Settlement analyses of underground circular tunneling in soft clay

An increasing number of tunnels have been constructed worldwide due to urbanization and industrial purposes. In the past few decades, tunnel boring machines (TBM) have been used to drill in increasingly difficult geotechnical conditions such as soft ground like soft clay. Because of the difficulties associated with this, it is crucial to estimate ground surface settlement induced by tunnelling. This is to ensure the safety of tunnel construction as well as to minimize the associated impact on surrounding infrastructure. In tunnel construction, prediction of ground settlement is frequently estimated using a specified tunnel volume loss, and by applying semi-empirical methods and relying on engineer's experiences. One of the key constant parameters in the semiempirical method, K, is generally estimated using basic soil classifications, which has the potential to lead to inaccurate judgement from engineers. Better estimation of the constant K has had limited attention by other studies. This paper uses a force relaxation technique and the finite difference program, FLAC, to estimate the circular settlement profile and a K value for a range of different scenarios. A number of particular cases are numerically simulated with variation in the factors that influence the tunnel transverse settlement including tunnel depth to diameter ratios (C/D) and clay strength ratios (γD/cu). Results from the study compare favourably with previous empirical and analytical studies. A range of K values is proposed for different soil strengths and tunnel dimensions.