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.