GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT
HYBRID DESIGN OF ROCK SUPPORT - CASE STUDY
A hybrid design approach for rock support in hard and layered rock masses is discussed, based on a study of tunnelling on the new Skarvberg road tunnel, in Norway.
Jorge Terron-Almenara1, Erlend Skretting2, Karl Gunnar Holter 2 and Are Håvard Høien3
1 - Department of Geoscience and Petroleum, Norwegian University of Science and Technology (NTNU), Trondheim, Norway
2 - Norwegian Geotechnical Institute (NGI) 3 - Norwegian Public Roads Administration (NPRA)
ABSTRACT Hard rock formations with layered, anisotropic geological structures are common in Norway and design for rock support under such ground conditions has been traditionally done with an empirical approach, assisted with engineering rock mass classification. In most cases, rock stability has been achieved using fibre- reinforced sprayed concrete combined with rock bolts, and sometimes ribs of reinforced sprayed concrete (RRS concrete) and bolt spiling.
However, the discontinuous character and more
complex ground behaviour of layered rock can lead to design challenges, as experienced in construction of the new Skarvberg road tunnel in northern Norway. A significant portion of the tunnel exhibited consistent overbreak, delamination problems and a tunnel failure. In this article, the ground behaviour and rock support
performance at different locations of the tunnel are investigated to find a basis for design optimisation of rock support under similar ground conditions. A site investigation programme formed the basis for further assessments and analyses with a hybrid design methodology and the numerical code UDEC (Itasca). The work identified the main challenges of
classification systems to capture ground behaviour and derive optimal support design in layered rocks. It has resulted in the development of, among others, a specific ground behaviour classification for layered ground and a new, optimised RRS-support concept for layered rock masses of very poor quality (e.g., rock mass quality (Q) ~ 0.1–1).
Above, figure 1: The new Skarvberg tunnel. a Location along the E69 highway in Finnmark County, b Tunnel alignment plotted over an aerial photograph of the terrain surface, including identified weakness zones (represented as solid black lines with double cross-ticks) and rosette plots of joint sets, and c Geological profile (modified from NPRA 2017). The two test locations for stress measurements are marked with blue dots and the two study locations with red dots
16 | September 2025
1 INTRODUCTION Layered hard rock masses with anisotropic geological structure are common and typically exhibit persistent and weak bedding planes, which are a significant source of mechanical weakness. For tunnelling, the ground behaviour of layered rock is therefore different and more complex than that of blocky rock masses without a dominant weak orientation. Such complexity results from the interaction of several engineering geological parameters, such as joint spacing and persistence, bed thickness, mechanical properties of joints, tunnel span, roof geometry, and in-situ rock stresses, as well as the characteristics of the rock reinforcement.
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