An overburden height of about 1.5 times the tunnel diameter is known to be critical with regard to the stability of an underground excavation. The near-surface zone is usually affected by heavy weathering and represents the border area between hard and soft rock. Due to the variability of the rock mass parameters, a most accurate determination is required to allow for an adequate geotechnical design. For these geological conditions, it is vital to take a more "creative" approach to determine rock mass parameters rather than sticking too close to standard tests and existing classification systems. For numerical calculations and for the interpretation of monitoring results different strengths of the rock mass are taken into account as the ground behavior is sensitive when varying the input parameters.
The geotechnical design of an underground excavation nowadays is a well-known procedure regulated by the Guideline for the Geotechnical Design of Underground Structures with Conventional Excavation (Austrian Society for Geomechanics 2010). Laboratory tests of rock to derive rock mass parameters are executed according to national standards. The latter also provide classification systems to describe rock and rock mass for engineering purposes. This practice is suitable for most cases, but some rock types require a different approach to describe them sufficiently for engineering purposes. Most of them lie within the border area between hard and soft rock. Regarding the geotechnical design, it also represents the transition zone between "stable" and "unstable" conditions. These kinds of rocks therefore demand special attention, but just in these cases the determination of rock mass parameters is extremely difficult.
We encountered the issue described above during the excavation of the Pummersdorf Tunnel, a 3.45 km NATM-Tunnel near the federal capital of St. Pölten. The prevailing overburden height of approximately 1.5 times the tunnel diameter (D) is critical with regard to the stability of an underground excavation (Vavrovsky 1987 and Poisel 2005). Rock failure in the crown up to the surface forming a chimney is an often monitored collapse phenomenon. This is a very critical failure mechanism, especially in horizontally layered rock. As it was observed during various tunnel excavations in the past, even little differences in the rock mass strength led to a collapse of the cavity (Vavrovsky 1993). These differences were often so little that they could not be determined on site by the geologist or the tunnel engineer.
The paper emphasizes the challenges of dealing with ground types described above for geologists and tunnel engineers.