The use of compensation grouting in tunnelling: a case study

At the London Docklands Light Railway Lewisham Extension (DLR-LWE) twin tunnelling project, compensation grouting was carried out beneath sensitive structures to prevent excessive settlement due to tunnelling. However, excessive grout injections might exert extra loads on the tunnels underneath. This issue has become a major concern for tunnelling and grouting contractors after a report by the UK's Health and Safety Executive in 2000, which listed grouting force as one of the possible factors that might have caused the collapse of the London Heathrow Express tunnels in 1994. This paper presents the extensive field monitoring data compiled by the tunnelling contractor on surface and building settlements as well as tunnel lining deformations caused by compensation grouting. It will be shown that the final corrected building settlements, incorporating compensation grouting, were about 5 mm. In the case of greenfield settlement the maximum observed settlement was less than 15 mm, a criterion specified by London Underground Ltd. The settlement trough parameters were found to deviate slightly from the proposed ones, probably because of the layered ground conditions. Field monitoring data revealed that the ground disturbance caused by the first tunnel construction had resulted in more ground settlement during the construction of the second tunnel. It was difficult to understand the observed modes of tunnel lining deformations caused by compensation grouting, owing to the monitoring techniques used and the layered ground conditions. Nevertheless, the maximum observed tunnel diameter change was only 8 mm, less than 1% of tunnel diameter change. In conclusion, the surface and building settlements as well as the tunnel lining deformations were all within the criteria specified by London Underground Ltd. This indicates that the tunnelling contractor had carried out good tunnelling work.