The emergence of small footprint underground metro stations
An economic alternative for public metro systems, providing major cost savings and minimal surface disruption.
New opportunities are being harnessed in the design of metro systems to save money and reduce disruption to the public during construction. Today, most metro projects around the world are constructed with stations in large cut and cover boxes, with small running tunnels between them.
This arrangement leads to large excavation footprints required at the surface (typically around 30 x 200-250 meters (100 x 700-1100 feet) or longer with crossovers). In busy urban areas such footprints are often only available within major roads, and also disrupt side streets due to their length. Furthermore, in order to safely excavate, considerable utility diversion and protection must be installed and agreed upon with utility owners – a problem that has led to delays on many a project.
During operation, maintenance access is limited to stations during operational hours due to the tightness of space in the running tunnels. Furthermore, storage of trains is usually at a surface depot located above ground away from the urban area, which is not operationally optimal for metros with tidal flows to and from a downtown area.
Efforts to use smaller surface footprints and use conventional tunnelling techniques to create additional space below ground are sometimes required, but such schemes are expensive and typically restricted to locations with suitable ground and significant surface space constraints (such as New York or London).
However, a new and more economical alternative has emerged which removes the large footprint station, characterized as follows:
- The tunnel is large enough – around 12 meters (40 feet) in diameter to have two tracks and platforms contained within the tunnel (see picture).
- Plant and equipment can be located in rooms in the tunnel, just beyond the end of the platforms.
- By removing the platforms and plant rooms from the station box, its footprint is vastly reduced.
- Crossovers and sidings can be provided in the tunnel with no additional cut and cover boxes required.
- Emergency egress and maintenance access can be provided within the tunnel profile.
The benefits of this arrangement are as follows:
- Huge reductions in cost and surface disruption from station excavation.
- Utility protection/diversions are minimized.
- Cross passages or escape shafts, which are usually required every 250 meters (800 feet) are eliminated, benefiting safety, cost and schedule in construction.
- Crossovers and sidings are more readily provided, allowing for more optimal operational configurations.
- Safe in-tunnel maintenance access during normal train service can be provided.
- Significant future flexibility, with potential to reconfigure track layouts and add stations at a future date.
This philosophy has been successfully used on Barcelona Line 9, where the following benefits were realized:
- Economies of scale meant that the large single bore provided more internal space than two smaller running tunnels, at lower cost.
- Station footprints were circular shafts of 90-110 feet, offering huge cost savings.
- Decoupling station excavation and tunnelling provided schedule benefits.
- Settlement and attendant risks were much reduced.
As a consultant with proven experience in small footprint stations such as Bond Street and Tottenham Court Road in London, CH2M is excited to use this new approach in new locations around the world, saving both the public realm and the public purse.