 165 litres/second and the installation of approximately 1km of diversion pipeline by direction drilling.
• Installation of 750m of tunnel support using steel sets and timber lathes.
• Construction of a rail system used for removal of the existing pipe and spoil, and also for new pipe installation.
• Removal of 750m of reinforced concrete pipes and 1900 cubic metres of spoil within a confined, 1.8m-high tunnel.
• Installation of 750m of DN1000 polyethylene pipeline by winching a 57-tonne string of pipe into place on rails.
• Backfilling the 2500m3 annulus around the new pipe by pumping lightweight foam concrete through a 50mm delivery pipe down vertical shafts onto the tunnel.