GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT
Above, figure 4: Basic failure modes of Voussoir rock beams. a Buckling or snap-through, b crushing, c shear, d diagonal tensile rupture. (modified after Diederichs and Kaiser 1999)
With the use of a numerical approach based on
the Distinct Element Method (DEM), detailed analyses of tunnel stability in layered, discontinuous rock masses can be performed. However, numerical methods applied for the analysis of discontinuum ground also have limitations, as noted by Starfield and Cundall (1988), Oliveira and Pells (2014), and Walton and Sinha (2022). Hence, there is a lack of specific design methods that
integrate different methodologies of assessing ground behaviour plus adequate rock support designs for anisotropic, layered ground. The design gap led research work (Terron-Almenara
et al. 2023) to develop a hybrid approach, built on a retrospective stability analysis of more than one hundred hard rock tunnel cases, including anisotropic rock masses, from Norway and internationally. Here, the new Skarvberg tunnel is studied as an
example to demonstrate the limitations of the current empirical methodology used in Norway (NPRA 2020, 2022a) for tunnel rock support design in layered rocks. Two main objectives of the study were: to find a basis for developing improved design recommendations for layered ground; and, practical implementation and testing of a hybrid approach.
Table 1: Rock mass class A/B C D E F
G The performance of the current design
methodologies was numerically studied at the new Skarvberg tunnel by comparing the tunnel stability with empirically based and hybrid-based designs. As a result, this article presents a new design procedure for tunnel rock support in layered rock masses, based on the application of a hybrid methodology (Terron-Almenara et al. 2023) in the anisotropic rock masses at Skarvberg. In this context, a new classification for layered rock
masses called Layered Ground Behaviour Classification (LGBC) is also proposed. This is intended as a supplementary design tool. Finally, a new and optimised rock support concept -
consisting of modified ribs of sprayed concrete called Crown-RRS (C-RRS) - is proposed for the reinforcement and support of horizontally layered rock masses of quality Q 0.1–1. The numerical analyses performed for the proposed C-RRS support concept showed tunnel stability and an associated potential for a reduction in rock support consumption. Hence, in addition to an improvement in tunnel
stability, optimisation with the use of hybrid designs can also pose a beneficial contribution to the environment.
Description
Good to exc. good Fair
Poor Very poor
Extremely poor Exc. poor
Support class I
II
III IV V
VI
Q-value 10–100 4–10 1–4
0.1–1
0.01–0.1 < 0.01
Prognosis (%) 10 45 30 15 0 0
Actual (%) 1.7
11.9 34.8 45.5 5.8 0.3
Above, table 1: Forecast versus actual distribution of rock mass classes for the new Skarvberg tunnel in relation to the NPRA classification of rock masses and rock support classes (NPRA 2022a)
18 | September 2025
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