GROUND SUPPORT | HYBRID DESIGN ROCK SUPPORT
This limited anticipation of some sort of instability or
trend that might have anticipated the failure at Profile 3 + 162. The available deformation records at the failure area were derived from comparison of scanned tunnel profiles before and after the event, which gave a total deflection of the centre roof of about 150mm–200 mm. Four multi-point borehole extensometers (MPBX)
were also installed vertically in the centre roof to study the behaviour of the layered rock over time and the extent of loosening zones. The measurements were also utilised for calibration process in the numerical analyses of tunnel stability. The measured vertical displacements at different
anchor depths in extensometers at Profiles 2 + 227 and 2 + 231 were measured for a period of three months. Considering that on average, a 4m long excavation round was taken per day and the characteristics of the hard and anisotropic rock mass (Q ~ 0.3–0.4), displacements appear to occur in connection to block movements, its associated dilation, and possible rock detachment. Some differences in the distribution of vertical displacements measured at the anchors placed at different depths were also observed. Such response may be interpreted either as rock mass movements mass occurring at any depth between 4m and 12m, as the beneficial reinforcement
effect of the 5m long rock bolts stabilising the arch roof, or a combination.
6 ANALYSIS METHODOLOGY Four numerical models were run to study the ground behaviour and the performance of different support designs Table 5. The pairs of models A1–A2 and B1–B2 cover an empirical and a hybrid design approach for both study sections, Profiles 3+162 and 2 + 227. A summary of the analysis methodology based on
a hybrid approach (Terron-Almenara et al. 2023) is presented in Table 6. ● Step 1. Identification of ground behaviour and assessment of potential failure mechanisms based on the evaluation of, among other rock mechanical parameters, the rock mass structure and competence, jointing conditions, tunnel geometry, and the in-situ stresses. The ground conditions are categorised to the classification presented by Terron-Almenara et al. (2023) for hybrid design. The LGBC is utilised to select the LGBT category for best to the rock mass conditions.
● Step 2. Specific site investigations are recommended in such hybrid methodology for anisotropic rock masses when rock mass quality Q < 0.4 and/or GSI < 40. The resulting set of rock mechanical properties is then utilised, especially in the analytical solutions in Step 4, and in the numerical calculations in Step 5.
● Step 3. Empirical and basic estimates of rock support design are derived from well-established classifications for hard rock tunnelling like the RMR, the Q-system, or the empirical classification proposed by NPRA (NPRA 2022a). The aim was to obtain a basic estimate of empirical rock support design concerning rock bolting, thickness of sprayed concrete, and the design of RRS arches.
● Step 4. The authors selected three analytical solutions based on limit equilibrium analysis of Voussoir roof beds, like the ones proposed by Lang and Bischoff (1982), Diederichs and Kaiser (1999), and Abousleiman et al. (2021), chosen since their joint application can capture the actual ground and support behaviour of the jointed roof beds.
● Step 5. The numerical analyses in the models summarised in Table 5 are conducted with the numerical code UDEC v5.0 (Itasca 2011). It is a two- dimensional numerical program based on the distinct element method (DEM), treating the rock mechanical problem as an assemblage of distinct, interacting, and deformable blocks; it solves the equations of motion and contact forces between blocks (Cundall 1980), permitting simulation of large displacements together with block deformation (Eberhardt 2023).
Above, figure 7: A Tunnel face at Profile 2 + 227 (Photo: Bøgeberg and Skretting 2021) b Interpretation of the geological conditions based on face mapping. Tunnel azimuth ca. 180°. Legend for lithology and structures as in Figure. 6
26 | September 2025
The four models A1, A2, B1 and B2 were calibrated before performing the analyses. The exerted loading conditions on the support systems in each of model were then compared with the calculated resistance and capacity of such support systems. Since the mode of shotcrete failure in hard rock masses not subject to high
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