Investigation of ground surface settlement in twin tunnels driven with EPBM in urban area

Nowadays, with the increasing population of large cities, the need to expand public transportation, especially metro systems, is greatly increasing in urban areas. Therefore, excavation of new tunnels near the existing ones or other excavations located nearby has become inevitable. Excavation of such tunnels in urban areas should be done by considering the effects of these tunnels on buildings and other urban structures. Significant factors affecting interaction between tunnels, as well as the characteristics of surface settlement, are the existence of mixed ground (soil-rock) at tunnel faces or fault zones in the direction of tunnel excavation which have not been clearly investigated by researchers. These parameters have a great effect on the amount of maximum surface settlement and shape of surface settlement curve. Although several studies aim to analyse the interaction between newly excavated and existing tunnels and its effects on surface settlements, this subject certainly needs further investigation. This study mainly focuses on the effects of the interaction between twin tunnels mainly opened in fault zones and mixed ground on the basis of surface settlement measurements by using Earth Pressure Balance Machine (EPBM). Both numerical and empirical methods are used in this study. Observed data are used to test the validity of the results obtained from three-dimensional numerical modelling. The results from numerical methods were in good agreement with the real data. The results of this study reveal that the amount of maximum surface settlement and shape of surface settlement curve are strongly related to spacing between tunnels, fault zone thickness and type of tunnel face material. The interaction factor is almost zero when spacing is larger than 4D (D is tunnel diameter). Independent of fault zone thickness, the effects of the fault on longitudinal surface settlement continue 25 m from both sides of the fault centre.