Changes in Settlement Troughs Induced by Subway Tunnel Excavations in Weak Geological Environments in Istanbul
Abstract: Due to increasing urban populations, many modern cities attempt to solve problems associated with theincreasing demand for public transportation by constructing these systems underground. To create new metro lines,multiple underground excavations must be carried out in several locations; these are often close to each otherin shallow and weak geological environments due to geometric limitations as well as access-related restrictions.Settlement troughs can occur as a result of these excavations, potentially causing serious damage to the structureswithin the settling area. This study evaluated the data collected by monitoring the extent of surface deformationduring the excavation process in four different locations on the Kirazlı-Olimpiyat-Başakşehir Metro Line as wellas two different locations on the Ataköy-İkitelli Metro Line; all of these sites were located on the European side ofIstanbul. The geometry of the surface settlement troughs that formed due to these tunnel excavations was examined;a total of 12 sections were investigated in this study. An analysis of the monitoring data reveals that, in similar geological environments, successively excavated tunnels with the same geometry and tunnelling methods can disturbthe geological environment through which the tunnels are constructed; in particular, if the second tunnel passesthrough the same section, this results in the formation of an increasing and asynchronous surface settlement trough.This result indicates that the material parameter (K), which affects the size of the settlement trough formed inscenarios with twin subway tunnels with close axes and that are excavated at shallow depths, has different valuesat the wings of the settlement trough that forms after the tunnels have been excavated, and that this change in Kinfluences the shape of the surface settlement trough. This study suggests that predicting any changes in K, whichis dependent on the characteristics of the surrounding rock, is important in order to mitigate any potential risks,especially at the design stage of any project that requires the excavation of multiple tunnels at shallow depths and inweak geological environments.