When tunnelling in poor ground or faulted rock under high overburden frequently deformations caused by the excavation exceed the deformability of conventional support systems. An economical approach to handle the problems associated with large deformations has been used especially in the Eastern Alps during the last 25 years. This comprises longitudinal gaps in the relatively thin shotcrete lining, a dense rockbolt pattern, and continuous monitoring of displacements.
A low cost solution to provide flexibility of the lining has first been introduced by Rabcewicz at the Tauerntunnel in Austria, by dividing the shotcrete lining into several segments, and leaving gaps between the segments.
The obvious disadvantage of the support system is the missing thrust transmission between the single "segments", resulting in al low utilization of the lining. Under unfavourable geological conditions this may lead to relatively high initial displacements in the excavation area.
When excavating through very heterogeneous rock mass, the high initial displacement rates may lead to unfavourable stress concentrations in relatively stiffer formations, which might fail in a brittle manner when overloaded.
Recent experience on two major railway projects has led to a further development of the support system.
To better utilize the capacity of the lining, low cost steel yielding elements (SLC = stress lining controller) are installed in the longitudinal ga...(this text has been cut automatically)