Search Results (20)

During the installation of prefabricated vertical drains (PVDs) in soft soil foundations, the smear effect induced by mandrel shoe penetration can severely damage the soil structure and reduce permeability, thereby becoming a key factor restricting foundation consolidation efficiency. Previous studies have generally neglected the smear disturbance caused by the geometry of the mandrel shoe. Although existing studies have conducted numerical and theoretical analyses on the smear effect induced by PVD installation, most of them are still based on equivalent circular simplifications and are therefore unable to characterize the anisotropic disturbance induced by a triangular mandrel shoe. To address this limitation, a three-dimensional CEL penetration model considering the real triangular geometry was established, and the traditional cavity expansion theory was directionally modified. The effects of penetration rate, geometric angular structure, and soil type of the triangular mandrel shoe on the smear zone were systematically investigated. The results show that, with increasing penetration rate, the near-field peak stress and far-field displacement increase simultaneously; from slow penetration to fast penetration, the near-field peak stress increases by approximately 42%. By quantitatively defining the critical threshold corresponding to a sharp 50% attenuation in radial displacement as the boundary of the strong smear zone, it was found that increasing the size of the mandrel shoe significantly amplifies the geometric corner effect, and the near-field disturbance range increases by about 21% compared with that of the small-sized case. The larger the size, the more pronounced the anisotropic disturbance characteristics become: the stress concentration effect and displacement splitting in the vertex direction are further enhanced, causing the disturbance range in that direction to far exceed that in the side direction. Soil properties are the key medium parameters controlling the smear zone. Owing to its relatively high stiffness index and skeleton strength, Clayey Silt shows the largest displacement range, whereas Common Clay exhibits the smallest smear zone because of its stronger structural constraint. The modified theoretical model agrees well with the CEL numerical simulation results, verifying its effectiveness under conditions that consider the geometric characteristics of the mandrel shoe. This study provides a theoretical basis and numerical support for the structural design of mandrel shoes in soft-ground PVD construction. Full article

In the treatment of soda-residue-stabilized soil with high water content using drainage boards with vacuum preloading, the boards often prone to clogging and bending under lateral pressure, reducing their hydraulic conductivity and affecting the soil reinforcement. In this study, the structure of the standard plastic drainage board (filter membrane + filter core) was improved, and three types of new anti-clogging plastic drainage boards with different structures were developed (Type X: geotextile + filter core, Type Y: geotextile + wire mesh + filter core, Type Z: geotextile + filter membrane + filter core). Permeability tests were subsequently used to determine the optimal structure. In-lab vertical draining tests with vacuum preloading were carried out on the selected model to study the change in water content, vacuum pressure, surface settlement, vane shear strength, and pore water pressure of soil with drainage board insertion depth, providing a reference for the application of new anti-clogging drainage boards in engineering. The results showed that: (1) the type Y anti-clogging plastic drainage board (geotextile + wire mesh + filter core) exhibits the most balanced performance in terms of permeability, anti-clogging ability, tensile strength and bending strength and is suitable for vacuum preloading of soda residue with high water content; (2) the mechanical properties and anti-clogging performance of drainage boards are highly dependent on their structural configuration. Introducing a wire mesh between the filter core and the geotextile significantly enhances the tensile and bending strength of the drainage board without noticeably compromising its drainage performance; (3) the insertion depth of the drainage board significantly affects drainage efficiency, vacuum transmission rate, and strength development of the soda residue. The effective reinforcement range of the drainage board is not limited to the insertion depth but also extends below the bottom of the drainage board. Full article

One of the key indicators of the foundation soil consolidation is the smear effect brought on by the insertion of a Prefabricated vertical drain (PVD), which also smears the extent of disturbance. Prior research primarily examined the impact of the diameter of the Prefabricated vertical drain sleeves, ignoring the impact of pile shoe size on smear effect. The penetration process of pile shoes of varying sizes in layered soils was simulated using transparent soil model experiments, and Particle Image Velocimetry (PIV) technology was used to visualize and assess the soil disturbance caused by the pile shoes. Theoretical and experimental data are used to suggest and analyze the correction coefficients for the geometric characteristics of pile shoes using the Mohr–Coulomb criterion and reaming theory. The study’s findings demonstrate that transparent soil and the PIV method can successfully capture the dynamic evolution of the “inverted cone” in the smeared area, which is consistent with the theory of cylindrical pore expansion’s prediction. The horizontal disturbance range will increase as the equivalent radius of the pile shoes increases, and it is 4.5d for pile shoes with an equivalent radius of 1.5 mm and 5d for pile shoes with an equivalent radius of 2.0 mm. The discontinuity of the soil layer interface will be made worse by pile shoes with a high equivalent radius, making the phenomenon of stress concentration more noticeable. Its quantitative analysis demonstrates the reasonableness of the correction factor λ, which offers a trustworthy tool to quantify the perturbation effect of the pile shoe size. A correction factor λ is proposed so that the error between the corrected theoretical value and the test value is less than 5%. Full article

