Abstract
The general geology of Norway makes most of its tunnels to be constructed mainly in strong rock intersected by weakness zones of different sizes and characteristics. The Norwegian support tradition is, to the largest degree as possible, to reinforce the rock to make it self-bearing. In weak rock, this reinforcement has been accomplished by using bolts, sprayed concrete and ribs of reinforced concrete (RRS). RRS are normally designed with 6 rebars mounted on brackets that are attached to rock bolts with a center to center distance of 1.5 m covered in sprayed concrete. The spacing between the RRS in the tunnel direction is usually 1–3 m. In recent years, the application of RRS has gradually changed from following the blasted tunnel profile that formed unarched RRS that reinforced the rock to using RRS with an arched design that supports the rock. Following this development was an increase in the use of materials, as the amount of sprayed concrete used is now considerably larger and the rebar diameter changed from 16 to 20 mm. This change has also caused an abrupt increase in the support measures used for decreasing rock quality, from simple reinforcement by bolts and sprayed concrete to load-bearing arches. The authors believe that a more gradual transition is logical and this article will discuss and evaluate the current Norwegian support strategy by reviewing international theory, performing parameter analysis and presenting data from current and previous Norwegian road tunnels, with a focus on rock mass quality and deformations. It is concluded that arched RRS is not necessary for all cases where it is used today, and that evaluation of the need for load bearing arched RRS should be based on deformation considerations. Norwegian conditions comprise the basis for the discussion, but the problem at hand is also of general interest for hard rock tunnelling conditions.
Highlights
In Norwegian tunnelling, stability challenges are usually related to zones of weak rock
Considering the reduction of E-modulus and uniaxial compressive strength for rock masses compared to rock mass quality, according to Hoek and Diederichs (2006) and Hoek et al (2002), one can assume that the reduction in critical strain is dependent on rock mass quality
The introduction of reinforced ribs of sprayed concrete (RRS) in the Q-value support chart and in the Norwegian Public Roads Administration (NPRA) support table have caused a considerable increase in support in Norwegian road tunnels
Summary
In Norwegian tunnelling, stability challenges are usually related to zones of weak rock. Typical challenges can include wide weakness zones in sub-sea fjord crossings 100–300 m (m) below sea level, minor zones and jointed rock in urban areas with typically a 5- to 100-m overburden, overstressing of solid rock and weakness zones for tunnels under high mountains or along steep valley sides (up to a 1000-m overburden or more). When rock reinforcement requires more than bolts and sprayed concrete, a lean support construction is used to reinforce and keep the rock mass in place, rather than heavy support constructions, for example, fully casted lining. This lean support is usually comprised of reinforced ribs of sprayed concrete (RRS) (see Fig. 1).
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