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Winner:Moor House draught relief shaft

THE 2004 FLEMING AWARD - Use of the observational method and robust risk management ensured successful completion of a shaft under a new office building in central London.

The Moor House draught relief shaft is a 40m deep, 8.2m diameter shaft beneath the new Moor House office building in central London. It is the first element of the Crossrail 1 project to be built.

The Crossrail 1 project, the fate of which is yet to be decided by the UK government, will link Paddington in the west of London to Whitechapel and Docklands in the east by means of deep tunnels, with above ground sections further out of London and a spur to Heathrow.

The key aspects of the Moor House draught relief shaft were construction programming and planning; design complexity, construction complexity and the use of the observational method, and the interaction between clients, owners, designers and contractors.

It was recognised during the tender phase that the draught relief shaft was a key element in the successful completion of Moor House.This resulted in the shaft being built from inside the basement of the completed structure during the cladding and fit-out stage.

This programming decision allowed the shaft to be taken off the critical path for the development. It also provided the opportunity to build an acoustic shroud around the work area at the head of the shaft, making 24 hour working possible without causing disruption to local residents.

Detailed design of both the shaft and the piled foundations of Moor House considered the impact on, and from, the future Crossrail construction - tunnels, adits and the 40m deep, 40m square station box next to Moor House (Torp-Petersen et al, 2003 and Morrison et al, 2004).

The constraints placed on the construction of the shaft by the Moor House piles - already installed and loaded - required that ground movements were kept to a minimum.

Water bearing sand and silt layers of the Lambeth Group further complicated this constraint. If allowed to seep into the shaft during excavation, water would have caused unacceptable and large ground movements.

The magnitude of the acceptable ground movement was assessed using finite element model calculations while the risks associated with groundwater seepages into the shaft were investigated by a large scale dewatering trial.

Results allowed the construction specification to be set in an observational method framework.

The monitoring regime for feedback into the framework included piezometers and inclinometers, with further monitoring of groundwater conditions in trial pits dug ahead of the shaft in the Lambeth Group deposits.

A contingency system - secondary pressure grouting to 'restress' the ground - was also developed for if ground movements exceeded allowable levels (in the order of 20mm).

Shaft construction started with additional reinforcement being placed in the slabs of the Moor House basement to accommodate temporary work loads from installing the shaft and the dewatering system, which was turned on before shaft excavation began.

Shaft construction was carried out with precast fibre-reinforced concrete segments using an underpinning sequence.The segmental rings were, with the exception of the first ring, of constant 350mm thickness and had special low friction bearing to help stress-free distortion of the shaft during the future Crossrail construction.

Two of the shaft rings built just above the horizon of sandy bands at depth incorporated inclined ports to allow, if indicated from the observational data, supplementary dewatering from within the shaft.

The project team recognised the sensitivity of the shaft as it passed through water bearing layers and adopted a robust risk management regime - a plan that paid off early one Sunday morning.

The shaft was at a critical depth, the dewatering system switched off and the telephone line from the automatic telephone warning system was cut. This potentially disastrous sequence of unlikely events was managed by the presence of the 24-7 cover of the dewatering contractor, put in place especially to take the shaft through the ground where the consequences of collapse due to water ingress were most severe.

Paul Morrison, Arup Geotechnics (Acknowledgement: I would like to thank friends from City University for input to the presentation and also those organisations not represented on the competition team but who contributed to the project, namely WA Fairhurst & Partners, Greycoat and DL&E as well as colleagues in Arup Structures. ) References Torp-Petersen G, Zdravkovic L, Potts DM and St.John HD (2003).

The prediction of ground movements associated with the construction of deep station boxes. Proceedings of the ITA World Tunnelling Congress 2003 - (Re)Claiming the Underground Space. Amsterdam, 12-17 April 2003. Balkema.

Morrison P, McNamara A, Roberts T (2004). Design and construction issues relating to a deep shaft for Crossrail. Proc Institution of Civil Engineers, Geotechnical Engineering 157. October 2004.

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