Centrifuge and Numerical Investigation of Loose Hydraulic Fill Supported by DCM Columns

With an increasing amount of land reclamation being built in deep water, a recent design approach involving the use of hydraulic fill on floating deep cement mixing (DCM) columns has emerged. However, the load-transfer mechanism among the unreinforced loose earth platform, the floating semirigid columns, and the consolidating soft ground is not fully understood. This paper presents the findings of a centrifuge model test undertaken to investigate the time-dependent response of a group of DCM columns in soft clay under thick, loose hydraulic fill. The measurements obtained were further analyzed using a fully coupled three-dimensional finite-element model. The study revealed that the ratio of surface settlement to the subsoil settlement increases as the relative density of the overlying fill decreases. As consolidation progresses, the soft clay experiences a load-recovery phase despite being beneath loose sand with low tendency to dilate. These behaviors can be explained by a unique punching shear mechanism observed in the loose sand, which is characterized by the formation of a conical wedge above the column head, surrounded by zones of volumetric contraction. The effects of extending the DCM column head above the soft clay surface were studied by means of a numerical parametric study. It was revealed that settlements decrease with longer length of column extension for a given fill thickness. This could be a cost-effective alternative to dynamic compaction for improving the long-term performance of loose fill in deep water reclamation.