TECHNICAL | DRILL & BLAST, CONVENTIONAL


predominantly characterised by competent sandstone, locally interbedded with grey shale (Figure 3). In Zone I, the rock mass is generally strong and blocky,


resulting in ground behaviour dominated by kinematic block movement along discontinuities, with limited susceptibility to stress-induced failure. Zone II exhibits greater geological variability due to


Above: Figure 3. Observed tunnel face geology in Zone I and II alongside borehole core samples.


and a tendency to swell when exposed to water. Geomorphologically, the tunnel crosses a series of ridges and intervening valleys comprising the North Ridge near the North Portal, followed by a shallow Upper Valley, a broader and deeper Lower Valley (with the seasonal Nala River) and the South Ridge near the South Portal, as shown in Figure 2.


2.2 Geotechnical zoning for risk management To better assess and manage the tunnelling challenges, the alignment was divided into four geotechnical risk zones based on the geology, rock mass characteristics and anticipated ground behaviour during excavation (see Figure 2). Each zone reflects distinct lithological and structural conditions that govern excavation stability, design and construction challenges, and groundwater risks, as summarised in Table 1.


2.3 Geological suitability for permanent GFRP application Zones I and II of each tube have a combined total length of 2,500m, by far the majority of the length of the tunnel, which is 3,300m-long from portal to portal. They also form the longest continuous section outside the higher-risk Zones III and IV, extending for approximately 2,000m. The alignment, therefore, is


Table 2 - Summary of predicted failure mechanism and support system adopted in various rock classes


Support class


P1 P2 P3 P4 GSI 76+ 56-75 36-55 21-35


Predicted failure mechanism


Kinematic instability


Kinematic instability and localised shear failure


Kinematic instability and minor shear failure with 1.7m-thick plastic zone


Kinematic instability and shear failure with 7m-thick plastic zone


P5 16-20


Kinematic instability and deep shear failure with 10m+ thick plastic zone


Support measures


50mm + 50mm thick layers of SCL + 32mm SN bolts at 2 x 2m c/c


50mm + 100mm SCL + 32mm SN bolts at 1.5 x 1.5m c/c


SN bolts at 1.5 x 1.5m c/c + R38 Spiles pre-support where required


100mm + 150mm SCL + 38mm SDA bolts at 1.5 x 1.5m c/c + 6m-long R38 Spile pre-support at 400mm c/c + 75mm Face sealing


SDA bolts at 1.5 x 1.5m c/c + 12m-long 114mm Pipe-roof pre-support at 350mm c/c + 100mm Face sealing + Face bolts


100mm + 175mm SCL + 38mm 75mm + 150mm SCL + 32mm


alternating sandstone and shale units, including locally thicker, more weathered shale horizons and structurally disturbed zones associated with regional lineaments. While the interbedding results in locally reduced rock mass quality and increased deformability within shale layers, the overall ground behaviour was primarily associated with slabbing and localised shear within shale beds, rather than large-scale failure. The favourable ground conditions encountered in


Zones I and II presented opportunities to optimise both the excavation and support strategy and the permanent support design. Ground behaviour governed predominantly by kinematic block instability, with limited stress-induced deformation, enabled: ● Adoption of permanent GFRP rock bolts, which are well suited to ground conditions where support demands are predominantly tensile. The bolts act to stabilise kinematically controlled wedges and tie locally yielded ground into the surrounding elastic ground arch.


● Subsequent optimisation of the secondary lining, made possible using permanent rock bolts, including reduction in lining thickness and replacement of conventional rebar with SFRC lining, resulting in a more efficient permanent lining system.


● Acceleration of the excavation and primary support cycle through the elimination of lattice girders, enabled by real-time, survey-based profile control.


3.0 DESIGN FRAMEWORK 3.1 Zone I and Zone II 3.1.1 Design approach The alignment traverses rock masses of varying quality, classified using the Geological Strength Index (GSI). Here the alignment encounters blocky to moderately fractured rock (GSI 35-75) and highly fractured or weak rock (GSI 25-35). To address these varying conditions, a series of support classes (P1-P5) has been defined, each tailored to the predicted rock mass behaviour as defined below and support measures summarised in Table 2: ● Support Class P1 is applied in competent rock with minimal jointing, and the primary concern is brittle fracture under intermediate to high stress conditions. Rock bolts anchor any loosened rock into the intact mass, while the Sprayed Concrete Lining (SCL) stabilises blocks spanning between the bolts.


● Support Classes P2 and P3 are designed for moderately fractured and blocky rock where kinematic instability governs. Rock bolts tie the plastic zone back into the elastic natural arch and the SCL supports loosened rock between bolts.


● Support Classes P4 and P5 target highly fractured or weak rock masses. Closer rock bolt spacing, thicker


12 | August 2026


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