Investigation of Tunnel Face Stability and Deformation using Critical State Plasticity

The behaviour of the excavation face is one of the most critical aspects in urban tunnels excavated with conventional methods (ie, not with TBMs), as the majority of such failures are due to excessive face extrusion, often leading to face collapse. This paper investigates the stability conditions and deformational behaviour of an unsupported tunnel face through an extensive set of three dimensional finite element analyses. The Modified Cam-Clay constitutive model is used in modelling the behaviour of the ground. The obtained results are used in understanding how various geometrical (tunnel diameter, depth of overburden), as well as geotechnical conditions (ground strength and compressibility) affect the behaviour of the tunnel face. Using these results, face stability is quantified in terms of the magnitude of face extrusion by correlating an average normalized face extrusion with a face stability factor similar to that proposed by Prountzopoulos (2012). Finally, an attempt is made to calculate the factor of safety (FS) of an unsupported tunnel face.