Design of Rock Support in Hard and Layered Rock Masses Using a Hybrid Method: A Study Based on the Construction of the New Skarvberg Tunnel, Norway

Abstract

Hard rock formations with layered, anisotropic geological structures are rather common in Norway. The design of tunnel rock support under such ground conditions has been traditionally done with an empirical approach and assisted with engineering rock mass classification. In most cases, rock stability has been achieved with the use of fiber-reinforced sprayed concrete combined with rock bolts, sometimes supplemented with ribs of reinforced sprayed concrete (RRS) and bolt spiling. However, the discontinuous character and more complex ground behavior of layered rock can lead to design challenges as experienced during the construction of the new Skarvberg highway tunnel in Northern Norway. A significant portion of the tunnel exhibited consistent overbreak, delamination problems and a tunnel failure. In this article, the ground behavior and rock support performance at different locations of the tunnel are investigated to find a basis for design optimization of rock support under similar ground conditions. For this purpose, a thorough site investigation program was carried out, which formed the basis for further assessments and analyses with a hybrid design methodology and the numerical code UDEC (Itasca). As a result, the main challenges of classification systems to capture ground behavior and derive optimal support design in layered rocks were identified. As such, this study has investigated design optimization possibilities, which resulted in the development of, among others, a specific ground behavior classification for layered grounds and a new, optimized RRS-support concept for layered rock masses of very poor quality (e.g., Q ~ 0.1–1).