Prediction of Settlement in Existing Tunnel Caused by the Second of Twin Tunnels

In 1994 to 1996, three 9-m-diameter parallel tunnels were constructed crossing below twin existing tunnels at London Heathrow Central Terminal Area station for the Heathrow Express project at Heathrow Airport in the United Kingdom. Extensive monitoring was undertaken during construction. Settlement curves at various construction stages in the existing tunnels were measured. Marked asymmetry was noted in the settlement curves caused by construction of the two outer tunnels. It has been noted by several authors that the surface settlement trough caused by the second of two closely spaced tunnels is markedly asymmetrical. This has been accounted for by the fact that when twin tunnels are closely spaced, the ground in the region where the second tunnel is to be constructed has already been subjected to appreciable shear strains associated with construction of the first tunnel. This reduces ground stiffness, hence a higher volume loss for the second tunnel, producing an asymmetrical settlement trough with settlements greater on the side nearest to the first tunnel. The data from London Heathrow have been added to a database of case histories of twin tunnels prepared from reported information worldwide. A new empirical method was developed for estimating the settlement trough caused by the second of twin tunnels, along with an exercise demonstrating the use of the method, which is proposed for use as a preliminary predictive tool.