Perth Main Sewer - Northbridge to Highgate

Trenchless Australasia — March 2008

The West Australian Water Corporation has upgraded sewer pipes in the Leederville and Highgate area as part of a five-year program to provide a safer and more efficient wastewater system. The construction of the pipeline by Trenchless Technology resulted in a high profile construction project being undertaken with minimal disruption to traffic.

The existing Perth Main Sewer Section 6 – which lies between the access chamber (AC) 6025, corner Lincoln Street, Smith Street and AC N1555 at the corner Fitzgerald Street, Newcastle Street – was constructed in 1913 using unlined reinforced concrete pipes and was in very poor condition with significant internal corrosion. An upgrade was paramount for the Water Corporation.

The existing sewer had been gunited several times since the 1950s to counteract hydrogen sulphide attack. Despite this, it was in an advanced stage of decay and the structural capacity of the pipe to resist loads was significantly reduced.

The nominal pipe diameter was 1,370 mm (54”) with grades as flat as 1 in 3,400. The initial expectation was that this sewer would be re-lined. However, it was instead decided to microtunnel with new pipe on a new alignment, using 2,100 m of DN1500 reinforced concrete pipe.

Supply and installation 13 new plastic liner access chambers also took place, as well as link-in from existing sewer at four locations and grouting of the existing sewer main.

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Section 6 was the final stage in the Corporation’s Perth Main Sewer upgrade project, with tunnelling completed in October 2007 and the final stages of work completed in December 2007. Contractors MacCormick Civil & Tunnelling constructed approximately 2 km of sewer main pipes by microtunnelling from Fitzgerald Street in Leederville through to Smith Street in Highgate, replacing existing underground wastewater pipes.

The new pipeline was designed and constructed adjacent to the existing sewer with the minimal amount of separation required allowing for vibrations.

The tunnelling project was completed on time and within budget, and the Water Corporation has complimented the contractor for the exceptional project that was delivered.

Scope

A Value Management Study Workshop on this project was conducted to evaluate and identify a preferred option for consideration for the Pert Main Sewer Section 6.

To prevent a sewer collapse, associated overflow and impact on public health, the sewer pipes of Perth Main Sewer Section 6 had to be restored. While pipes with sufficient hydraulic capacity can be relined, conduits with insufficient capacities have to be replaced.

To guarantee middle to long term flow capacity, the sewer pipes should be able to transport the future flow in 30 years. Replaced pipes with larger diameters have to be able to transport long term sewer design flow safely. The maximal growth rate was derived from the Australian Bureau of Statistics and the Western Planning Commission and has been identified as 1 per cent.

From the Value Management Study, it was recommended that two options remained after more than a dozen were identified, subsequently refined to seven with a further five being eliminated. Discussions with the Contractor for the Perth Main Sewer Section 5, MacCormick Civil & Tunnelling showed that with the availability of the microtunnelling machine the sewer could be replaced with a DN1500 pipe at a relatively low cost, and so microtunnelling with new pipe on new alignment was chosen.

Decommissioning of Existing Main Sewer

The existing main sewer was decommissioned by filling with low strength grout slurry in similar fashion to that of the decommissioning of Perth Main Sewer Section 3.0 and Section 5.2

Features

* The new pipeline design incorporated two fixed tie-in points into the old main situated in the suburb of Northbridge – right next to Perth’s CBD. Site access was extremely limited and most of the tunnelling was designed in the roadways.
* The sewer was constructed in soft ground strata with 3 m high groundwater table and invert depths ranging between 3 m to 8 m.
* The microtunnelling comprised over 2 km of 1,500 mm CSR Humes RCPL pipe, constructed by closed face slurry shield microtunnelling using pipe jacking methodology.
* Caisson shafts were utilised to minimise potential environmental and dewatering issues. The caisson shaft methodology produces a shaft which is completely sealed with a water tight plug at the base.
* The construction methodology of the caisson shafts and drive lengths of 250 m resulted in minimal traffic disruption.
* MacCormick Civil & Tunnelling was the contractor for the whole project. Herrenknecht AVN 1200 micro tunnelling machines were utilised.
* On completion of tunnelling, new manholes were constructed followed by final connections to the existing sewer lines.

Other Options Considered

Option 1: Do nothing

Option 2: Open cut and replace on
existing alignment

Option 3: Open cut and replace on new alignment (adjacent to existing main)

Option 4: Reline existing DN1370 only

Option 5: Reline existing DN1370, open cut and replace flat top

Option 6: Reline existing DN1370 and microtunnel flat top

Appeared in issue: Trenchless Australasia - March 2008 Trenchless Australasia — March 2008