Grydale’s in-house engineering team completed ventilation system design modeling for Laing O’Rourke to determine the ventilation and dust control requirements for the development of Central Walk.
To adhere to ventilation regulations, the following minimum ventilation is required:
- 1.5 m³/min (0.025 m³/s) for each worker.
- 4m³/min/kW (0.067 m³/s/kW) of diesel equipment.
- Air flow velocity between a minimum of 0.3m/s and a maximum of 2m/s.
In addition, volumetric calculations were undertaken to determine the requirements for the ventilation of Central Walk during construction.
An estimation of the diesel equipment load of approximately 300kW was used as a worst-case scenario. This load would require a volumetric flow rate of 20.1m³/s.
The cross section of the excavation working area was calculated to be 476ft² (145m2)and to achieve the minimum air flow velocity of 0.3m/s would require a volumetric airflow of 43.5m³/s.
Taking the greater of these, 43.5m³/s was determined as the requirement for the excavation of the working area to provide the minimum level of ventilation.
A Grydale JMS-50-MES (mobile, electric, skid) dust collector was selected as the best solution to provide the ventilation and dust control requirements for excavation works.
The JMS-50-MES was mounted on the surface at the main access shaft, with ducting running down the shaft with a 90-degree bend, then lengthening out to run along the drive to provide dust extraction during excavation. Locating the dust collector at the surface reduces plant within the excavation area.
The Variable Speed Drive (VSD) on the JMS-50-MES allowed the dust collector to operate using reduced power during the initial excavation works and for power to be turned up as excavation advanced further from the surface, when greater airflow was required. This feature provides power savings and reduces operating costs.
Ventilation System Design
The ventilation system was designed in a sequence of stages in line with the excavation methodology.
For the initial stages of excavation, air was drawn into the working area through the main ventilation access shaft. Ducting from the dust collector followed the excavation at the tunnel face, ensuring all dust was captured and removed from the work area.
There are eight escalators and shafts along Central Walk, four on the southern side and four on the northern side of the Central Walk excavation area. When works for each of the escalator sections commenced on the southern side, the coinciding escalator shafts were opened to the surface, and dust extraction was required where excavation was taking place.
Once excavation for each escalator sump was completed, extraction ducting was returned to the furthest excavation face. Air was then drawn into the working area through the access and escalator shafts.
Once Fibre Reinforcement Panel (FRP) works began in areas behind the main excavation area, a brattice wall was installed to separate the fit-out area from the excavation area. To ensure airflow was still present in both areas, openings in the brattice wall were required to allow air to flow from the FRP area into the excavation area. The openings in the brattice wall between the work areas needed to have a cross sectional area below 82ft² 25m², otherwise the FRP area would not receive any ventilation.
Following excavation of the southern side of Central Walk, excavation started on the northern side. When the four escalators on the northern side were excavated, ducting could remain at the eastern end of the walkway to ventilate the remaining work area. Air was drawn though the main access shaft along with each escalator shaft as they were excavated. Ducting was erected to manage dust in the areas being excavated.
In addition to the JMS-50-MES, small portable axial fans have been used to direct air flow into corners and dead ends to ensure no stagnation occurred and that air was continually circulated.
Construction at Central Station commenced in 2018 and excavation is expected to be completed in October 2021 (subject to change due to the COVID-19 pandemic), with track installation to follow excavation works.
The new Central Station and Central Walk are scheduled to open to the public in November 2022. Construction work will continue along the 18.6 miles (30km) length of the Sydney Metro, City and Southwest project before services start in 2024.