Three-dimensional numerical studies of "NATM" tunnelling in stiff clay

The use of tunnels for various purposes has become more and more popular in many congested metropolitan cities worldwide. However, tunnelling may induce excessive ground movements and adverse effects on nearby structures and piles. Therefore, it is necessary and vital to gain the fundamental understanding of tunnelling-induced ground movements, the stability of tunnel heading and tunnel interaction to nearby structures. In this study, three series of elasto-plastic coupled-consolidation numerical analyses were conducted to study the three-dimensional stress transfer mechanisms, ground movements, lining stresses, the stability of the tunnel heading with the use of soil nails, and the influence of tunnelling on a loaded pile (including pile movements and stresses) during a “NATM” open face tunnel excavation in isotropic and anisotropic stiff clays. Based on the study of the effects of soil stiffness ratio (n = Eh'/ Ev') and coefficient of earth pressure at rest (K0) on ground movements, it is clear that for a given K0, the ground settlement trough becomes shallower and wider as n reduces from 1.6 to 1.0, whereas for a given n, the ground settlement becomes shallower and wider as K0 increases from 0.5 to 1.5. Generally, a stress influence zone extends 2.0D in front of tunnel face and 2.0D behind of the tunnel face. The effects of n do not have significant influence on lining stress. However, K0 has significant influence on the distribution of lining stress around the tunnel. The stress coefficient Kxz(= σxx'/ σzz') for soil elements rises to 1.8 at the crown and invert, while it falls to 0.5 at the springline immediately behind and around the tunnel face (original K0= 1.0). On the other hand, the stress coefficient Kyz(= σyy'/ σzz') rises above 1.0 for all elements around the tunnel. The changes of Kxz and Kyz demonstrate a very non-uniform stress distribution around the tunnel heading during tunnel excavation. The use of soil nail in a tunnel heading does not only improve the stability of the tunnel heading, it also reduces surface settlements due to reduction of stress relief at tunnel face. Results show that an optimum axial rigidity of soil nail ((EnAn)opt= 300MN in this study) for stabilizing a tunnel heading does exist. Beyond this optimum value, the stabilizing effect does not increase further. Tunnelling-induced ground movements near a loaded existing pile do not follow a normal Gaussian distribution. Additional settlement takes place at the pile head due to the development of plastic yielding around the tunnel and pile tip. The factor of safety of the pile with a working load of 1850kN drops from 3.0 to 1.5 due to tunnelling effects