Reclamation and Ground Improvement of Soft Marine Clay for Development of Offshore Terminal 4, JNPT, Navi Mumbai

Abstract

A container yard of around 100 Ha area was developed as part of the fourth terminal for Jawaharlal Nehru Port Trust (JNPT) in Navi Mumbai by means of reclamation in sea over soft, compressible marine clay. Ground improvement of the subsoil was warranted in order that the finished reclamation satisfies the stringent serviceability criteria. Extensive offshore geotechnical investigation campaign comprised conventional borehole sampling, laboratory tests and various in situ field tests such as CPTu, field vane shear tests, etc. Investigations revealed clay thicknesses of 4 m–22 m, with top layer showing undrained shear strengths less than 7 kPa and increasing with depth, underlain by weathered basalt. The ground was improved using prefabricated vertical drains (PVDs) with preloading. Radial consolidation and cone dissipation tests were carried out to establish the coefficient of horizontal consolidation (ch). The marine reclamation is confined on all sides by a perimeter bund with revetment, Stability analyses carried out for this bund showed that it was necessary to have overfill beyond the reclamation cope line along with tension geotextiles so as to achieve the prescribed factors of safety against slope failure. After removal of the preloading, the overfill was cut back to the final geometry and revetment was provided along the perimeter. Extensive geotechnical instrumentation and monitoring were conducted using multilevel magnetic extensometers, settlement gauges, inclinometer and piezometers to monitor the behaviour and performance of ground improvement. Field ch and smear coefficients were back-calculated. Around 200 confirmatory boreholes (CBH) were carried out after completion of ground improvement to ascertain the post-improvement and validate the design parameters, thus eliminating the risk of post-construction settlement.