Kolkata Underground Car Park – Perfect Example of Using Sheet Piles as a Permanent Structure and With Top Down Construction Methodology
Kolkata, formerly known as Calcutta, is India's fourth largest city, a famous historic landmark, and the capital of West Bengal state. A period of economic stagnation followed the country's independence decades ago but has been replaced by a boom engendering rapid growth in all domains, including road traffic. The population of the eastern Indian mega-city currently exceeds 15 million.
Several construction stages were considered when designing the sheet pile wall
The construction of the shopping mall and car park was launched with a soil investigation carried out by local companies Geotest and Drillers & Engineers. Special attention had to be paid to the groundwater levels that regularly rise from their habitual elevation of 1.5 m below ground level to El. 0.0 m or higher during the monsoon season.
Steel sheet piles are a preferred choice for construction sites in confined spaces
The perimeter wall of the carpark was built with 628 AZ 26 sheet piles
- Phase 1: Excavation & dewatering to El. -4.80 m, upper strut at El. -0.30 m
- Phase 2: Excavation & dewatering to El. -7.80 m, strut at El. -4.30 m, water level at El. -1.50 m
- Phase 3: as per phase 2, water level at El. 0.0 m (monsoon season)
- Phase 4: Final stage, bottom strut at El. –7.30 m, maximal water level under hydrostatic conditions.
The top-down method reduced the period during which the construction area was unavailable for public use
The highest bending moment (465 kNm/m) and deflections (63 mm) occur during phase 4. The technical department proposed an 18 m long AZ 26 sheet pile in steel grade S 355 GP as a suitable profile. The steel stress verification of the AZ 26 characterised by a section modulus of 2,600 cm3/m showed an existing stress of 179 N/mm2. The allowable stress amounts to 355 N/mm2, resulting in a safety factor of 2.0.
The verification after an assumed lifetime of 50 years is based on the loss of steel thickness due to corrosion given by tables in EC 3. As the inner side of the basement structure is coated, loss of thickness had to be considered only on the outer side. A loss of 1.75 mm was determined assuming the presence of aggressive natural soil. The recalculated reduced section modulus of the AZ 26 sheet pile due to corrosion was 2.300 cm3/m. The safety factor following a half-century's exposure to aggressive natural soil thus amounts to 1.76.
The sheet piles were installed with a resonance-free vibratory hammer
The owner of the project eventually opted for AZ 26 sheet piles in 6 and 12 m lengths to allow shipment in containers. The piles were spliced on the job site. Some 1,110 tons of 18-m-long AZ 26 single piles in steel grade S 355 GP were installed. C 9, C 14 and Omega 18 connectors were used to join the piles at the corners of the wall. The AZ 26s were driven with a resonance-free vibratory hammer suspended from a crawler crane. They form a 167-m-long parallelogram with a width of 15 to 27 m and a perimeter of about 376 m. The vertical loads of the superstructure are carried by 500-mm-diameter concrete columns.
A separate foundation system carries the main vertical loads
The side of the AZ piles facing the shopping mall and car park was coated for aesthetic reasons. Kolkata's first fully automatic car park was made watertight by seal-welding the Larssen interlocks of the AZ 26 sheet piles. A fire analysis showed that fire protection could be assured by an ordinary sprinkler system.
The sheet piles of the car park and shopping-mall levels were seal-welded and coated