GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT
Table 7:
Parameters Design stage Q-value
Support design
Illustration of support design (*
)
Vertical roof displacement (mm)
Max. axial force RRS (kN)
Max. shear force (V) on RRS (kN)
Max. bend. mom. (M) on RRS (kNm)
Max. axial load (Qb in bolts (kN)
FSc-bending RRS
FSc-shear RRS FSb
)
147 931 53
125
190 0.7
5.6 (~ 1) (*) (1) Radial rock bolts, (2) Bolt spiling, (3) Sfr 150mm, (4) Si-RRS 30/6, (5) C-Si-RRS 30/6
Above, table 7: Comparison of ground behaviour and support performance for models A1, A2, B1, and B2. Values in between brackets represent the factor of safety (FS) for the general bolt-loading condition in the tunnel roof
The approaches exhibit limitations in characterising
ground behaviour. A hybrid approach, combined with numerical calculations performed with UDEC, showed it was possible to establish a better and wider description of the failure mechanisms. This study demonstrated that a significant optimisation of the support design would be possible, as shown in the studied models A2 and B2. The presented results show the importance of
conserving or improving the self-bearing capacity of the layered and jointed rock or Voussoir beams to arch and take part in the ground load. In addition, a comprehensive site investigation campaign was found to be important. Rock reinforcement ahead of the face with spiling
together with rock bolts and shotcrete is important, and actively contributed to ground stabilisation and tunnel
stability. Ground load taken by the rock mass itself and/or by the bolts is then only transferred to the RRS support to a limited extent, hence allowing for design optimisation. The results showed a significant reduction of the load
on support systems in rock mass qualities Q 0.3–0.4, which permitted an optimisation of the ordinary full-profile Si-RRS-arches into a support concept or proposal to reinforce only the crown (Figure 9). The satisfactory behaviour and stability observed in the numerical results for C-RRS-support should enable the use of this or equivalent designs in tunnel projects with ground conditions comparable to those studied in model B2. More case studies in hard, layered rocks will contribute to development of the approach.
DECLARATIONS: The authors confirm that there are no known conflicts of interest associated with the publication of this article, and that are
no financial or non-financial interests that has affected the outcome of this work. This paper was originally published online in Rock Mechanics and Rock Engineering Journal, from Springer, in August 2024, under a Creative Commons Attribution 4.0 International Licence (
http://creativecommons.org/licenses/by/4.0/). Open access funding provided by NTNU. This article has been prepared under a PhD project jointly financed by NPRA and the Norwegian Railway Administration (Bane NOR). The authors also wish to acknowledge the financial support of Sweco Norge AS and the Norwegian Tunnelling Association (NFF). As permitted under the particular open access facility, this version of the original paper has been abridged and edited for space, and images adapted to house-style. The original paper is available in full at
https://doi.org/10.1007/s00603-024-04082-3. Datasets generated during this study are available from the corresponding author (Jorge Terron-Almenara) upon reasonable request.
47
845 96
51
155 2.6
9.7 1.3 (1.6) 8
1093 187
59
159 2.1
2.3 1.25 (4)
11.6 949 97
46
110 2.7
5.9 1.6 (2) Model A1
Empirical design (failed support)
0.1
Sfr 150mm; B c/c 2m, L 4m; Si RRS 30/6 c/c 2m
Model A2
Hybrid design 0.1
L 5m/4m; Si RRS 35/6 c/c 1.5m Sfr 150mm; B c/c 1.5 m, Model B1
Empirical design (after failure) 0.3–0.4
L 5m/4m; Si RRS 30/6 c/c 1.5m Sfr 150mm; B c/c 1.5m, Model B2
Hybrid design 0.3–0.4
Sfr 150mm; B 1.7m × 1.5m, L 4m/3m, C-Si RRS 30/6 c/c 1.5m
30 | September 2025
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