Tunnelling in Alpine Fault Zones - Excavation and Support Strategies

Tunneling through faulted rock in general is associated with stability problems, high deformations and frequently changing stress conditions. Depending on the characteristics of the fault zone, different strategies for excavation and support of the tunnel must be applied.
With the increased demand in Alpine base tunnel the issue of safe and economical tunneling is increasingly important. But also in tunnels with medium overburden squeezing conditions have been observed. Strategies applied in different regions are rather controversial. Some
groups try to convey, that a certain system fits to any conditions. Unfortunately tunneling through fault zones is not that easy.
Extensive experience gained over the last decades from tunneling in the Austrian Alps as well as other alpine regions, combined with a continuous research has led to increased understanding of fault zones, as well as to a number of improvements in short term prediction and supports. Due to the heterogeneity of Alpine fault zones, the short term prediction of the rock mass behavior is an important issue. Several techniques have been developed to increase the reliability of short term predictions.
High displacements in general lead to unacceptable loads, respectively strains in conventional tunnel linings, leading to problems with stability and safety. In Austria a low-cost system has been developed, which allows one to control the stresses in the lining by integrating
yielding elements into the lining. This system has been successfully applied on several projects. Improvements also have been made to grouted bolts, enhancing their performance.
Case histories are used to demonstrate the variability of different fault zones, and the development of the technology. It is emphasized, that only the combination of excellent technique, good workmanship, continuous engineering on site, up to date monitoring and interpretation techniques, as well as a suitable site organization lead to a successful mastering of the difficulties encountered, when tunneling through fault zones.