HYBRID DESIGN ROCK SUPPORT | GROUND SUPPORT
Above, figure 5: Qualitative categorisation of ground behaviour in hard, horizontally layered rock for unsupported, D-shaped tunnels. Approximate tunnel span is 10m. Joint spacing and rock mass quality decrease from category I–III
2 NEW SKARVBERG HIGHWAY TUNNEL A major upgrade has been undertaken by the Norwegian Public Roads Administration (NPRA) on the E69 highway to North Cape, including construction of the subsea North Cape tunnel and the Honningsvåg tunnel, completed in the late 1990s, and more recently the 3.5km-long Skarvberg tunnel (see Figure. 1a). The new Skarvberg tunnel is D-shaped, 12.5m-wide, and was excavated by drill-and-blast. Its rock support design is based on empirical rock mass classifications. The main elements of the permanent rock support are shotcrete and rock bolts, as well as RRS support arches. The geology was characterised mainly as hard rock
with a distinct anisotropic, layered rock mass structure. The rocks belong to the Kalak Formation, mainly composed of sequences of metasandstones and mica schists disposed sub-horizontally and intersected by sub-horizontal, 0.1m–1m thick intrusions of metagabbro. The rock mass structure is formed by three main joint sets as shown in the rosette diagrams made from joint registrations during construction (Figure. 1b). From aerial photography over the tunnel alignment,
up to nine weakness zones were identified (Figure. 1b). The geometrical correlation between the structures recognised on the surface and those encountered (and mapped) as weakness zones at the tunnel level, during construction, confirmed the weakness zones to be vertical to subvertical (Figure. 1c). In addition, minor fracture zones were also encountered, having similar strike and dip but a lower degree of jointing and shearing.
Tunnel construction saw a significant deviation
between forecast and encountered distribution of rock mass classes, and rock support. While only 15% of ‘very poor quality’ rock mass of Class IV was forecast in the tender studies, based on Q-logging, the proportion met during construction was 51.6% of at least Class IV (see Table 1). This difference was mostly related to the frequent
occurrence of tunnel roof instabilities, caused by the combination of layered rock mass structure and insufficient horizontal confinement in long sections of the tunnel, and limitations of empirical systems used in tender design to forecast such behaviour. Several tunnel sections were supported with RRS
arches and bolt spiling. Despite the heavy rock support measures in locations with very poor rock mass quality, a roof failure occurred at Profile 3 + 162 (see Figure. 2). The failure occurred in a tunnel section with mapped Q 0.1 and supported with single (Si)-RRS 30/6 spaced c/c 2m, radial bolts 4m-long, at c/c 2m, and bolt spiles in the crown. Failure occurred in the centre of the roof and extended about 9m behind the tunnel face. The centre part of the RRS arches in the flat tunnel roof was visibly buckled downwards, causing the development of tensional fractures in the shotcrete. In addition, frequent instabilities during the
excavation of the tunnel were observed in the form of delamination, rock fall, and over-break when the rock mass was exposed immediately after each advance. Several more site investigations were conducted and the findings used by the contractor`s specialist consultant
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