Abstract:
This paper highlights salient features of an excavation 38 m (Length) by 10 m (Width) by 24 m (Depth) carried out in Central Kolkata. Secant pile wall (SPW) formed using a combination of alternating 1200 mm & 800 mm diameter piles was used. Six layers of struts and waler beams, including bottom slab were utilized as lateral restraints to piles. The geotechnical profile for excavation zone was predominantly silty sand to sandy silt. The profile is characteristic of river channel deposit usually encountered in Kolkata. A notable variation in terms of SPT and grain sizes, in soil character was observed in the boreholes conducted at close proximity at this site. The groundwater table was present at 4-5 m below ground level.
Temporary stage analysis (including seepage analysis) and design were carried out in FEM softwares. Design checks for clay bursting, piping, bottom heave, etc were also carried out. Embedment ratio for pile was approximately 1.58. Extensive instrumentation and monitoring were carried out during the excavation period to check parameters such as ground movement, pile movement, piezometric heads, etc as per review levels. Piezometers with different tips showed that there were seams with relatively high hydrostatic pressure when compared with adjacent layers. Pumpout tests were also conducted inside the shaft at the beginning of the excavation to check the adequacy of pile cutoff. During excavation, ground improvement using Tube-a-manchette grouting technique was also required to be carried out as per field observations.
The paper discusses some of the major challenges faced as well as measures taken to counter them, during deep excavation carried out in silty soils in a very congested Urban setting in the heart of a Megacity.
Abstract: Theme 1