Preloading and prefabricated vertical drains design for foreshore land reclamation projects: A case study

The Changi East Reclamation Project in Singapore comprises land reclamation in foreshore conditions. As the foreshore area was underlain by a thick layer of soft marine clay, a high magnitude of primary and secondary consolidation settlement was expected, owing to fill and future live load. An average fill thickness of 10 m was predicted to contribute to a magnitude of settlement greater than 2 m over a period of several decades. In order to minimise this expected future settlement, it was necessary to accelerate the consolidation process to complete the majority of the settlement during construction stage. A combination of prefabricated vertical drains (FVDs) with preloading ground improvement was successfully applied in this project. The technique comprises the installation of prefabricated vertical drains and the subsequent placement of surcharge to accelerate the consolidation of the underlying marine clay. This paper discusses the theories of radial drainage and the preloading technique, and considerations and design methodologies for the ground treatment of marine clay with prefabricated vertical drains in such foreshore land reclamation projects. In addition, predictions of magnitude, time rate of settlement and performance assessment of prefabricated vertical drains are also discussed. The design predictions at a case study site were compared with the field instrumentation results in order to verify the design approach used. It is found that PVDs with preloading are the most effective method for improving soft clay under land reclamation, based on the soil conditions present at the case study site.