OasisLMS

This paper discusses through case studies, some of the challenges that were faced and how they were resolved, during construction of adits by New Austrian Tunnelling Methodology (NATM), at various depths of soft soils encountered in the megacity of Kolkata, India. Two case studies at deep and shallow depths have been taken up. The first one is Cross Passage construction with adit crown at overburden depth of 21m approximately. The second one is construction of Ventilation Adit with crown at overburden depth of about 7m. The geotechnical profile varied from silty clay / clayey silt / sandy silt, with low consistency, for the case study locations. The field SPT (N) varied from 15-26 for cross passage zone at greater depth, whereas it was in the range of 5-7 for the adit at shallow depth. The excavation size was 4m height with oval shape for cross passage and 4.6-5.4 m height with circular/D-shaped for ventilation adits, respectively. A combination of steel ribs and shotcrete with wire mesh was used as temporary support in both the cases. For shallow depth NATM, double layer Self drilling anchors were installed at the crown to support the excavation. Excavation was done in stages with sequence of heading followed by benching, with lag. Face stability was maintained with grouting and soil nails.  Reinforced concrete was envisaged as permanent lining. Both temporary and permanent stage design analysis was carried out in FEM software. Close monitoring was done during the excavation phase in accordance with the design limits set, as excavation was close to buildings and also below main road with heavy vehicular traffic. Ground improvement, for improving the shear strength as well as to reduce the permeability of soil, was extensively done as a pre-requisite before excavation.

Kaustuv Bhattacharya, ITD Cemembtation India Ltd.; Manish Kumar, ITD Cemembtation India Ltd.; Prodyot Ray, ITD Cemembtation India Ltd.; Nirbhay Singh, ITD Cemembtation India Ltd.

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