Construction of tunnels beneath Heathrow Airport for London’s new Heathrow Express Rail Link, to connect the Airport with Paddington Station, began in 1994. The tunnels were excavated using tunnel boring machines while the tunnel crossovers and stations for the various terminals were constructed using Sprayed Concrete Linings (SCL), previously known as the New Austrian Tunnelling Method (NATM). Since the SCL method was unproven in London Clay, a trial tunnel was undertaken prior to the start of the project. The trial tunnel tested various sequences of construction and allowed the ground settlements and displacements to be monitored.
GCG’s initial role was to analyse the results of the trial tunnel and advise Charles Haswell & Partners and London Underground Ltd (LUL) on the effects of tunnel construction on the existing LUL Piccadilly Line tunnels. Eight and a half metre diameter station tunnels were to be constructed directly beneath the Piccadilly Line and finite element modelling was used to predict the potential tunnel settlements and distortions.
The station tunnels in the Central Terminal Area collapsed in October 1994 causing much uncertainty over the reliability of the SCL technique in London’s ground conditions. After an investigation, the project was restarted the following year and successfully completed in 1998. GCG was asked to provide expert witness advice to Balfour Beatty on the detailed cause of the collapse. State-of-the-art numerical analysis was used to model the construction sequence and examine soil behaviour. This evidence played a key role in the resolution of the associated legal proceedings.