Knowledge of deformations and stresses induced around tunnel excavations is of utmost importance when assessing their stability and support requirements. This paper presents a case study of the design of the Piora exploratory tunnel in the Swiss Alps. Tunnel excavation was done by a TBM to a diameter of 5m for a distance of 5.5km. The case study is conducted at the chainage 1.619km, where overstressing and breakout phenomena were observed, using 2-dimensional finite element analysis. Because of the nature of the flat lying rock strata, a numerical modelling approach for bedded rocks was adopted and implemented. The model for bedded rock is based on the constitutive model for jointed rock mass suggested by Amadei and Goodman (l98Ia); it takes into consideration the bed thickness, as well as the joint normal and shear stiffness. The numerical results obtained are compared with those obtained from field monitoring of displacements; and show good agreement.
The large capital investment of mining and tunnelling projects requires greater assurance of their satisfactory performance in the long term. Therefore more rigorous techniques are required in the design process. The increasing physical scale of underground work operations has also had a direct effect on the need for safe and efficient geotechnical design. Thus, the ultimate objective· in the structural design of an underground opening is to control rock displacements around the excavation. Analytical techniques are required to evaluate each of the possible modes or response of the rock mass, for the given excavation conditions and proposed geometry. However, as the problems take on greater reality and the complexity of the design condition increases, the use of analytical methods becomes limited to the point where other, more realistic methods must be sought. Thus, the complexity of many practical engineering problems makes it necessary to use numerical methods