Innovative Tunnelling Practices Adopted to Mitigate Various Execution Challenges in a Rail Tunnel Construction in India-Paper

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The paper describes construction of two NATM railway tunnels, 0.96 km and 6.62 km long, on India’s Obulavaripalle–Krishnapatnam Port line. Weak, fractured rock, groundwater seepage, forest restrictions, and three sections with only 4–6 m of overburden created significant construction risks. Designers proposed cut-and-cover construction for these low-cover sections, but acquiring additional forest land and managing streams and flood risk would have delayed the project.<br /><br />Instead, the team strengthened the ground and excavated the tunnels using carefully controlled methods. Before tunnelling, they installed about 3,000 cement-grout holes and 3,000 steel micro-piles to reinforce the overburden. Stream beds were grouted, and a reinforced-concrete apron protected the main stream crossing. Inside the tunnels, workers used face grouting and sealing, closely spaced lattice girders, spiling, rock bolts, mesh, and shotcrete. Blast lengths were reduced from about 3 m to 1 m in the low-cover zones, with smaller explosive charges to limit disturbance.<br /><br />Instrumentation and frequent surveys monitored tunnel convergence, shotcrete stress, and ground-surface settlement. Monitoring recorded a maximum surface settlement of 9 mm, supporting safe progress. Seepage was managed with cotton waste and cement capsules, followed by an impermeable lining system.<br /><br />The three low-cover sections were completed in five months. Compared with conventional cut-and-cover construction, the approach reduced estimated costs from INR 25 crore to INR 10 crore and saved about a year, while avoiding additional forest-land acquisition. Both tunnels were completed in 25 months. The authors conclude that, with appropriate design, support, and monitoring, this ground-improvement and controlled-excavation approach can be a practical alternative to cut-and-cover in similarly challenging terrain.

Keywords

NATM railway tunnels

India

ground improvement

low overburden

forest restrictions

cement grouting

steel micro-piles

shotcrete support

surface settlement monitoring

controlled excavation