ABSTRACT
The loads were enormous – around 30,000 tons under towers and 13,500 tons uplift at anchorages.
Investigations included boreholes at sites and load-bearing tests. Bores on the Howrah side indicated soft material, sand at 18 m below ground and clay after 5.8 m of sand. On the Calcutta side, stiff clay was found 29 m below ground, after layers of water-bearing sand and silt.
Load-bearing tests included driving a 1.4 m screw, followed by driving 1.8 m diameter CI cylinders into the clay layer, and loading them until settlement was observed.
For the tower foundations, large multi-chamber caissons were adopted on each bank (55.32 m by 24.8 m), with 21 chambers (7 × 3) each 6.3 sq m in area, with outer walls 1.78 m and inner 1.6 m thick with cutting edge. The caissons had provision for compressed air working.
Both caissons were to be driven into the stiff clay layer, for Howrah at RL –62.53 and that of Calcutta at RL –79.13.
Two separated anchorage blocks 8.2 m by 16.4 m were provided to resist uplift forces with 1.5 as the factor of safety. A system of girders as grillage were embedded in concrete for transferring the uplift forces. Provision was made for future level adjustments by jacking.
