A series of finite element analyses, conducted on the basis of modified triaxial tests incorporating radial drainage, were carried out to investigate the lateral deformation and stress state characteristics of prefabricated vertical drain (PVD) unit cells under vacuum preloading. The analyses revealed that the inward horizontal strain of the unit cell increases approximately linearly with the vacuum pressure (Pv) but decreases non-linearly with an increase in the initial vertical effective stress (sigma ' v0). The variations in the effective stress ratio, corresponding to the median excess pore water pressure during vacuum preloading of the PVD unit cell, were elucidated in relation to the Pv and sigma ' v0 using the simulation data. Relationships were established between the normalized horizontal strain and normalized effective stress ratio, as well as between the normalized stress ratio and a composite index parameter that quantitatively captures the effects of vacuum pressure, initial effective stress, and subsoil consolidation characteristics. These relationships facilitate the prediction of lateral deformation in PVD-improved grounds subjected to vacuum preloading, utilizing fundamental preloading conditions and soil properties. Finally, the proposed methodology was applied to analyze two field case histories, and its validity was confirmed by the close correspondence between the predicted and measured lateral deformation.

In view of the challenges posed by construction on deep soft coastal ground, this study introduces the precast drainage pile (PDP) technology. This innovative approach combines precast pipe piles with prefabricated vertical drains, installed through static pile pressing and subsequently subjected to vacuum negative pressure for the consolidation of surrounding soil. To evaluate the efficacy of PDP technology, a comparative analysis was conducted between precast pile and PDP, incorporating field testing and numerical simulation. The investigation focused on the evolution of excess pore water pressure, deformation, and pile bearing capacity. Results indicated that vacuum negative pressure drainage could induce rapid initial dissipation of pore water pressure, followed by a slower rate. Excess pore pressure decreased more rapidly and significantly closer to the drained pile, aligning with drainage consolidation theory. After 5 days of consolidation, the PDP exhibited a 16% increase in ultimate bearing capacity compared with the undrained pile. Numerical simulation outcomes closely matched field measurements. The enhancement in pile bearing capacity was found to correlate hyperbolically with drainage time, culminating in a 26.5% ultimate increase. The research achievements facilitate the development of new pile technologies in coastal soft soil areas.

The rail network invariably encounters soft subgrades consisting of shallow estuarine clayey deposits. Cyclic loading generated by the passage of trains causes deformation and corresponding development of excess pore water pressure (EPWP), which dissipates during the rest periods between two consecutive trains. This paper presents an experimental study describing the effect of yield stress and EPWP responses upon intermittent cyclic loading (i.e. with rest periods), and the associated consolidation with the combination of vertical and radial drainage by way of a prefabricated vertical drain (PVD). Based on the laboratory data, the normalised yield stress for cyclic loading (NYCL) is introduced as an insightful parameter to define a novel empirical relationship between the yield stress, cyclic stress amplitude and the initial effective stress. The experimental results indicate that, as the NYCL increases, the peak EPWP decreases and, during the rest periods, the EPWP reaches a stable equilibrium faster without causing further settlement. Furthermore, this study demonstrates that the accumulated EPWP caused by cyclic loading can be further reduced when using a larger width of PVD for a given unit cell radius. An analytical model inspired by empirical parameters for predicting EPWP is proposed, capturing the effects of NYCL and the PVD characteristics.

Preloading with vertical drains is a well-proven ground improvement technique suitable for soft clay deposits. This technique involves installing vertical drains that cause soil remolding in the immediate vicinity of its surface, creating a disturbed zone known as the smear zone. This study aims to investigate the impact of overburden stress on the characteristics of the smear zone. Very often, the vertical drains penetrate to depths up to 20 m. In such situations, the soil experiences varying stresses, leading to different responses to the drain-induced soil disturbances. Very few studies have investigated how overburden stress affects the smear zone. Through a series of experiments encompassing vertical and radial consolidation tests, along with vane shear tests, this paper evaluates the variation in shear strength and consolidation properties within the smear zone, simulating different depths. Employing three overburden pressure intensities (25 kPa, 50 kPa, 100 kPa, equivalent to depths of about 4 m, 8 m and 16 m), the study establishes that the remolding effect intensifies with depth. Furthermore, it demonstrates the influential role of the smear zone on the undrained shear strength properties of the improved ground, highlighting its variability based on overburden stress.

To accelerate the dissipation of excess pore water pressure, enhance the bearing capacity of piles, and mitigate long-term settlement in soft ground, a novel green and lowcarbon pile foundation technology, termed the precast drainage pile (PDP) technology, is proposed. This innovative approach integrated precast pipe piles with prefabricated vertical drains (PVDs) attached to their sides. The piles were installed using static pile pressing and were subsequently subjected to vacuum-induced negative pressure to facilitate soil consolidation, which enhances the resource utilization rate of pile foundations and promotes the sustainable utilization of soft soil foundations. To investigate the bearing characteristics of the PDP, this study combined the shear displacement method for piles with the consolidation theory of soft soil foundations. A calculation model for the load-settlement behavior of precast piles, accounting for the influence of vacuum-induced soil consolidation, was derived, establishing a method for analyzing the load transfer mechanism of PDPs. The reliability of the theoretical model was validated through comparisons with engineering test results. Building on this foundation, the influence of factors such as consolidation period and pile length on the bearing characteristics of PDPs was analyzed. The results demonstrated that, compared to a 10 m precast pile without drainage, the ultimate bearing capacity of single piles with drainage durations of 3, 7, 14, and 28 days increased by 7.3%, 12.7%, 20.3%, and 29.6%, respectively. Furthermore, under a 7-day drainage condition, the bearing capacity of piles with lengths of 10 m, 20 m, and 30 m increased by 12.7%, 12.8%, and 13.1%, respectively. Overall, the findings of this study provide a theoretical basis for the research, development, and design calculations of this new sustainable pile technology.

