Over-station developments are an attractive proposition thanks to soaring real estate prices, especially in central London. But these projects throw up unique challenges.
- Mitigate rail vibration
- Make use of existing piles
- Separate construction work from rail operations
- Take advantage of DfMA
In crowded and ever-growing cities, sites for development are increasingly sought after.
But with swathes of land occupied by rail stations – and thousands of people travelling through them each day – it makes sense to develop over-station projects to deliver new facilities to local communities.
Soaring real estate prices mean local authorities and landowners seek to realise the maximum return on investments on their properties and better serve their communities.
Transport for London, for example, aims to realise £3.4bn in non-fare revenue to reinvest in the transport network, much of which will come from land development.
However, these developments come with challenges not found in other city-centre developments, which not only inform project design but also impact project delivery.
Mitigate rail vibration
Mitigating vibration can be a key consideration to ensuring the viability of over-station developments, depending on the proposed use.
Where possible, this should be done at the source of the vibrations – the wheel/track interfaces – although this is not always practical in adjacent or air rights projects where the station is itself not involved in the development, or due to cost considerations.
This process begins by examining the rail line for any wear from use or for distortions to the tracks as a result of ground movements over time.
Track maintenance and realignment by the rail operator will correct these distortions and go some way to reducing vibration.
“Using existing piling for new built assets is not only possible but highly desirable”
The next stage of the process examines whether the tracks can be isolated to prevent the transmission of vibration to nearby assets.
Rigid track sleepers with resilient mountings can be installed progressively during engineering hours and may be sufficient.
Resilient clips or floating slab tracks can provide further mitigation, but would require a significant closure of the railway.
As for the over-station asset, elastomeric bearings between columns and the foundations may be necessary to mitigate vibration, although this will have implications for the design of the structure and allowance will need to be made for maintenance/replacement during the asset’s life.
Make use of existing piles
Using existing piling for new built assets is not only possible but highly desirable, as reuse can bring cost savings and sustainability to the project.
Removing old piles from the ground can be highly problematic and may disturb the ground so much that it becomes impractical to install new piling cost effectively.
Materials testing can confirm pile strength; non-destructive testing can confirm the integrity and lengths of existing piles; and back calculations can ascertain the loads that the piles previously supported.
Combining this information with geotechnical analysis will demonstrate what the current piles can carry in the future, and additional piles can be inserted where existing infrastructure allows in order to support additional loads.
However, there are other considerations in the reuse of existing piles: in situ foundations combined with the insertion of new piles create congestion in the ground that affects the design of utility layouts.
New loads transmitted via a building’s foundations into the ground will result in potential settlement and heave movements.
Even small changes can be enough to affect track alignments, causing an increase in vibration, so it is important to understand when and where these movements will occur and their magnitude.
Where an over-station development uses both newly inserted and existing foundations, there will be differential settlement.
This needs to be carefully evaluated throughout the proposed construction sequence, which typically involves loads being removed during and following demolitions and then increased again following construction.
Separate construction work from rail operations
It is crucial that construction of over-station developments doesn’t impinge on functioning railways, and this imperative will affect construction techniques and programming.
If working over the rail line, a crash deck is usually needed which allows construction to take place without the risk of tools, components or materials falling onto the tracks.
While a temporary crash deck can be constructed, existing structures may be sufficient if investigation confirms this.
Permanent works can be designed to provide the required levels of protection for new over-station developments.
“The simpler site logistics associated with DfMA will mean less disruption to residents across the area affected by the project”
If constructing part of the primary structure immediately above the platforms, then pile designs with a large bearing capacity will allow for a structure spanning greater lengths across the rail lines, avoiding the need to insert piles at more regular intervals and limiting how long the tracks need to remain closed.
Large capacity can be achieved by capping clusters of multiple small diameter piles, avoiding use of large piling rigs.
Taking this approach gets around height restrictions for plant used close to live rail lines, with potentially significant benefits for the project programme and cost.
Take advantage of DfMA
As well as bringing material and labour cost savings, the use of design for manufacture and assembly (DfMA) can cut onsite assembly time through the offsite production of prefabricated components.
The Northern line extension project – which will add two new train stations and 3.2 km of twin-bored tunnels to the London Underground network – will make ample use of prefabricated beams, columns, lattice slabs, twin walls and other elements, including the intent to adopt modular M&E components.
Use of DfMA on the project is expected to not only reduce construction time, but the simpler site logistics associated with DfMA will mean less disruption to residents across the area affected by the project.
Finally, working with the UK’s rail stations often involves researching historical plans and records, which provide invaluable information on the existing structure and infrastructure.
However, such documents may contain errors or inconsistencies and should not be relied upon in isolation.
While historical documents can act as a very valuable guide, they should be accompanied by selective intrusive investigations and laser scanning to confirm the actual arrangement.
Maurice Gidwani is technical director at Mott MacDonald