The Metro Tunnel is a key rail infrastructure project currently under construction in Melbourne, Australia that includes the construction of twin 9-km 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 tunnelling between the two CBD stations is being undertaken by roadheaders.
Site geology comprises interbedded Siltsone and Sandstone known as Melbourne Formation. The mineralogy in its unweathered state typically comprises 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 project. Grydale were later contracted for all dust collection systems required for the project.
Initial engineering consultation converting the design from an Overlap or Cross-Over Extraction methodology to the now in-situ proven Full Extract methodology. The complete extract system is designed to remove all contaminants from the tunnel at the face and supplies fresh clean air for Civil and Mechanical and Engineering Works.
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 design was separated into 13 stages, representing the tunnel area changes with dust collectors for each stage, with additional axial fans to boost airflow in the latter stages of construction.
Melbourne Metro provided some unique challenges for the ventilation design: space and noise constraints plus the location within the CBD. The ventilation system was designed around the last dig sequence where maximum air flow is required. The ventilation design utilises scrubbers for each stage, with additional axial fans to boost airflow in the latter stages of construction.
The Melbourne Metro Stations will be built as trinocular caverns, where three overlapping tunnels will be mined by the road-headers to total platform width will be around 19m – one of the widest metro platforms in the world. Two temporary acoustic sheds up to 20m high enclose construction sites within the CBD to minimise impacts from construction of stations on residents, businesses and local community. A system of louvres on the outside of the acoustic sheds help to control clean air flow into the tunnel. The ventilation system was also designed for environmental noise limits whilst balancing required air flow into the tunnel.
Since construction began Grydale have designed, manufactured and supplied the following dust collection systems, in line with the ventilation design:
- CBD North – 4 x JMS-60-MES (60m3/s electric, drag skid units).
- CBD South – 5 x JMS-60-MES (60m3/s electric, drag skid units). CBD South had further space constraints, so these units were customised to have four roof mounted fans to provide a reduced footprint so scrubbers can be located within the acoustic shed.
- Parkville – 1 x Grydale JMS-30-MDT: (30m3/s diesel, track unit) and 1 x JMS-10-MES (10m3/s electric drag skid unit) were rented to provide short term dust extraction at source within a positive pressure ventilation system.
The JMS-60-MES dust collection units run 415kW fan motors on Variable Speed Drives which has allowed up to 40% power savings. The excavation sequence has been programmed into the dust collectors via the Programmable Logic Computer (PLC) so as excavation advances power progressively increases. Ventilation design for the CBD underground stations utilises a mix of negative and positive pressure systems, whilst design for Parkville and Anzac stations, and the portals, use positive pressure ventilation. The latter works are supported with mobile source extraction units for shotcrete and other works, where dust or fibre controls are required in addition to the supplied air ventilation.