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.

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.

Horizontal drains (HDs) are used widely for effective groundwater management in agricultural lands and geotechnical applications. These drainage systems facilitate water flow through perforations in the HD pipe wall, causing a head loss known as entrance head loss (he) across the pipe wall. This he typically is added to the groundwater table, impacting the achievement of the anticipated groundwater level. The extent of he is influenced by both the perforation arrangement of the pipe and the material used for the pipe envelope. In this study, a numerical model, which was validated using a physical sand tank model, was utilized to simulate HDs and assess the impact of perforation arrangement and envelope material permeability on he. The analysis revealed a significant influence of the perforation arrangement on he, with closely spaced smaller perforations exhibiting lower he values than widely spaced larger perforations. Under the specified model conditions, he accounted for 13%-43% of the head loss. Envelope wrappings, such as geotextiles, commonly are employed in HDs to prevent particle loss from the surrounding soil through the perforations. The study demonstrated that geotextiles can substantially reduce he, thereby enhancing the overall performance of HDs. Notably, the effect of he becomes negligible when the permeability of the geotextile exceeds 500 times that of the surrounding soil. An investigation of 25 geotextiles available on the market revealed significant anisotropy in permeability; the in-plane permeability could be as low as 1/500th the cross-plane permeability. This discrepancy can lead to a substantial increase in he. Consequently, the study suggests a selection criterion for geotextile envelope permeability to minimize he, emphasizing the need for careful consideration when choosing materials for HD applications.

This paper offers valuable insights for advancing the consolidation of dredged slurry, alleviating blockage, and the application of vacuum preloading-airbag pressurization (VP-AP) technology in engineering practice. This study conducted laboratory model tests on the consolidation of sludge using prefabricated vertical drains-vacuum preloading, prefabricated horizontal drains-vacuum preloading, and VP-AP methods to further investigate the effectiveness of the VP-AP technique in strengthening the consolidation of dredged slurry. Comparative analysis was conducted on the macroscopic and microscopic differences in soil drainage characteristics, settlement, water content, shear strength, and soil particle morphology under the three treatment methods. The results show that the VP-AP method surpasses traditional vacuum preloading techniques in soil consolidation, effectively guaranteeing the consolidation of deep soil layers and enhancing the uniformity and stability of foundation strength. In addition, the microstructural analysis reveals that the VP-AP method can effectively mitigate the decrease in drainage efficiency caused by the clogging of the PVD and improve the structure of the soils, allowing a significant increase in the mechanical properties of the soils. In conclusion, the VP-AP technology demonstrates significant advantages in drainage efficiency, consolidation effectiveness, thus it has a widespread application potential in engineering practices for treating soft ground and deserves further in-depth study.

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.

Horizontal drains have been widely installed along expansive soil slopes to maintain slope stability. However, these drains typically get clogged with clay particles after several years of operation and must be maintained and replaced regularly. This paper proposes a new type of horizontal drain with a replaceable tubular filter element (RTFE) to overcome the time-consuming nature and laborious replacement procedure of existing horizontal drains. Tests were conducted to compare its drainage performance with that of a conventional horizontal drain. The effects of horizontal drain clogging on the pore water pressure and slope stability were analyzed using the equivalent permeability coefficient of the expansive soil considering the adverse effects of cracks that are randomly distributed in the soil when the matrix suction exceeds the air-entry value. This coefficient was then used as one of the input parameters in the finite element analysis (FEA) for a hydro-mechanical coupling simulation. A replacement standard for the tubular filter element was established according to the numerical results, and the replacement method was explained. The study results showed that the RTFE-equipped horizontal drain was evidently superior to the conventional horizontal drain owing to the advantage of quick replacement. It can also effectively preserve the soil and prevent infiltration deformation caused by the loss of skeleton particles, implying a more economical, effective, and controllable means for the dewatering of expansive soil slopes. This study provides references for the construction and management of engineering projects involving horizontal drainage systems.

Horizontal drains are gradually introduced to the vacuum preloading method to improve dredged slurries by adding geotextiles to alleviate the blockage in the consolidation process. This study considers the consolidation of slurries enhanced by the vacuum preloading method with geotextile combined horizontal drains based on a double-layered consolidation model. The model approximates geotextile as a special soil layer possessing an equivalent consolidation factor. An analytical solution of the layered consolidation model is obtained using the Laplace transform and the finite Fourier transform method. The effectiveness of the solution is verified by comparing it with the one-dimensional double-layered consolidation solution and the one-dimensional consolidation with a partially permeable boundary. Through comparison with laboratory experiments, the model shows good fitness with the test results in the literature. The influences of related parameters, including the drain arrangement densities, soil parameters, and geotextile parameters, are discussed on average consolidation degree and pore water pressure. The influence mechanism is explained regarding drainage path and vacuum pressure transfer. Findings demonstrate that geotextile facilitates vacuum transfer and promotes soil consolidation, especially when the smaller density of drains' deposition and lower soil permeability are applied.