Challenges of Construction of Adits by New Austrian Tunneling Methodology in Soft Soils of Kolkata-Paper

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The paper examines two New Austrian Tunnelling Method (NATM) adits built in the soft alluvial soils of Kolkata: a cross passage beneath about 20 m of ground and a ventilation adit beneath about 7 m. The soils were mainly silty clay, clayey silt and fine sandy silt; the shallow site was weaker, with SPT values of 4–7, compared with 15–26 at the deeper site.<br /><br />Both excavations proceeded in stages, using heading followed by benching, with steel ribs and mesh-reinforced shotcrete as temporary support. The shallow adit also required double-layer self-drilling anchors at the crown. Grouting and soil nails helped stabilize excavation faces, while ground improvement around the excavations reduced water ingress and strengthened the soil. Reinforced concrete was planned as the permanent lining.<br /><br />Two-dimensional finite-element analyses informed support design and monitoring limits. Instrumentation tracked ground settlement, tunnel deformation, face movement and groundwater pressure, particularly because the works were close to buildings and beneath a busy road. Maximum surface settlement reached 30% of the alarm level at the deep cross passage and 76% at the shallow adit; additional grouting was used at the latter. Although shallow excavation produced greater settlement, its zone of influence was narrower—about twice the excavation diameter, compared with six times for the deeper works.<br /><br />The authors emphasize thorough geotechnical investigation, advance mapping of nearby structures, appropriate ground improvement and continuous monitoring as essential to safe NATM construction in urban soft ground.

Keywords

NATM

soft alluvial soils

Kolkata tunnels

cross passage

ventilation adit

shotcrete support

ground settlement

geotechnical investigation

ground improvement

instrumentation monitoring