PREDICTING THE EFFECT OF EXCAVATION SEQUENCE ON GROUND MOVEMENTS DURING NATM TUNNELING USING FINITE ELEMENT METHOD

摘要

The underground excavation leads to ground movements in terms of surface and subsurface settlements which may cause damage to structures. The Karaj Urban Railway (KUR) tunnel in Iran had been constructed in accordance with the principles of the New Austrian Tunneling Method (NATM). Induced displacements are empirically controlled by adjusting the excavation rate, distance between tunnel face and support, partial face excavation and closure of invert. This paper discusses the effects of the excavation sequence and face distance on the surface settlement of KUR tunnel by carrying out three-dimensional finite element modeling. Initially, the finite element modeling is carried out to simulate step by step excavation sequence of KUR tunnel which had been constructed in soft soils by NATM method. The settlements obtained from monitoring of Karaj Urban Railway tunnel had been used to validate the modeling work. It can be concluded that the excavation of the top head gave a significant effect on the total surface settlement compared to the excavation of bench and the corner. The optimal face distance between the top head and the bench was found to be about 35 m. The soil movements at the surface started to occur when the tunnel heading in top head excavation was about 20 m away from the end point of excavation. This should be taken into consideration when installing the instrumentations for monitoring purposes.