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.