In this paper, finite element (FE) modeling is conducted for a high-speed railway embankment on soft soils in Sebou, Morocco. Discrepancies arise between predicted and measured behaviors when using standard creep models. To address this, an advanced anisotropic creep constitutive model, known as Creep-SCLAY1S, is applied for comparison, focusing on the prefabricated vertical drain (PVD) treated soft soils. This advanced model incorporates fabric anisotropy, soil structure, and time-dependent behavior. The time-dependent soft soil creep model (SSCM) is also employed for further comparison. Numerical predictions are then compared with field instrumentation data. Results indicate that Creep-SCLAY1S offers improved predictions of in situ measurements, particularly post-construction, and provides a more accurate peak excess pore pressure during the embankment's rapid surcharge phase.

This paper puts forward a vibrable prefabricated vertical drain (V-PVD) that combines vibrators on PVD to alleviate the clogging on PVD and enhances the reinforcement effect of vacuum preloading method. To validate the reinforcement effect of V-PVD, a full-scale on-site test was conducted including four zones with different V-PVD installations. The ground surface settlement and pore water pressure in each zone were monitored. In addition, a comparative analysis was conducted on vane shear strength and water content before and after soil reinforcement. The test results indicates that the vibrable prefabricated vertical drain in vacuum preloading method can effectively improve the soil reinforcement effect. The ground surface settlement increased by 20.9% to 43.8% compared to conventional vacuum preloading method, and the dissipation value of pore water pressure increased by 17.1% to 58.6%, and vane shear strength increases by 5.9% to 24.5%. The activation of the vibrator helps to remove clogging around PVD, and the more vibrators installed on PVD surface, the better the soil reinforcement effect is achieved. However more vibrators installed on PVD, the drainage area on the PVD surface was influenced and drainage efficiency reduced initially, which implies that a reasonable installation of vibrator should be considered in practice.

When stone columns or vertical drains are applied to improve soils, it is common to face situations where the soft soil layer is too thick to be penetrated completely. Although consolidation theories for soils with partially penetrated vertical drains or stone columns are comprehensive, consolidation theories for impenetrable composite foundations containing both two types of drainage bodies have been few reported in the existing literature. Equations governing the consolidation of the reinforced zone and unreinforced zone are established, respectively. Analytical solutions for consolidation of such composite foundations are obtained under permeable top with impermeable bottom (PTIB) and permeable top with permeable bottom (PTPB), respectively. The correctness of proposed solutions is verified by comparing them with existing solutions and finite element analyses. Then, extensive calculations are performed to analyze the consolidation behaviors at different penetration rates, including the total average consolidation degree defined by strain or stress and the distribution of the average excess pore water pressure (EPWP) along the depth. The results show that the total average consolidation rate increases as the penetration rate increases; for some composite foundations with a low penetration rate, the consolidation of the unreinforced zone cannot be ignored. Finally, according to the geological parameters provided by an actual project, the obtained solution is used to calculate the settlement, and the results obtained by the proposed solution are in reasonable agreement with the measured data.

Soft clays are prevalent in coastal areas of Australia, exhibiting low bearing capacity and considerable settlement upon loading, and must be improved as subgrades to meet the increasing demand for railway transportation soft soils beneath railway embankments are often subjected to train-induced cyclic loads generating higher excess pore water pressure (EPWP), reduced bearing capacity, and deformations under poor drainage conditions. These often lead to reduced efficiency in transportation and maintenance costs. Thus, it is important to investigate the behaviour of soft soils subjected to heavy cyclic loading and preventative actions. This paper provides a comprehensive review of the role of Prefabricated Vertical Drains (PVDs) in mitigating failures associated with railway subgrades under cyclic loading.

The demand for increased axle loads and speeds of trains can diminish the stability of track substructure, leading to potential particle migration or slurry pumping under critical drainage conditions. This paper primarily focuses on the role of geosynthetics in mitigating the risk of soil fluidization potential under cyclic load. Laboratory experiments were conducted to evaluate the effectiveness of geosynthetics including geotextiles, geocomposites, and prefabricated vertical drains (PVDs). The laboratory study indicates that subgrade instability primarily occurs due to the migration of fines towards the subgrade surface and the substantial increase in moisture content (MC). Dynamic Filtration Tests (DFTs) reveal that geocomposite inclusion in rail tracks can reduce the fluidization potential of soft soils and the combined prefabricated vertical drains-geocomposite system can be used to mitigate the critical excess pore water pressure (EPWP) that accumulates in shallow or deeper soil layer due to activated radial drainage paths.