This study incorporates a nonlinear seepage relationship into Biot’s consolidation theory and simulates the consolidation of a single vertical drain under vacuum preloading using the finite element method. The model, simplified via the equal-strain assumption, is validated against theoretical predictions. Under the axisymmetric Biot’s framework, consolidation behavior is analyzed in detail. The results show that in the early stages of consolidation, excess pore water pressure in the vicinity of the prefabricated vertical drain (PVD) does not fully dissipate and may even increase, indicating the occurrence of the Mandel–Cryer effect. As the consolidation process advances, the consolidation front gradually extends outward, and the void ratio near the PVD decreases rapidly, leading to the formation of a clogging zone. In contrast, the reduction in the void ratio in the non-clogging region is relatively slow. The progressive development of the clogging zone significantly impedes the overall consolidation rate. Furthermore, this study explores the influence of key parameters—including the compression index, permeability coefficient, well diameter ratio, smear effect, and well resistance—on the formation of the clogging zone and the Mandel–Cryer effect. Full article

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. Full article

Prefabricated vertical drains combined with heating is a new approach to improving the mechanical properties of soft clay foundations. Rising temperatures cause the formation of concentric and radially aligned soil regions with distinct heterogeneous characteristics. This results in incomplete contact between adjacent soil layers, with the water in the interstices impeding heat transfer and manifesting as a thermal resistance effect. Based on the theory of thermo-hydro-mechanical coupling, a two-dimensional dual-zone axisymmetric marine soft soil model improved by a prefabricated vertical thermo-drain has been established. A generalized incomplete thermal contact model has been proposed to describe the thermal resistance effect at the interface of concentric soil regions. The effectiveness of the numerical solution presented in this paper is verified by comparison with semi-analytical solutions and model experiments. The thermal consolidation characteristics of concentric regions of soil at various depths under different thermal contact models were discussed by comprehensively analyzing the effects of different parameters under various thermal contact models. The outcomes indicate that the generalized incomplete thermal contact model provides a more accurate description of the radial thermal consolidation characteristics of concentric regions of soil. The influence of the thermal conductivity coefficient on the consolidation characteristics of the concentric regions soil is related to the thermal resistance effect. Full article

A full-scale embankment on soft clays improved with prefabricated vertical drains (PVDs) have to be analyzed in 3D conditions due to a great number of vertical drains under an embankment. However, 3D analysis is very complex, time-consuming, and needs a powerful computer. Therefore, axisymmetric vertical drains have to be converted into equivalent plane-strain conditions for 2D analysis. Different matching approaches based on unit cell concept have been developed in the literature and the matching can be achieved by modifying the drain distance and/or soil permeability according with relatively simple instructions. This paper investigates verification of three different matching approaches to be used in the numerical analysis of full-scale embankment built on multiple vertical drains. The elasto-plastic soft soil model was used in the numerical analysis, and the results are compared with the laboratory and field measurements. The results of numerical analysis demonstrate that the matching methods are in extremely good agreement with the measurements if the effect of both the smear zone and discharge capacity are taken into consideration. It is seen that these methods provide practical solutions and important advantages to geotechnical engineers. Full article

