Green Square is one of the City of Sydney’s key urban renewal precincts, which is being transformed from old industrial land into a major new residential, retail and cultural hub. The Green Square Stormwater Drain (GSSD) is the culmination of a strategic alignment between City of Sydney and Sydney Water to provide flood protection in the Green Square area. Through a process of optioneering and hydraulic analysis, a new 2.5 km long underground drain consisting of multiple 1800 mm diameter pipes was installed by microtunnelling, and an open trench box culvert was replaced with channel widening via an anchored retaining wall in the final 300 m from Maddox St to Alexandra Canal. The new drain augments the existing trunk drain system and reduces flood hazard, allowing Australia’s largest urban renewal project to proceed.
The channel widening section of the GSSD was originally intended to be constructed into the bank of the existing open channel. A constructability assessment for installation of the box culvert within the narrow corridor between the existing open channel and adjacent buildings indicated that open trench box culvert construction would not be cost effective. This paper describes an innovative solution, where the existing channel was widened using an anchored retaining wall, replacing the proposed box culverts.
The trenchless (microtunnel) solution offered an alternative, value for money approach with significantly reduced environmental impact and achieved comparatively minimal community disruptions.
This paper also describes the ground engineering challenges and solutions employed on the site which included difficult ground conditions, landfill and addressing impacts of wall construction on adjacent infrastructure such as roads, bridges and buildings. Ground engineering risks were successfully managed through detailed scoping of investigations, numerical modelling of designs and adoption of observational methods during construction. The specification requirements, design, installation, monitoring and performance of the successful microtunnel drain and anchored wall system are discussed.