Alexandria (inner-East Sydney), New South Wales


Sydney Water and City of Sydney



  • Pipelines


2015 – 2018

Contract value

$100 – 150 million 


Green Square Trunk Stormwater
(Drying Green Alliance)

Fast facts


DN1800 Microtunnelling

5 no.

caisson shafts

100 %

of works self performed


Green Square is one of the City of Sydney’s key urban renewal precincts earmarked for development, and given it was prone to flooding, a new stormwater drainage system was required.

The alliance’s scope included the design and construction of a 4km drain and deep caissons for gross pollutant traps to address flooding issues within the Green Square urban renewal area to enable future development.

The 4km DN1800 concrete pipeline was self-performed by microtunnelling in twin and triple parallel formation with each line no more than 800mm apart.

Part of the alignment included long curved sections to avoid existing significant services.

Ground conditions were classified as restricted and hazardous containing asbestos with construction undertaken in highly variable ground conditions consisting of saturated fill, sand, clayey sands, peat and some rock.

To facilitate microtunnelling, 5 large shafts up to 12m in diameter were constructed with cast in-situ caissons for the TBM launch and retrieval access, and construction of drainage structures below groundwater in highly non-cohesive soils. Ongoing dewatering was mitigated by using the ‘wet’ caisson sinking technique.

The alliance, made up of Sydney Water, City of Sydney, Seymour Whyte / Rob Carr JV, UGL, Parsons Brinckerhoff and RPS Manidis Roberts delivered:

  • the safe construction of a 4km concrete pipe stormwater drain and deep shafts in varying, contaminated and water-charged geology
  • a significant reduction in disruption to local roads, building and utilities through complex sequencing and mitigation strategies by using a micro TBM in lieu of traditional open trenching methods
  • a significant cost reduction by changing the construction method to an innovative trenchless methodology
  • analysed/modified the TBM and jacked pipe segment design.

Project gallery

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