Consolidation mechanism of deep cement mixed column improved ground

Consolidation process of Deep Cement Mixed (DCM) column improved ground is a complex issue due to the interdependent behaviour of the DCM columns and the surrounding natural soil. The existing consolidation models for DCM column improved ground have disregarded the vertical drainage in natural soil to simplify the analytical model development. Even though the permeability of cement stabilised soil is greater than that of the natural soil in most situations, vertical drainage in natural soil remains a significant parameter controlling the overall consolidation behaviour of the composite ground due to the large surface area of soil compared to the DCM column area. Therefore, vertical drainage in natural soil should be taken into account in the analytical solutions for consolidation of the composite ground. This paper presents an in depth investigation into the consolidation mechanism of DCM column improved ground considering the equal strain condition. Drainage in surrounding soil in both radial and vertical directions, stress transfer from surrounding soil to DCM column and vertical drainage through DCM column were taken into account in proposing the consolidation mechanism. The consolidation behaviour is first explained using a numerical model with equal permeabilities for both DCM columns and surrounding soil. Then the cases with different permeabilities in DCM columns and natural soil are described. Results of this study show that the stress transfer from surrounding soil to DCM column does not affect the dissipation of pore water pressure in both materials. Also it was found that the difference between the pore water pressure reduction and the effective stress increment is equal to the stress transferred from the surrounding soil to the DCM column during consolidation.