Environmental effects of tunnel excavation in soft and shallow ground with EPBM: the case of Istanbul

In metro tunnel excavations, it is important to control surface settlements observed before and after excavation, which may cause damage to surface structures. Otherwise, the metro tunnel cannot perform the task expected, and the advantages of the tunnel are lost. In this study, short-term surface settlements and their effect on buildings are examined in three zones for twin tunnels, which have been excavated between the Otogar and Kirazlı 1 stations of the Istanbul Metro line, and are 5.8 km in length. Geology in the study area is composed of fill, stiff clay, dense sand, very dense sand, and hard clay, respectively, starting from the surface. Tunnels are excavated by using two Earth Pressure Balance Tunnel Boring Machines (EPBMs) with a 6.5 m diameter as twin tubes with 14 m distance from center to center. The EPBM in the right tube is followed about 100 m behind the other tube. Segmental lining with 1.4 m of length is currently employed as final support. The results of this study indicate that, with EPBM excavation method in shallow depths and soft grounds, the damage done to the environment prolonged project costs up to 15.8% more. In addition, tunnel conditions prolonged the project schedule to 29.3%.