GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT


Table 6: STEP OBJECTIVE 1


Identification of GBT and failure mechanisms


INPUT


span, rock competence, joint conditions, in-situ rock stress


Rock mass structure, joint spacing, tunnel


METHOD


Classification of GBT proposed in Terron- Almenara et al (2023) LGBC in Section 4.3


OUTPUT


Potential failure mechanism in the rock mass; Associated load condition (approx distribution & size)


2 Set up of geotech model 3


Estimate of basic support needs based on classification systems


Mech parameters of rock, joints and rock mass; In-situ stresses;


Ground deformation


measurements of in-situ stresses, mapping, monitoring of deformation


investigation holes, Rock testing,


Classification systems - RMR, NPRA, Q-system


Limit equilibrium solutions based on Voussoir analogue


Detailed geotech model considering in-situ ground conditions


An empirical-based estimation of the rock mass quality and basic needs of tunnel support


based on tunnel stability and support


Design optimisation


performance in bedded rock


Geological mapping and inspection of the rock mass at the face


4


ground behaviour and stability of bedded rock


Analytical study of


Bolt design, loading, and in-situ stresses


Geometrical parameters of bedding and tunnel; Rock strength and deformability;


5


Numerical analysis of ground behaviour, support performance and design optimisation


deformation monitoring; tunnel support based on output from Steps 3 & 4


Tunnels geometry; geotechnical model;


calibration; Evaluation of ground response in


UDEC analyses; Model


terms of displacements and stresses;


Assessment of support loading and design optimisation


Optimised tunnel support design; Potential reduction of rock support consumption


Above, table 6: Analysis methodology for the hybrid design of rock support in models A2 and B2 in layered rock, based on Terron-Almenara et al. (2023). Description of the analysis steps is numbered and presented below in the text


believe that the involvement and integration of


different design tools and approaches as suggested in a hybrid approach by Terron-Almenara et al. (2023) have contributed to the identification of possible deviations and to reduce the level of uncertainty. Before excavation in the models, the models were run


in elastic conditions to equilibrium. Then, the full face excavation was computed, and the support installed sequentially behind the face. Bolt spiling ahead of the face was also modelled at the two studied locations. With regards to the modelling of RRS support, UDEC


permits the construction of composite beam support that may simulate RRS in the tunnel direction. The modelling of RRS-support requires definition of normalised beam sections to derive equivalent beam properties


8 GROUND BEHAVIOUR AND SUPPORT PERFORMANCE


8.1 Summary of Results The results for model A1 (Figure 8) clearly indicate that the failure of the tunnel support occurs due to flexural failure of the RRS support. The unfavourable combination of a wide excavation span combined with a flat roof and too short bolt lengths, too large centre distance of bolts, and insufficient horizontal confinement did not contribute to rock mass arching. This resulted in an excess of ground load not taken by the rock mass nor by the roof bolts, and then transferred onto the RRS. By using a hybrid approach, as in model A2, proposing


an improved support design has been possible; in that, rock support elements are dimensioned to become loaded to a more optimal level. Hence, an improved load sharing in the rock support can be realised. Obviously, model A2 requires a generally heavier support design than that in A1. In turn, it provides stability whilst limiting oversupporting.


28 | September 2025 In terms of the shear loading of rock bolts in the four


UDEC models, the calculations showed that the shear load was, in general, rather low, mainly in the range of 5kN–8kN for all four models with one maximum finding of 18kN (in model A1). When comparing models B1 and B2, the results


suggest an improvement of the rock arching in the latter, which naturally results in a slight reduction of the support loading. The latter improvement in the loading conditions of the hybrid support in the B2 model can be visualised from the comparison of loading values of the support system between models B1 and B2 in Table 7. Besides the limited magnitude in the load reduction of the support systems in the hybrid B2 model, the improved loading condition has still allowed for an optimisation of the rock support design from the use of modified RRS arches. A summary of the results is presented in Table 7.


9 FINAL DISCUSSIONS AND RECOMMENDATIONS The study of ground behaviour and support performance in the new Skarvberg tunnel has enabled the evaluation of rock mass conditions influencing the design of rock support in layered rock masses of poor quality. As such, the application of a hybrid design


methodology allowed for design optimisation as so indicated in the numerical results for models A2 and B2. Layered rock mass conditions require a more


comprehensive characterisation of the ground and the combination of different analysis methods. With a rational combination of the mentioned approaches and methods, including the Voussoir beam models, the authors were able to predict the ground behaviour of layered rock and derive more optimal and cost-effective support designs.


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