Stability analysis of a large tunnel driven in a stiff, fissured soil mass

This paper describes structural geology investigations and stability analysis of a large urban tunnel excavated in a stiff, fissured soil mass using the "New Austrian Tunneling Method" in Sao Paulo City, Brazil. The orientation of the soil mass fissures was mapped by the construction supervision team. Stereographic projections were used to define the dominant families of joints. The conclusion, based on field data and stability analysis, is that the main excavation was safe, without any need of additional systematic support, in roof, lateral walls and at the exposed face (in the centre-downtown direction of excavation adopted). These predictions were confirmed by construction of the tunnel with no accidents due to fissuring of the soil mass.