TECHNICAL | DRILL & BLAST, CONVENTIONAL


Table 5 - Production rates: conventional versus optimised design


Design option


A - Traditional B - New Design


Production gain


sandstone (Zone I)


100m/month 120m/month


Blocky


and grey shale (Zone II)


75m/month 90m/month


20% faster production rate (2.4months/year time saving)


Within the approximately 500m-long Nala stretch,


a 25m-long section of extremely poor ground was encountered, comprising highly disintegrated shale and mudstone with soil-like behaviour, zero RQD and GSI values below 10. Despite the adoption of a tightly controlled pilot heading, minor face instabilities were observed during the initial advances, with soft, clay- rich material intruding locally into the face even after the installation of face bolts and face-sealing SCL. No instability was observed above the pipe roof, as its spacing had been reduced from 350mm to 250mm at the DRM based on probe drilling results. The pipe roof system was designed and verified to span into the face, providing effective crown confinement and preventing upward migration of instability. These observations highlight the severity of the ground conditions rather than deficiencies in the excavation approach. Following the initial face instability (Figure 10), the


Above top: Figure 10. Illustration of pilot tunnel face exhibiting instability and after contingency measures implemented.


Above bottom: Figure 11. Graphic showing


indicative plan of the twin tube tunnels, each at a different stage of excavation, below the Nala River valley section.


encountered as excavation commenced at the onset of the monsoon. The design mandated systematic pre-excavation grouting (PEG) ahead of each advance to maintain face stability and control inflows. Joint apertures, fracture networks, and joint infill characteristics were assessed to select appropriate grout materials: ultra-fine cement grout was used in mudstone and siltstone, while colloidal silica grout was adopted in shale to effectively penetrate fine, clay-rich fissures. Monitoring holes drilled ahead of the face were used to assess inflow conditions and verify grouting effectiveness. Grouting was implemented in a two-stage (primary and secondary) pattern, supported by laboratory and field trials to optimise grout mix, pressure, injection sequence and consumption, ensuring consistent sealing performance.


ACKNOWLEDGEMENTS


The author sincerely thanks Dr. Anmol Bedi, Mr. Marco Invernici, Mr. Thomas MacGowan and Mr. Gaurav Mamgain from the Bedi Consulting team for their guidance, knowledge sharing, and support. Gratitude is also extended to the client, National Highways Authority of India, the design team at Infinite Civil Solutions and Mr. Rajeev Pathania, along with all members of the contractor Dilip Buildcon team for their support and openness to implementing innovative methods on this project. The broader project team’s expertise was critical to the successful adoption of optimised designs and pioneering innovations. The author would also like to dedicate this paper to Mr. Bikram Singh Rana, in


recognition of his commitment, professionalism and valuable contribution to the successful delivery of the project. His dedication and contribution to the project will be remembered with great respect.


face was immediately stabilised using a 150mm-thick steel fibre-reinforced SCL and 5m-long GFRP face bolts. In subsequent advances, a face-wedge excavation and support approach was adopted, limiting excavation strictly to the contour and maintaining confinement of the retained wedge. This methodology proved highly effective in controlling face stability in the disintegrated, water- bearing ground and demonstrated the robustness of the pilot tunnel strategy in safely navigating this high-risk Nala section during monsoon conditions.


6.0 CONCLUSIONS The DVEXT-15 demonstrated that large-span NATM road tunnels can be safely and efficiently delivered in highly variable and adverse Indian ground conditions through disciplined, data-led design and construction control rather than conservative overdesign. To that end: the DRM was established as a powerful risk-management tool; delivery of India’s first successful application of permanent-grade GFRP rock bolts in a road tunnel was achieved, and with multiple benefits to lining thickness, construction period and embodied carbon reduction; optimised excavation and support sequencing in Zones I and II achieving 20% faster construction; proved that quality assurance, not material choice, governs performance; showed the critical importance of systematic probing, pilot tunnelling and PEG as active risk-mitigation measures; demonstrated that pilot tunnels and staged excavation are also powerful enablers of safe innovation; and provided a transferable framework for Indian tunnelling projects that integrates international best practice with local ground realities.


18 | August 2026


Interbedded sandstone


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