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

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The presentation examines two adits built using the New Austrian Tunnelling Method (NATM), also known as the Sequential Excavation Method, in Kolkata’s soft soils. NATM uses an observational “design as you monitor” approach, combining staged excavation, temporary support and continuous monitoring.<br /><br />The deep case was a cross passage beneath approximately 20 metres of overburden, excavated to about 3.9 m high by 3.7 m wide. The shallow case was a ventilation adit beneath roughly 7 metres of overburden, with a 4.6 m excavation diameter. Both encountered predominantly silty clay and clayey silt, but the shallow site had weaker ground and local silty sand, making it more vulnerable to instability and water ingress.<br /><br />Support systems were tailored to conditions. Both used steel ribs, mesh-reinforced shotcrete and staged heading-and-benching excavation. The shallow adit also required double-layer self-drilling anchors, face nails or grouting, and ground treatment. Grouting was performed at both sites to control water ingress and stabilize soil.<br /><br />Finite-element analysis assessed support forces and face stability under conservative groundwater and surcharge assumptions. Calculated face-stability factors of safety were 1.69 for the deep excavation and 1.25 for the shallow excavation with anchors and face grouting.<br /><br />Monitoring included settlement markers, extensometers, optical targets and piezometers. Maximum surface settlement reached 30% of the alarm level at the deep site and 76% at the shallow site; additional grouting was undertaken at the latter. Key risks included live utilities, a historic brick sewer, nearby old buildings on strip foundations, water ingress and settlement beneath busy roads.<br /><br />The case studies emphasize thorough geotechnical investigation and mapping of structures and utilities before excavation, careful selection of support and ground-improvement measures, and continuous monitoring with clear alert, action and alarm thresholds.

Keywords

NATM

Sequential Excavation Method

Kolkata soft soils

adit excavation

ground support

shotcrete

steel ribs

grouting

settlement monitoring

finite element analysis