WestConnex M4-M5 Link – tunnels design, Sydney, Australia

Australia’s largest span road tunnel caverns, to get Sydney moving now and into the future

WestConnex is Australia’s largest road infrastructure project delivering 33-kilometres of motorway and road infrastructure through Sydney’s west. The M4-M5 Link is the third and final stage, that provides the missing link between the New M4 at Haberfield and M8 at St Peters, creating a western bypass of the CBD which will allow the full benefits of WestConnex to be realised.

A joint venture of Aurecon and Jacobs is providing detailed design and construction phase services for Acciona Samsung Bouygues Joint Venture on the M4-M5 Link Tunnels project.

This follows Aurecon’s public participation and feedback role in response to community engagement during the feasibility and planning phases of the M4-M5 Link Tunnels and Rozelle Interchange projects.

The WestConnex M4-M5 Link Tunnels

At a glance:

  • Provides a link between the first two stages of WestConnex; M8 (at St Peters) and New M4 (at Haberfield)
  • Will be approximately 7.5km long and accommodate up to four lanes of traffic in each direction
  • Includes ramps between the main tunnels and the surface interchange at St Peters as well as the Ramps between the main tunnels and the surface connection at Wattle Street, Haberfield
  • Connection provisions to the future Rozelle Interchange Iron Cove Link and eventually to the proposed Western Harbour Tunnel
  • An underground complex of ventilation tunnels at St Peters, together with a surface ventilation facility

The tunnel excavations will span almost 34 metres in order to accommodate the 5-lane traffic sections at the eight underground caverns where traffic will merge and diverge at the interchange ramps along the route. At 34 metres wide, the largest tunnel cavern is large enough to hold a Boeing 737-300 nose to tail across the vast space.

Digital engineering application in tunnel design

An integrated digital 3D model was developed for the tunnel and internal structures to establish the optimum spatial requirements and space proofing for the large span and tunnel length. Space proofing ensures that all required functions are accommodated within the planned tunnel volume.

Optimising tunnel support

Shotcrete is sprayed against the excavated surface to form a structural shell for rock support on the project. The Aurecon and Jacobs design engineers used sophisticated modelling techniques to analyse the compressive membrane action of the shotcrete shell, followed by field testing to validate the modelling.

By accounting for the compressive membrane action, engineers optimised the thickness of the shotcrete for rock support.

Solving the rockbolt challenge

In tunnelling, rockbolts are used to reinforce the rock around the excavation. As the M4-M5 Link Tunnels span up to 34 metres at the widest point, a rockbolt challenge emerged. A span of such width would traditionally use long cable-bolts, but these are slow to install and cost significantly more compared with standard rockbolts.

To address this challenge, engineers designed a system where longer bolts will only be installed in the central section of the tunnel roof, with standard rockbolts used on the tunnel shoulders. This removes the need for time-consuming and costly, long cable bolting across all parts of the tunnels.

An additional lane at the St Peters interchange

Increased demand to future-proof tunnel projects is leading to more complex large span tunnels and caverns. This presents significant engineering challenges. The design joint venture’s engineers on this project had to design a road tunnel to connect into transport networks that are proposed to be built in years to come.

To do this, they optimised the tunnels’ vertical and horizontal geometries against a number of constraints. These included existing Sydney Water and Sydney Metro tunnels, buildings with basements, areas of shallow rock cover and existing cut and cover transition structures at Haberfield and St Peters, which the tunnel ramps needed to connect through.

At St Peters, this optimisation of road alignment and innovative tunnel support design allowed the Jacobs and Aurecon designers to increase the number of exit lanes from the main tunnels to the St Peters Interchange from three to four lanes, and thereby provide substantial additional future capacity at this crucial connection point with Sydney Airport and Port Botany via the Sydney Gateway project when it is delivered in the coming years.

The M4-M5 Link Tunnels and Rozelle Interchange projects form a critical part of WestConnex, connecting the first two WestConnex stages; M8 and New M4. By easing congestion and connecting communities, the new motorway will provide crucial support for Sydney’s long-term economic and population growth.

Unfortunately, you are using a web browser that Aurecon does not support.

Please change your browser to one of the options below to improve your experience.

Supported browsers:

To top