A Case Study On The Design Of Transition Zone For Cement Deep Mixing For A Port Reclamation Project

This paper describes a large container terminal project constructed in reclaimed land comprising deep soft clay that extends to RL-32 m chart datum. The wharf deck is a piled structure, and a 24 m wide area immediately behind it was constructed in 2008 using a Cement Deep Mixing (CDM) technique to improve the soft soil to provide stability. Post-construction settlement in this area was expected to be less than 35 mm in 20 years. However, the container yard some 30 m behind this area was only treated with prefabricated vertical drains (PVD) and surcharge, and post-construction settlement in this area was expected to be over 300 mm.

The design of the transition zone between the wharf and the container yard to limit differential settlement is described in this paper. In the transition zone, the CDM columns were progressively shortened so that the columns were only partially penetrating to give progressively larger post-construction settlement towards the container yard. The project was complicated by the fact that the surcharge design for the container yard was completed by another consultant and the work carried out by another contractor. In addition, the surface drainage within the area was bi-directional, making the design of the transition zone more challenging. To provide confidence to the client, reliability analyses were carried out to assess confidence levels for the predicted post-construction differential settlement to meet the design objectives.