Situated within the context of a soft ground foundation at an iron ore mining site, this study investigates the impact of substantial surcharges on the settlement of such foundations and the adjacent infrastructure. By employing the finite-difference numerical software FLAC3D 6.0, a series of three-dimensional simulations were conducted to assess the stress response and deformation of gallery pile foundations, shallow foundations, and mine shed pile foundations to step loading. This study integrates the analysis of soil strength augmentation under considerable stress and its attenuation characteristics under significant deformation. Various reinforcement measures, such as the implementation of stone columns, prefabricated vertical drain, and surcharge preloading techniques, were examined for their capacity to consolidate the foundation, reduce settlement, and mitigate impacts on adjacent structures. The results reveal that horizontal displacements in the pile and shallow foundations escalate progressively with additional surcharge throughout the operational period. The most pronounced horizontal deviation in the pile foundations is observed at the juncture between sand and silt strata. Stone columns act effectively as a barrier to the sliding surface, consequently reducing the influence of surcharge on the movement of the foundation. Full article

By comparing different settlement forecast methods, eight methods were selected considering the creep of marine soft soils in this case study, including the Hyperbolic Method (HM), Exponential Curve Method (ECM), Pearl Growth Curve Modeling (PGCM), Gompertz Growth Curve Modeling (GGCM), Grey (1, 1) Model (GM), Grey Verhulst Model (GVM), Back Propagation of Artificial Neural Network (BPANN) with Levenberg–Marquardt Algorithm (BPLM), and BPANN with Gradient Descent of Momentum and Adaptive Learning Rate (BPGD). Taking Lingni Seawall soil ground improved with prefabricated vertical drain-assisted staged riprap filling as an example, forecasts of the short-term, medium-term, long-term, and final settlements at different locations of the soft ground were performed with the eight selected methods. The forecasting values were compared with each other and with the monitored data. When relative errors were between 0 and −1%, both the forecasting accuracy and engineering safety were appropriate and reliable. It was concluded that the appropriate forecast methods were different not only due to the time periods during the settlement process, but also the locations of soft ground. Among these methods, only BPGD was appropriate for all the time periods and locations, such as at the edge of the berm, and at the center of the berm and embankment. Full article

During the consolidation period of vacuum preloading drainage of dredged mud slurry, the clogging behaviors of the filter drainage structural layers and the core boards of prefabricated vertical drains (PVD) determine the drainage capacities of PVD. However, currently, there is a lack of comprehensive research on the evaluation criteria for these two clogging behaviors. Therefore, based on typical dredged mud slurry, typical geomembranes, and raw material core boards with different bending forms, relevant macro and micro-scale experimental studies have been carried out in this study. The research results show that (1) with the application of the gradient ratio test method, the clogging behaviors of filter membranes of PVD under graded vacuum preloading can be effectively simulated. Also, in the design of graded vacuum preloading, characteristics of equivalent pore sizes and pore structures should be emphasized to investigate the suitability of filtration and drainage performance of PVD filter membranes. (2) The compressive yield strength of core board grooves is a key factor influencing the reduction rate of flow capacity. The reduction rate of flow capacity and well resistance increment can be used as comprehensive indicators reflecting the clogging behaviors of core boards, while the bending angles and bending rates of core boards can be used as specific technical indicators. (3) The proposed clogging evaluation criteria for PVD are as follows: a filter membrane gradient ratio (GR) > 4.0, a core board bending rate >60% and a core board bending angle < 45°, or a reduction rate of flow capacity of bending drainage board > 90% or well-resistance increment > 9. Also, these criteria can be incorporated into the control indicators for drainage performance of PVD used in such types of foundations. Full article

Prefabricated vertical drains (PVDs) with staged riprap preloading have been widely used in soft soil ground improvement and embankment construction. However, ground treatment effectiveness evaluation is still a difficult problem due to multiple factors. Considering this, in situ monitoring and numerical simulation were conducted to study the deformation and strength characteristics of marine soft soil ground treated by PVD-assisted staged riprap under the Lingni Seawall construction in China. Monitoring and analysis of results showed that use of PVD-assisted staged riprap resulted in a good improvement effect. In particular, in the PVD-treated zone within 10 m in depth, corresponding to a half-length of the PVD, the average radial degree of consolidation reached up to 75–100%, and the soil strength increased significantly by 200–700%. Moreover, numerical simulation showed that the linear 1-dimensional drain element of PVD closely met the engineering accuracy requirements with good consistency with the monitoring data. Compared with a totally solid element model, the numbers of elements and nodes were reduced and the calculating efficiency and model accuracy were increased by using a PVD linear element, which provides a basis for building large complex finite element models. Full article

