As a relatively new method, vacuum preloading combined with prefabricated horizontal drains (PHDs) has increasingly been used for the improvement of dredged soil. However, the consolidation process of soil during vacuum preloading, in particular the deformation process of soil around PHDs, has not been fully understood. In this study, particle image velocimetry technology was used to capture the displacement field of dredged soil during vacuum preloading for the first time, to the best of our knowledge. Using the displacement data, strain paths in soil were established to enable a better understanding of the consolidation behavior of soil and the related pore water pressure changes. The effect of clogging on the deformation behavior and the growth of a clogging column around PHD were studied. Finite element analysis was also conducted to further evaluate the effects of the compression index (lambda) and permeability index (ck) on the soil deformation and clogging column. Empirical equations were proposed to characterize the clogging column and to estimate the consolidation time, serving as references for the analytical model that incorporates time-dependent variations in the clogging column for soil consolidation under vacuum preloading using PHDs.

Nowadays, the utilization of prefabricated vertical drains (PVDs) or prefabricated horizontal drains (PHDs) in combination with vacuum preloading (VP) has emerged as a prevalent and effective strategy for treating dredged slurry. Nevertheless, both of these methods possess certain inherent limitations. In this study, three groups of parallel model experiments are conducted to compare the effectiveness of PVDs, PHDs and PHDs-PVDs under step VP in treating dredged slurry. Firstly, the water discharge, settlement and pore water pressure are monitored during the experiments. Then, the shear strength and water content of the soil at various locations after experiments are measured and the soil profiles at different cross sections are gauged. Additionally, soil excavation is conducted to evaluate the deformation characteristics of PHDs and PVDs. Finally, a scanning electron microscopy analysis is to assess the clogging of filter membranes. The results indicate that the proposed method can combine the advantages of both PHDs and PVDs, effectively enhancing the treatment effectiveness of the slurry. These findings elucidate the dewatering and reinforcement mechanism of PHDs-PVDs-VP and provide valuable insights for its practical engineering application.

Prefabricated horizontal drains and vacuum preloading have advantages in the consolidation of ultra-soft dredged sludge and soils for maintenance dredging, reclamation, and ground improvement in coastal regions. While laboratory tests and field trial projects have been reported, a convenient analysis and design method is still unavailable. This study proposes a new simple method for the settlement analysis of soft soils considering horizontal drains, vacuum preloading, creep, and large-strain effects. A unified equation is constructed to account for various layouts of horizontal drains in consolidation. A new explicit method is developed to consider the large-strain deformation with the nonlinear evolution of permeability and compressibility of ultra-soft soils under vacuum preloading. The viscous compression is taken into account using a simplified Hypothesis B method. The proposed solution also facilitates convenient consideration of multiple layers of soils and drains subjected to staged loading. The proposed method is examined by a series of physical model tests with different horizontal drain dimensions. Finally, the method is applied in the analysis of two well-documented field cases in Hong Kong and Japan, which confirms its effectiveness and accuracy.