Squeezing rock conditions at an igneous contact zone in the Taloun tunnels, Tehran-Shomal freeway, Iran: a case study

Squeezing rock conditions at the contact zone of an andesitic-basaltic body and tuff country rocks in the Taloun tunnels were investigated and analyzed. Evaluation of the rock mass properties illustrates the fact that they were significantly reduced at the contact zone, especially when wet. Detailed monitoring and measurements of tunnel-wall convergence at the contact zone in the Taloun service tunnel, during the 10 months following excavation and installation of initial support, prior to installation of heavy support, showed greater than 3% of the normalized tunnel closure. This confirms moderate squeezing conditions at the contact zone. The measured displacement was even higher than that of the fault zone in which deformation was decreased during the first month and eventually stabilized. Similarly, numerical modeling of the deformation at the contact zone not only confirmed a higher value of the tunnel convergence but also demonstrated the reduction of in situ stress and development of plastic zones across the contact zone. These data are also believed to account for the squeezing condition at the contact zone. It is expected that this condition will be significantly increased in the main road tunnels due to the fact that these tunnels are twice as wide as the service tunnel. Therefore, proper and timely support must be applied. Numerical analysis of the support at the contact zone showed that the stress due to bending moment is greater than that of the axial forces on the lining. This calls for certain support measures in the form of permanent lining and two layers of steel bars to compensate for the tensile stress exertion on the lining.