The use of horizontal drains assisted by vacuum loading is an effective method for speeding up the consolidation of dredged soil slurry. However, few studies developed models for the large strain consolidation of clayey slurry with prefabricated horizontal drains (PHDs) under self-weight and vacuum loading considering the effects of nonlinear compression and creep. This study introduces a PHD-assisted finite strain consolidation model considering nonlinear compression and limited creep by incorporating an improved elasto-viscoplastic constitutive equation. Firstly, the governing equations for the consolidation of very soft soil with PHDs were derived and solved by the finite-difference method. Subsequently, the proposed consolidation model was verified by comparing the calculations with the finite element solutions, a laboratory model test, and a field trial performed in Hong Kong. Good agreement with the numerical solutions and measured results indicates that the proposed model can capture the consolidation features with PHD combining staged filling and time-dependent vacuum loading. Then, the proposed model was used to estimate a self-weight consolidation test and field test in Japan to show the performance of the proposed model. Finally, parametric studies were conducted to explore the influence of nonlinear compression and creep on the consolidation of soft soil with PHDs.

Soft clay is identified to have some unfavorable characteristics when used as the foundation for construction work. This is due to the low bearing capacity and high compressibility which often leads to prolonged consolidation settlement. Therefore, soil improvement is necessary, and this can be achieved through the Vacuum Consolidation Method (VCM). This research focused on conducting laboratory-scale model tests to determine the characteristics of clay soil improved by vacuum consolidation in terms of its physical properties, hydrolysis, and compressibility. The results showed that the water content decreased as soil approached the Prefabricated Vertical Drain (PVD). A similar trend was also observed for the compression index value; this is because local density occurs in the soil near the PVD as a result of the movement of fine particles towards the PVD, which fills the empty pore space. Moreover, soil permeability in both vertical and horizontal directions reduced, and the soil became anisotropic after VCM improvement. The results of this research can be a consideration in the use of the VCM method for soft soil improvement, especially the problem of clogging zones that occur due to the suction pressure of the vacuum pump, which can reduce the performance of the VCM.