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Metros & Urban Rail

Going underground: saving surface buildings while tunnelling

Going underground

Metro systems, by their nature, are needed in heavily developed urban areas. These areas of cities also tend to be the longest established areas, containing older buildings which often reflect the history and culture of the city, or more recent structures which are highly regarded.


Anthony Bennett, Aurecon’s tunnels leader, looks at proven approaches to saving the building while developing underground rail infrastructure.

Invariably there will be challenges in implementing the underground works of stations and tunnels near these valuable buildings.

  • Do we have to choose between the need to provide a modern transport system so that the city works, and retaining the surface structures?
  • Can we save the building while constructing the metro?

This problem has been addressed through a number of techniques that have been developed and used successfully. The appropriate solution will depend upon the value of the structure weighed against the cost, and the degree to which the structure is affected by the Metro works.

Move the building

This sounds a daunting prospect, but might be the only option, for example, if an open excavation is required. While moving timber structures is relatively common, especially in the housing industry, moving a more substantial masonry structure is a serious undertaking. In Adelaide, the facade of the former Marine and Harbour Building was moved 34 metres in 1979 to make way for the construction of a new building.

More recently in 2011, the two storey Rob Roy hotel (at one stage named the Birdcage) was moved, less its basement, and then returned to its original position, to allow the construction of the Victoria Park tunnel in Auckland. These moves have required the strengthening of the structures using concrete beams and steel rod ties, which allowed the building to be jacked up and then skidded along rails.

A variation on this approach is to dismantle the structure and re-build it. This has allowed Captain Cook’s cottage to move from Yorkshire to the Fitzroy Gardens in Melbourne, and London Bridge to be found in Arizona. However, this is unlikely to be acceptable in many cases, as some features, such as the elaborate plaster work of the Rob Roy hotel, do not lend themselves to this method, and it could be considered that some of the historical value of the building is lost in such drastic interference.

Underpin

In cases where the metro work is likely to clash with, undermine, or cause unacceptable movement at the foundations of building, underpinning can be installed. Methods such as piling using short masted rigs from within basements, and the construction or insert of beams under the walls can be used to transfer the support of the structure to deeper more stable strata.

Such a technique was used to protect buildings during the construction of the Brisbane Rail tunnels, and to allow the construction of a new subway connection underneath the historic Perth Railway station to the new MetroRail Station. This method is more conventional than moving buildings, and can, especially if there is a basement, cause less disruption of the building.

Prevent movement at the tunnel

Masonry buildings are susceptible to damage if their footings move, and tunnelling can lead to ground movement from a number of causes including closure of the ground around the tunnel as it is excavated, and deflection of the ground support. These effects can be minimised by using very stiff support systems, designed to limit the ground movement rather than merely to carry the loads.

Such stiff support systems in reasonable ground can comprise the use of wall beams or drift beams to support heavy steel ribs called sets. Control of ground movement at the face and beyond the face can be achieved using canopy tubes, steel tubes drilled ahead of the tunnel, allowing the face to be excavated under support.

In very soft ground conditions, it is likely that the tunnel is excavated using a tunnel boring machine, with the advantage that the face can be supported directly, the lining is constructed within the TBM, meaning that there is no unsupported length of tunnel, and also that the inflow of ground water can be restrained, which assists in controlling the drawdown of the water table, a second mechanism for ground movement in soft ground.

Monitoring against predictions and superficial repairs

There is an increasing casebook of precedents and more accurate analytical methods available. Therefore, with the prediction of ground movement for tunnelling methods and ground types, and the correlation between movement and building damage, the assessment of the risk of building damage from tunnelling is reasonably reliable.

When these predictions are tested by monitoring the movements over the tunnel as it advances, it is possible to assess whether it will be necessary to take any special precautions, or whether the movements will be acceptable. This analysis might well show that the most economical approach is to tunnel conventionally, and to undertake the repairs of any minor cracks that develop in the surrounding buildings.

This approach needs to be managed to maintain the support of affected people and the reputation of the project owner and contractor, not to mention the designer.

To return to the original question, these methods demonstrate that it is possible to retain your valuable buildings, and build your Metro too.

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