Increasing development of infrastructure in Indonesia has driven the need for effective ground improvement methods to accelerate the consolidation of soft soil, which is estimated to occupy around 10% of the country’s land area. A prefabricated vertical drain combined with vacuum preloading is among the most effective methods for this purpose. However, the prefabricated vertical drain creates a smear zone in the surrounding soil area during installation. This study examines the effectiveness of a newly developed mandrel system in reducing the smear zone during prefabricated vertical drain installation. Large-scale consolidation tests at a macro level and microstructure analysis using scanning electron microscopy at a micro level were employed to investigate the effect of soil water content and shear strength. The results show that the water content and shear strength of the soft soil gradually increased in the inner smear zone and transition zone, while both decreased in the radial distance. Furthermore, the soil structure underwent a transformation in which the particle area and pore area became a closed flake structure, and apparent agglomeration occurred. The test results indicate that the newly developed mandrel system can effectively reduce the smear zone. The macro to micro test results demonstrated that the mandrel system is successful in reducing the smear zone effect. Full article

This article provides a comparative knowledge of predicted and measured settlements of road embankments with prefabricated vertical drains (PVDs). The emphasis of this study was to investigate and back-analyse the ratio of horizontal permeability in an undisturbed area to a smeared zone, which affects the behaviour of vertical drains. Two models of PVDs in soft ground were developed by utilising a plane strain 2D numerical approach based on the equivalent permeability. Suggestions for the improvement of numerical accuracy of the soft ground beneath road embankments have been made in regards to the obtained results. The employment of the equivalent horizontal permeability in numerical analysis produced significantly similar results to those of the measured values. Furthermore, a smear effect permeability ratio of 300 produced a considerably accurate result with a model based on the equivalent horizontal permeability and measured data. Lastly, the smear effect ratio of 6 using the equivalent horizontal permeability approach was employed in order to predict the behaviour of vertical drains in the soft grounds under road embankments. Full article

The prefabricated vertical drain (PVD) is an essential means to mitigate the settlement of soft soil foundations in coastal areas of South China. The commonly used elastoplastic analytical method cannot directly reflect the interaction of different PVDs and the resulting displacement of soft soil. At the same time, these elastoplastic analysis and numerical simulation methods are greatly influenced by the values adopted for rock and soil material parameters. In this paper, we present a stochastic medium model that can directly reflect the interaction of different PVDs and the resulting displacement of soft soil. It is not affected by the characteristics of rock and soil themselves and can also reflect the actual deformation process of soft soil. According to engineering practice, the settlement curves of soft soil foundations in coastal areas of South China with PVDs exhibited distinct normal distribution characteristics, which was consistent with the description of settlement by the stochastic medium model. Hence, based on the stochastic medium model, this paper analyzed the settlement mechanism of PVDs and established a stochastic medium model for the settlement calculation of PVDs. A function for the soft soil foundation in the coastal area of South China cross-section settlement curve was presented by back analysis of the PVD model. We chose the stochastic medium model based on this methodology to explore the interaction between different PVDs. The above models were then applied to an expressway in South China. Comparing actual settlement monitoring values to calculated values obtained with the PVD model, the error between the two models was less than 15%. This research provides a new settlement calculation method of PVDs in soft soil foundations in the coastal area of South China and a new basis for designing soft soil foundations. Full article

The vacuum preloading method is commonly adopted for improving the soft ground that the embankment of the railway line is laid on. The PIV (Particle Image Velocimetry) technique is a powerful tool in observing the formation of the soil column, a phenomenon that is unique to the dredged slurry when treated by vacuum preloading. However, it is not clear to what extent the motions of the slurry particles can be represented by the PIV tracers. In this paper, a mesoscopic model has been established by using the CFD-DEM method to reproduce the vacuum consolidation process of the slurry, in which the PVD (Prefabricated Vertical Drain) membrane, the slurry particles, and the tracers are described by the DEM, and the pore water is governed by the CFD method. Eight computational cases that can cover a broad range of material parameters governing the PIV model tests on the dredged slurry have been designed and studied by the established model. The representativeness of the PIV tracer is evaluated by comparing the statistic displacement of the tracer to that of the slurry particles. It is found that for the commonly used tracer, the carbon powder, can reliably represent the particle motions of the slurry since the difference in displacements of the tracer and the slurry particles is smaller than 6.5% if the diameter ratio between the tracer and the slurry particle is within 1.8. Full article