The Metro Tunnel is a key rail infrastructure project currently under construction in Melbourne, Australia that includes the construction of twin 5.6 miles (9km) rail tunnels between South Kensington station and South Yarra with five new underground stations. While TBMs are being used to build most of the Metro Tunnel, station box excavation and tunneling between the two Central Business District CBD stations is being undertaken by roadheaders.
Site geology comprises of interbedded Siltsone and Sandstone known as Melbourne Formation. The mineralogy in its unweathered state typically comprises of a mineral assemblage principally of quartz (35% up to 59%).
The Cross Yarra Partnership, a consortium comprising of, John Holland Pty Ltd, Lendlease Engineering Pty Ltd and Bouygues Construction Pty Ltd, initially contracted Grydale to provide a ventilation design system for the construction of the Metro Tunnel. Grydale was later contracted to supply all dust collection equipment required for the duration of construction works.
Initial engineering consultation converted the ventilation design from an overlap or cross-over extraction methodology to the now in-situ, proven extract methodology. Within full extract ventilation systems, air is drawn through the tunnel using negative pressure from the outside and is exhausted via ducting to a dust collector to deliver clean air to the atmosphere. This system removes all contaminants from the tunnel at the face and supplies fresh clean air for Civil, Mechanical and Engineering Works.
The Metro Tunnel provided some unique challenges for the ventilation design, including space and noise constraints plus the location of excavation works within the CBD.
The design was created using Ventsim Design 5 to simulate ventilation, airflows, pressures, heat, gases,
radon, fire and other key ventilation data to be considered, along with managing the financial constraints of the project.
The ventilation design was separated into 13 stages, representing the tunnel area changes, and was designed around the last dig sequence, where maximum air flow is required. The ventilation design utilises dust collectors for each stage, with additional axial fans to boost airflow in the latter stages of construction.