Dust Control in Underground Station Construction
Underground Tunnel Stations. Location and Structure.
Urban metro and railway stations are often constructed underground to reduce the outside area being occupied, minimising land acquisition requirements and disturbance to existing above ground traffic during construction.
Urban metro locations are planned in locations that will provide easy access to important urban facilities. They are accessed via entrances/exits leading up to ground or street level.
Some stations provide interchanges to transfer passengers between lines or transport systems. In these instances, stations may include the development of multi-level platforms. In transfer stations there are often additional connecting tunnels and larger concourses to reduce walking times and manage larger passenger numbers.
Where underground stations are to be developed in already developed urban areas, for example where existing metro systems are being extended, there are additional implications for dust and noise control to be considered during excavation works.
Station Excavation. Methodology.
Although Drill and Blast is still a widely used excavation methodology for main tunnel construction, there is a move towards mechanical excavation for stations and caverns, in particular within existing urban areas. Mechanical excavation is regarded as safer, requires reduced ground support and fewer personnel.
The mechanical excavation of underground stations using Roadheaders is now widespread, they are also utilised for the rehabilitation of existing tunnels and underground caverns. Roadheaders are capable of excavating precisely in soft to medium rock without weakening the surrounding rock. They are flexible and mobile allowing them to excavate smoothly a range of size, shape and types of tunnel openings.
Following the excavation of stations and caverns they are reinforced with sprayed concrete, rock bolts and final concrete lining. These processes also generate significant dust that needs to be controlled. Grydale Mobile Dust Collectors are designed to be able to manage shotcrete fibres via a heavy drop-out box without having a detrimental impact on the filter life increasing their cost effectiveness.
Station Ventilation. Extract System.
An extract ventilation system is where air is drawn through the tunnel from the outside atmosphere using negative pressure and is exhausted via ducting to a dust collector to deliver clean air to the atmosphere. Air inflow needs to balance with air exhaust volume. Dust collectors can be located in acoustic sheds on the surface with ducting extended to the excavation at the face. Louvres on the shed balance airflow into the tunnel or shaft. This approach reduces plant equipment in the tunnel. Pressure losses ca be overcome by running a high-pressure fan on the dust collector to produce pressures up to -5kPa.
Extract ventilation systems remove all contaminants from the tunnel face and supply fresh clean air throughout the tunnel.
Sometimes full extract systems are perceived as being an expensive ventilation system design as capital is outlaid at the start of the project. This is not the case in short tunnels or for station/cavern excavation, where they can be installed as permanent solutions for the duration of the project. Constantly renewing clean air into the tunnel at minimum air flow plus treating dirty air before it is introduced to the atmosphere through a single contained duct are additional benefits.
Grydale dust collectors features a Variable Speed Drive (VSD) that can be turned up / down which reduces their operational cost significantly as they only utilise the power required to achieve the required air volume.
Full extract ventilation systems provide the following benefits:
- Reduced risk of contact with contaminants at the tunnel face.
- Reduced risk of contaminated air mixing with the surrounding tunnel air.
- Leakage occurs into the ductwork only.
- Dust collectors can be located with other dust generation works.
- Reduced noise pollution into the tunnel and reduced at the surface by attenuation through silencer and ducting.
Case Study. Melbourne Metro – Station Excavation.
Full Extract Ventilation System. Dust Control.
The Metro Tunnel provided some unique challenges for the ventilation design, including space and noise constraints plus the location on excavation works within the CBD. The ventilation 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 utilised JMS-60-MES dust collectors for each stage, with additional axial fans to boost airflow in the latter stages of construction.
Acoustic Sheds. Surface Mounted Dust Collectors.
Temporary acoustic sheds enclosed construction sites within the Central Business District (CBD) to minimise the impact on residents, businesses and the local community, from the construction of the stations. 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 while balancing required air flow into the tunnel.
Excavation Methodolgy. Trinocular Caverns.
The Melbourne Metro Station were built as trinocular caverns, where three overlapping tunnels were mined by roadheaders to total platform width that will be around 19m – one of the widest metro platforms in the world.
Spot Extraction. Track Mounted Dust Collectors.
Track mounted dust collectors were rented to work alongside excavation equipment to extract dust at the source during out of sequence excavation works. This provided a cost-effective solution for the project. Click here to read more about Spot Extraction.
Case Studies. Station Excavation.
JMS M-Series. Mobile Dust Collector Range.
Grydale JMS M-Series is an extensive range of internationally patented mobile dust collectors.
Air Volume Options. 6m3/s – 60m3/s.
Grydale JMS M-Series is an extensive range of mobile dust collectors with air volumes from 6m3/s (12,000 CFM) to 60m3/s (125,000 CFM).
All dust collectors feature a centrifugal fan that is optimised for the application air flow and pressure requirements. Centrifugal fans offer superior energy efficiency and enhanced durability and use only one motor and one impeller to develop system pressure.
All units have an independently certified filtration efficiency of 99.99% at 0.67 micron. This means that they are capable of filtering both Silica Dust and Diesel Particulate Matter (DPM).
Mobility Options. Tracks, Stepping or Drag Skid.
Grydale JMS M-Series dust collectors are compact integrated systems mounted on a single compact mobile base. This provides significant space saving advantages on construction sites where space is limited. Units can be repositioned with limited downtime in construction works, as the complete unit can be moved easily using one of the three available options:
Track Mounted Units – Grydale hold international patents for manufacturing the only self-propelled, mobile dust collector on the market. Track units offer full mobility with variable speeds between 2-4 k.p.h. to increase the speed and efficiency of operations. A key feature is the tracks are hydraulically driven by the diesel engine and operated via remote control. Track mobility allows dust collection units to move and operate alongside dust generating activities, reducing the volume of air requiring treatment and improving the efficiency of whole site operations.
Hydraulic Stepping Systems – units can be moved using the hydraulic stepping system which powers four legs that lift the unit up, while a pair of skates help to move the unit forward and backwards. Movement can be repeated to achieve the desired travel distance.
Drag Skid Mounted Units – using the designated lifting lugs at each corner of the machine skid mounted units can be lifted or dragged into position.
Power Options. Electric, Hybrid of Diesel.
Electric units use 380-480V or 1000V to power the centrifugal exhaust fan, whilst hydraulics are used to power the auger and rotary valve discharge system.
Hybrid units use a T3 or T4F/T5 Diesel Engine to power the hydraulic system while electric is used to drive the centrifugal exhaust fan, auger, rotary valve discharge system and screw compressor.
Diesel units use a T4F/T5 Diesel Engine to power the hydraulic pump driving the centrifugal exhaust fan, tracks, auger, rotary valve discharge system and screw compressor.
More Information? Contact Us.
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