GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT
to derive an adjusted support design. The encountered
rock conditions finally lead to an increased use of RRS- arches and rebar bolt spiles corresponding to factors of 13.4 and 6.6, respectively, higher than the tender forecast (Gildestad and Bakkevold 2021). The tunnel section at Profile 3 + 162 is further studied in this article, together with a monitored tunnel section at Profile 2 + 227. Locations are shown in Figure. 1.
3 DESIGN OF TUNNEL SUPPORT Horizontally layered rock masses of poor quality, in Norway The design of permanent rock support in Norwegian tunnels is based on an empirical approach. Rock support design in Scandinavian hard rock tunnelling is similarly based on the utilisation of the rock mass as a load-bearing structure. This is achieved by conserving - or improving - the self-bearing capacity of the rock mass with rock reinforcement measures, typically a combination of bolts and fibre-reinforced sprayed concrete that form part of the permanent support. In rock masses of poor quality the load-bearing
support normally has also the installation of RRS arches. In the Q-system, different configurations - or designs - of RRS support are available by adjusting the longitudinal centre spacing, thickness, the number of steel layers - Single (Si) or Double (D) - and the number of steel rebars in the RRS arches. This approach permits wide design - and construction - flexibility to suit different loading conditions. The design of rock support in Norwegian tunnels
must additionally comply with the national design guidelines for road tunnels as published by NPRA in Pedersen et al. (2010), and the subsequent revisions (NPRA 2020, 2022a). The guidelines are basically a simplification of the Q-system (Table 1). The guidelines, however, give no specific design
recommendations for rock support in anisotropic, horizontally layered rock masses. Instead, experienced geologists apply some mapping adjustments and recommendations to derive representative values of Q-parameters that reflect best the expected ground response of layered rocks. These can consider: joint roughness number (Jr
) and joint alteration number (Ja
roof, restricting deflection and providing the needed ground-RRS-arch interaction. In this context, the main functionality of the RRS arches is to hold ground pressure and provide a resistance force against radial deformations (thick black arrows in the figure). Bolt spiling is a pre-reinforcement measure of
the rock mass for poor conditions. The method of pre-reinforcement installs steel bolts above the tunnel crown and extending ahead of the face, prior to advancing the excavation (see also Figure. 3). Its stabilising effect has three main characteristics: (a) maintain the arched roof geometry; (b) reinforce both longitudinally and radially, generating a protective vault (grey-shaded colour in figure); and, (c) help load transfer and arching in the tunnel direction, thus a more even distribution of ground load. The bolt spiling method falls within the category
of an umbrella arch (UA) reinforcing system (Oke et al. 2014a). Design practice in Norwegian hard rock tunnelling is primarily based on experience and basic empirical design recommendations, advising use of bolt spiling when Q < 0.6. This leads to designs where the actual (and necessary) interaction between the ground, tunnel support and bolt spiling remains unaccounted for - or, in other words, the rock support design is principally independent of the beneficial (and known) stabilising effect of bolt spiling.
4 GROUND BEHAVIOUR OF HORIZONTALLY LAYERED ROCK MASSES The analysis of deformational behaviour in tunnels has been traditionally linked to the Convergence Confinement Method (CCM), an analytical elastoplastic model for weak and isotropic rocks. However, in hard and layered rock, the persistent and often weak planes of bedding have a discontinuous character that invalidates the premise and, therefore, the use of CCM. The following summarises the main geometrical and
engineering geological parameters controlling ground behaviour of layered rock.
)
of the most unfavourable joint set - i.e., the bedding planes; the value of the stress reduction factor (SRF) to represent the impact of in-situ stresses in relation to the main joint directions; and, assessment of representative rock quality designation (RQD) values to account for both the bedding and the cross-joints, such as by considering volumetric joint count (Jv), as suggested by Palmstrom (1982, 2000) and NGI (2015). A frequent design solution to reinforce and support
layered rock masses has been to use RRS arches combined with bolt spiling (a pre-reinforcement measure above and ahead of the face), resulting in a thicker and stronger beam of reinforced rock beds (illustrated with dark brown colour in Figure. 3). They also tie the load-bearing support (RRS) to competent and undetached ground above the tunnel
20 | September 2025
4.1 Failure Mechanics of Jointed Roof Beds The mechanics of tunnel roof beds have been studied at least since Fayol (1885) when the study of stability was mostly based on the classic beam theory for elastic and continuous beams. However, cross-joints in bedded rock formations reduce the allowable beam tensile strength and invalidate the continuum approach in favour of the jointed or Voussoir beam analogue approach. The essential principle of the Voussoir concept is that
jointed, discontinuous rock beams can develop moment resistance from the generation of a compression arch between the upper beam midspan and the lower abutment. With this approach, roof deflection and beam stress can be used to evaluate roof bed failure in relation to the intrinsic elastic properties of the rock, bed geometry, and span of the opening. Four basic failure modes of Voussoir roofs were
categorised by Diederichs and Kaiser (1999) (see Figure. 4).
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