A CASE STUDY OF DEEP SHAFT EXCAVATION IN RIVER CHANNEL SOIL DEPOSIT IN KOLKATA-PPT Presentation

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The case study describes a 24 m deep excavation in Kolkata’s river-channel deposits for a cut-and-cover tunnel box. The shaft measured 38 m by 10 m and was supported by a secant pile wall of 1,200 mm reinforced hard piles and 800 mm soft piles, with 200 mm overlap. Five levels of struts and walers provided support: steel systems at the first and fifth levels, and reinforced-concrete systems at the intervening levels. The tunnel base slab formed the final support.<br /><br />Ground investigations identified variable layers of silty clay, sandy silt and silty sand, with groundwater about 4 m below ground level. Some permeable bands and confined zones with elevated water pressures were difficult to identify through conventional borings.<br /><br />The team used two-dimensional finite-element modelling with the Hardening Soil model to assess staged excavation, wall–soil interaction and support forces. Predicted maximum wall deflection was about 60 mm, or 0.25% of excavation depth. The factor of safety against bottom heave was approximately 3.6. Seepage analysis found piping checks within acceptable limits, and dewatering was planned only inside the shaft for construction.<br /><br />Monitoring included piezometers, standpipes, inclinometers, optical targets, strut strain gauges and surface-settlement markers. Tube-a-manchette grouting was used near the shaft to limit settlement; adjacent buildings were monitored, with foundation grouting and temporary supports applied where needed.<br /><br />Construction challenges included high-collapse-risk sandy soils during pile boring, requiring casing, and occasional water ingress through pile joints, which required additional grouting. The experience highlights the importance of accounting for variable subsurface conditions and combining analysis, monitoring and timely mitigation to protect nearby structures during deep excavation.

Keywords

deep excavation

Kolkata

cut-and-cover tunnel

secant pile wall

Hardening Soil model

finite-element modelling

groundwater control

bottom heave stability

instrumentation monitoring

tube-a-manchette grouting