DRILL & BLAST, CONVENTIONAL | TECHNICAL


5.0 GROUND TRUTHS: CHALLENGES AND SOLUTIONS 5.1 Quality, not a mere concept: lessons learned from localised failure During excavation in Zone II, a cavity approximately 3m-deep developed from the crown to shoulder level in the forward of the Tube 02, while the rear drift was 18m behind. Although the anticipated ground comprised grey shale with GSI 30-40, the collapse was due to inadequate performance of the installed rock bolt plates combined with ≈4m-thick sheared clay band located around 3m below the crown. The cavity was immediately stabilised by sealing the


void with a 200mm-thick steel fibre-reinforced SCL. It was later backfilled with SCL to the original profile and self-drilling steel bolts were installed to stitch the SCL to the rock mass. An investigation was initiated at the direction of the DRM, which confirmed plate stripping failure: while the distal ends of the bolts remained anchored, failure occurred at the proximal ends (Figure 8). Subsequent testing identified inconsistent quality of the supplied bolt plates, with only a proportion meeting the specified design capacity, which directly contributed to the failure. This occurred despite supplier certification and ISO-accredited laboratory testing, demonstrating a gap between compliance testing and in-situ performance. The DRM directed immediate discontinuation of bolts from the original supplier. An alternative supplier was engaged, and targeted pre-construction trials were undertaken to verify performance prior to full production use. The affected tunnel length was treated as temporarily supported, and supplementary bolts from the new supplier were installed to restore the required safety margin. Instrumentation and Monitoring (I&M) played a


critical role in managing ground stability in this stretch, particularly during the benching stage. Post-rectification readings during the heading advance indicated rock mass behaviour within expected limits. However, during bench excavation, convergence monitoring detected ground movement. When the bench face reached the cavity location, the vertical displacements approached the 30mm action threshold (Figure 9), triggering an immediate temporary suspension of excavation and implementation of contingency measures, which included installation of additional rock bolts in the tunnel crown. Permanent GFRP application requires a more cautious


approach than conventional steel systems; the key lesson from this project is that their successful application depends on enhanced quality control, more frequent testing of production and materials, and ongoing DRM-led performance reviews, rather than any limitation on the suitability of GFRP bolts themselves.


5.2 Managing an unforeseen karstic hazard At Ch. 423+764.5 in Tube 01 (Zone I), systematic look- ahead probe drilling – which was mandated by the design to be performed at regular intervals – identified an unexpected high-pressure, water-bearing feature at the sandstone-limestone contact, approximately 2m above the temporary invert. The feature was not predicted


by surface mapping or intercepted by any boreholes and indicated the presence of a localised karstic conduit within the limestone. The probing revealed severe inflows of approximately 50l/s per hole from four probe holes (≈200l/s total), with water pressure estimated at around 10 bar based on jet throw distance (see Figure 9). In Tube 02, where the same contact was exposed slightly higher above invert level, only moderate inflows were observed, confirming a connected water pathway. The early identification of this karstic feature through


systematic probing was critical. Without it, blind face advance could have intercepted the cavity directly, posing a high risk of uncontrolled flooding and potential catastrophic face instability. The incident demonstrates the robustness and importance of the probing regime as an active risk-management measure rather than a passive observational tool. Water ingress was controlled through a staged


polyurethane (PU) grouting programme using a fast- reacting, high-foaming resin. Grouting was initiated from Tube 02, where pressures were lower and access safer, resulting in a measurable reduction of inflow in Tube 01, confirming connectivity. Injection was first carried out through existing probe


holes to arrest immediate flow, followed by systematic drilling and grouting of primary holes along the sandstone-limestone contact. Secondary holes were drilled between primaries where required. Grouting was terminated based on cessation of flow, pressure limits or agreed grout consumption criteria, restoring safe and controlled excavation conditions. Further, systematic probing was carried out at the end of each injection cycle, prior to advancing the face, to ensure complete sealing of the water path. Once probing confirmed a dry face ahead, excavation proceeded with support installed sequentially after each advance.


5.3 Nala:


PEG and face stability improvement In the Nala section, significant groundwater ingress was


Table 4 - Material and spoil savings achieved through design optimisation Design option


Spoil generated


A - Traditional B - New Design Savings


1,306,800m3 1,247,400m3 5%


Secondary lining concrete


66,660 m3 35,442m3 47%


Steel


1,177t 265t


83%


Above: Figure 9. Water inrush before and after PU injection.


August 2026 | 17


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