DRILL & BLAST, CONVENTIONAL | TECHNICAL


SCL and pre-support spiles are also incorporated. Rock bolts create a Reinforced Rock Arch (RRA) within the plastic zone, transferring loads from potentially failed rock to surrounding intact rock. Spiles, as a pre- support measure, retain small rock pieces in the crown and limit additional loosening.


3.1.2 Excavation & support sequence Excavation on each tube was carried out using a split heading and bench approach to enhance face and crown stability and limit wedge formation (see Figure 4). The New Austrian Tunnelling Method (NATM) was employed, with the excavation technique selected according to the prevailing ground conditions. In competent rock masses, excavation was undertaken using conventional drill-and- blast methods, while in weaker rock masses mechanical breakers were adopted to minimise disturbance to the surrounding ground. Excavated spoil was removed using front-end loaders and dump trucks. The primary reason for splitting the heading into


multiple drifts is to reduce the excavation area, thereby enhancing face and crown stability, improving control of deformations in weak rock masses and limiting potential wedge sizes in competent blocky rock. An initial SCL layer applied immediately after Drift 1 excavation, combined with rock bolting, anchors loosened blocks and reinforces the surrounding rock mass. Drift 2 is then excavated under similar support measures, followed by the jointless vault thickening layer installed one round behind, integrating both drifts.


3.2 Zone III and Zone IV 3.2.1 Design approach Where induced stresses exceed the global rock mass strength excess deformation and deep shear failure are predicted, particularly in soft shale units under high overburden and in the Nala crossing (where DVEXT-15 passes under the low valley with the seasonal Nala River), structural inverts or additional measures are required to stabilise the plastic zone around the excavation. For sections identified with potential deep plastic zones – and in the low cover Nala zone the standard support classes are not applicable – a Prescribed Support Design (SSD) is adopted instead.


3.2.2 Excavation & support sequence: Zone III Ground investigations indicated variable conditions with highly disintegrated rock, fractured sandstone and siltstone units, along with extremely week mudstone and shale units. Standing water was encountered in boreholes and, due to the absence of hydrological records, the design conservatively assumes coefficient of permeability k=1 × 10-


⁵m/s, with potential groundwater inflows up to


48l/min/m in the pilot tunnel during monsoon seasons. Given the pronounced geological and hydrogeological


uncertainty, a pilot tunnel excavation strategy was adopted. To characterise the ground variability, three distinct ground behaviour zones (A-C) were defined, each with a corresponding support and excavation sequence arrangement. The pilot tunnel thus serves a dual role,


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functioning both as a safe excavation sequence and as an exploratory adit that enables drainage, grouting and geological assessment prior to enlargement. Across all three ground behaviour zones, the primary


mitigation measure was systematic depressurisation of the rock mass through drainage holes drilled 15m ahead of the face, which ensured that at least one tunnelling cycle would be depressurised in advance. Systematic probing and pre-excavation grouting was carried out at the start of each cycle to assess the rock mass ahead and control water ingress, respectively. The encountered ground conditions and support measures adopted in all the three zones are presented in Table 3 and the excavation sequence adopted is illustrated in Figure 5.


3.2.3 Excavation & support sequence: Zone IV In zone IV, the tunnels pass through Green Shale under highly variable overburden conditions, ranging from approximately 11m near the South Portal up to around 100m along the alignment. Face mapping carried out during portal development works indicated very poor rock mass quality, with Rock Quality Designation (RQD) values ranging from 0 to 30%, typically around 10% or lower, and GSI values generally between 15 and 25. The shallow cover above the crown and intense weathering results in a high risk of crown collapse to surface. The presence of clay-rich


Above: Figure 4. Excavation sequence adopted in Support P1–P5.


Below: Figure 5. Excavation strategy adopted in Zone III.


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