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

Paper

Pdf Summary

The case study describes a 24 m-deep excavation for a cut-and-cover metro tunnel in central Kolkata. The shaft measured 38 m by 10 m and was excavated in river-channel deposits dominated by silty sand and sandy silt, with variable permeability and groundwater conditions.<br /><br />The retaining system used a secant pile wall of alternating 1,200 mm reinforced “hard” piles and 800 mm “soft” piles, with a 200 mm overlap. Five levels of steel or reinforced-concrete struts and walers, followed by the tunnel base slab, supported the wall. The pile embedment-to-excavation-depth ratio was approximately 1.58.<br /><br />Design involved staged finite-element analysis in PLAXIS 2D using the Hardening Soil Model, along with separate seepage analysis. Checks addressed wall movement, bottom heave, piping, exit gradients, and clay bursting. Predicted maximum wall deflection was about 60 mm, and the calculated factor of safety against bottom heave was approximately 3.6. The analyses indicated that the braced system was adequate, with dewatering needed inside the shaft for construction.<br /><br />Extensive monitoring tracked ground and building settlement, pile deflection, groundwater levels, piezometric pressures, and strut loads. Observed wall deflections remained below predictions. Tube-a-manchette grouting was used near the shaft to address water ingress, stabilize disturbed soil, and limit settlement. Nearby buildings were monitored, with foundation grouting and temporary supports applied where needed.<br /><br />Key challenges included thin, difficult-to-detect permeable layers with confined water pressures; maintaining bore stability in loose sandy soils; preventing water and soil ingress through wall joints; and protecting adjacent structures in a congested urban environment. The study emphasizes the need for careful ground characterization, staged design, instrumentation, and responsive ground-improvement measures in deep excavations within Kolkata’s variable river-channel deposits.

Keywords

deep excavation

cut-and-cover metro tunnel

Kolkata

secant pile wall

PLAXIS 2D

Hardening Soil Model

groundwater control

bottom heave stability

tube-a-manchette grouting

urban excavation monitoring