The WestConnex M8 is one of the largest multi-disciplinary projects that WSP* has undertaken in the infrastructure sector, drawing on the expertise of our local, regional and global resources.
With Sydney experiencing rapid population growth, increasing road congestion and transport challenges, road improvements are urgently needed for quicker, safer and more reliable travel. The WestConnext M8 project connects Kingsgrove via 9 km twin underground tunnels to a new interchange at St Peters, built on the former Alexandria municipal landfill site. By effectively duplicating the existing M5, it is expected to cut 30 minutes off a peak-hour trip into the city.
WSP’s involvement
This project is unique and complex, with a multi-level road interchange constructed over an industrial landfill, and multiple access points to major road tunnels. WSP* worked on this enormous project over six years, with wide-ranging involvement and cross-discipline interaction among the many aspects, including geotechnical, contamination, hydrogeological, landfill management and infrastructure, foundation improvement, design aspects, structures and services.
Investigation, sub-surface modelling, design and construction services
To assess the ground conditions for design and construction, the team conducted extensive geotechnical, hydrogeological and contamination site investigations across the full length of the corridor at over 500 locations, at depths up to 90 m. Parts of the site investigation were coupled with the project’s instrumentation and monitoring plan, incorporating instruments to monitor future tunnel impacts.
The landfill investigation was particularly challenging, requiring penetration through as much as 40 m of municipal and construction waste deposited over previous generations in a historical brick pit excavation. Prior to filling, multiple large block movements in the historical brick pit excavation were reported, which we investigated, assessed the risk and installed monitoring.
During the site investigation, we identified complex and variable ground conditions (e.g. variation of rock strength, degree of fracturing and rock mass permeability) along the project corridor. We conducted extensive 3D geological modelling to advise our construction and design partners on foundation levels, rock mass quality, and the location of major geological structures for tunnels and surface works including 10 proposed bridges. This informed subsequent design of roadway and ramp alignment, tunnel support, the hydrogeological model and the grouting assessments.
We undertook hydrogeological modelling of the project’s impact on the surrounding aquifers and proposed potential remediation options, including pre- and post-excavation grouting, hydraulic barriers through alluvial sands, and landfill capping systems to reduce leachate generation.
To manage the risks around earthworks and foundations within the landfill and brick pit zone, an extensive piled pavement was proposed to facilitate road construction. We worked directly with design and construction partners to optimise the extent and depth of the design elements. We also designed alternative cost-effective earthworks methods and treatments using rapid-impact compaction and surcharging for appropriate areas of roadway foundations, allowing earlier site access and transition embankments.
Our team undertook the instrumentation and monitoring design, installation and construction documentation for surface works and tunnel surface impacts. Instrumentation included above- and below-ground survey control for displacements and settlement, inclinometers, settlement plates and shape array systems to monitor construction and earthworks displacements. An extensive array of hydraulic monitoring points including vibrating wire and standpipe piezometers were used to assess the project’s impact on the regional gas and groundwater and to allow us to conduct regular quality sampling.
We also designed extensive temporary works packages (earthworks, pavements, foundations, retention systems) required during construction, and supervised construction of tunnels, slopes, excavations, crane and piling platforms.
Reforming a historical landfill and minimising waste
WSP* worked with the project architects, designers and regulatory stakeholders to reform the landform of the St Peters Interchange by incorporating the historical landfill into the final design. This supported the project’s strong sustainability goal of minimising haulage of excavated materials and waste away from the site.
Existing contaminated material was housed within the on-site containment cell and ongoing generation of leachate was reduced by the vertical barrier wall between the site and the adjacent Alexandra Canal that we designed. The incorporation of the landfill into the project and final landform required a capping system with extensive monitoring systems, geosynthetics, landfill leachate drainage and treatment, and gas capture systems to allow remediation and eventual closure of the landfill site (for which we developed the Landfill Closure Management Plan). We assisted with redesigning the landform and steeper batters (reinforcement for slope stability) that are typical for landfill closure projects, which reduced the need to remove material from the site and associated truck movements.
We also worked with our client to identify options to reuse clean tunnel spoil and excavated waste material for pre-load embankments, temporary works platforms and general earthworks.
A road that leads to a more liveable city
The entire WestConnex project is a 33 km, free-flowing network, safely and smoothly links the south-west and west of Sydney with the Centre Business District (CBD), and airport. As well as easing travel frustrations, the community will benefit from reduced emissions, less air pollution, increased parks and playgrounds, new walking and cycling paths, and a public art program – all contributing to greater sustainability and liveability of Australia’s largest city.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.
