A. Hitchcock, Mott MacDonald. This paper was first published in GE’s May 2003 edition.
Use of the observational method (OM) on projects where retained deep excavations require temporary propping is well-documented (Nicholson et al, 1999).The requirement to make savings in both project cost and programme by modifying the temporary works is a key driver for OM, and its use on the Airside Road Tunnel (ART) project at Heathrow Airport is an example of an instance where considerable savings are possible.
This paper describes how OM was implemented, through the technique of progressive modification (Powderham, 2002) on the first section of the west portal excavation, a 15m deep chamber from which a tunnel boring machine (TBM) would be launched. It details how an innovative contingency measure, laser controlled excavation and comprehensive wall convergence monitoring allowed careful control of the excavation phase, in which 60t of temporary steel propping was eliminated from the TBM chamber alone. This allowed a time saving of four to five weeks from the project programme, with improved site safety resulting from the need not to handle the heavy steel propping within a confined working environment. In addition, an enhanced understanding of the construction process led, through continuous improvement, to the realisation of further benefits on the remaining sections of the west portal and also the east portal.