Monitoring ground deformation around a tunnel heading in London clay

Following the collapse of NATM tunnels at Heathrow Central Terminal area, in Munich and Sao Paulo (HSE (1996)), there has been much debate concerning the purpose of instrumentation for shallow NATM tunnels in soft ground. The authors recently experimented with using chain deflectometers and inclinometers to obtain ground deformations ahead of an advancing NATM tunnel in London clay. The paper briefly describes the tunnelling situation. The details of the instrumentation, installation procedures and the performance of the instrumentation are discussed. It is finally shown that both inclinometers and chain deflectometers can be used to obtain reliable information on the ground deformations around and ahead of advancing NATM tunnels.