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Introducing the prefabricated horizontal drains (PHDs) into the permeable geotextile tubes has been reckoned as an efficient and economical method of dewatering slurries with high water contents. However, its application is limited by the poor understanding of the fundamental consolidation mechanism, often resulting in overconservative designs. With the proposed two-dimensional plane-strain consolidation model, a semi-analytical solution calibrated by laboratory test results is obtained. Furthermore, parametric analyses, for example, PHD pave rate, and element aspect ratio, along with the soil anisotropy coefficient, are also conducted to achieve the optimum design requirement. A critical condition corresponding to the optimum effectiveness of this consolidation technique is found, beyond which the consolidation efficiency decreases gradually. The greater the abovementioned parameters are, the later and milder the efficiency reduction occurs. The consolidation efficiency can keep at a high level until the end of consolidation when the values of these parameters are great enough. In the end, design charts and design recommendations are provided with immediate benefits for practical engineers. Key words: consolidation, slurry, horizontal drains, geotextile tubes, laboratory tests, Introduction Due to rapid modernization in China, the clayey slurries dredged from rivers or coastal regions were nearly 6 billion [m.sup.3] in 2016, increasing by 30% per year at least [...]

Vacuum preloading combined with prefabricated vertical drains (PVDs) system has been widely used to improve the soft clay with high water content. Clogging is usually formed around the PVDs during the vacuum preloading, impeding the propagation of the vacuum pressure and slowing down the consolidation process. To understand the forming mechanism of the clogging, particle image velocimetry (PIV) technique and particle tracking velocimetry (PTV) technique were adopted in the model test of vacuum preloading test. Through this study, three stages can be identified from the results of water volume discharge rate and maximum displacements versus time. In the first stage, the soil around the PVD is horizontally consolidated, which leads to the rapid formation of clogging. In the second stage, the formation of clogging slows down due to the loss of vacuum pressure, which further reduces the drainage. In the third stage, the clogging tends to be stable, and the drainage consolidation rate is significantly reduced. PTV results show that there is a difference in the displacement of large and small particles during improvement. Two methods were proposed to estimate the thickness of clogging zone, reflecting a growing layer of clogging zone compressed around the PVD. This study provides new insights to investigate the formation mechanism of clogging during vacuum preloading test. Key words: vacuum preloading, particle image velocimetry, particle tracking velocimetry, clogging, dredged slurry. Le prechargement sous vide combine au systeme de drains verticaux prefabriques (PVD) a ete largement utilise pour ameliorer l'argile tendre a forte teneur en eau. Un colmatage se forme generalement autour des PVD pendant le prechargement sous vide, empechant la propagation de la pression du vide et ralentissant le processus de consolidation. Afin de comprendre le mecanisme de formation du colmatage, la technique de velocimetrie par image de particules (PIV) et la technique de velocimetrie par suivi de particules (PTV) ont ete adoptees dans le test modele de l'essai de precharge sous vide. Cette etude permet d'identifier trois etapes a partir des resultats du taux de decharge du volume d'eau et des deplacements maximaux en fonction du temps. Dans la premiere etape, le sol autour du PVD est consolide horizontalement, ce qui conduit a la formation rapide de colmatages. Dans la deuxieme etape, la formation de colmatage ralentit en raison de la perte de pression du vide, ce qui reduit encore le drainage. Dans la troisieme etape, le colmatage tend a etre stable, et le taux de consolidation du drainage est considerablement reduit. Les resultats de la PTV montrent qu'il existe une difference dans le deplacement des grandes et petites particules pendant l'amelioration. Deux methodes ont ete proposees pour estimer l'epaisseur de la zone de colmatage, refletant une couche croissante de zone de colmatage comprimee autour du PVD. Cette etude fournit de nouveaux elements pour etudier le mecanisme de formation du colmatage pendant l'essai de prechargement sous vide. [Traduit par la Redaction] Mots-cles: prechargement sous vide, velocimetrie par image des particules, velocimetrie de suivi des particules, colmatage, boue de dragage., Introduction Vacuum preloading combined with prefabricated vertical drains (PVDs) has been widely used to improve the engineering property of the dredge slurry (Chai et al. 2005; Chu et al. 2006; [...]

In the process of treating the dredged slurry under vacuum preloading combined with the prefabricated vertical drains (PVDs), a dense “soil column” with low permeability will be formed around the PVD, which results in a poor drainage condition and an unsatisfactory treatment (i.e., the clogging effect). In this study, in order to predict the consolidation behavior of dredged slurry with different initial water contents, the compressibility and permeability of dredged slurry with different initial water contents are investigated. Based on the compression and permeability curves obtained by laboratory tests, the analytical solution to soil consolidation under vacuum preloading is derived. Both the clogging effect and the effect of the initial water content on the initial effective stress are considered. The developed analytical solution is validated through a series of consolidation tests of dredged slurry under vacuum preloading with different initial water contents. It shows that at a given vacuum preloading, the dissipation rate of excess pore water pressure in the dredged slurry decreases as the initial water content increases. The proposed analytical solution presents reliable predictions on the variation of settlement and degree of consolidation with time in the dredged slurry with different initial water contents under vacuum preloading.

Laboratory tests have been carried out for simulating dredged-slurry treatment combining air-booster and conventional vacuum preloading methods. Settlement plate, micro pore water pressure transducer and miniature vane shearing instrument are adopted for monitoring the settlement, the pore pressure dissipation and the soil strength during the test. The tests results demonstrate that the air-booster vacuum-preloading method can significantly improve the soil strength, pore-water pressure dissipation and settlement. In particular, the real-time responses of pore-water pressure during the air-pressurizing process have been obtained that, in combination with the numerical simulations, help derive the reinforcement mechanism of the air-booster vacuum preloading method, which involves using micro cracks generated by splitting soil between the booster tube and PVD using the pressurised air. Those cracks improve the soil permeability, and have lasting effect even after air pressuring ceased.

Treating the high water-content sludge with vacuum-assisted prefabricated horizontal drains (PHDs) placed inside, geotextile tubes have the advantage of multiple drainage paths, and geomembrane tubes have the superiority of high vacuum maintenance. Therefore, converting the geotextile tube into a geomembrane tube during slurry dewatering is an effective measure for greater dewatering efficiency and better dehydration effect. In this study, a profound plane-strain consolidation model considering two-dimensional seepage is developed for explaining the dewatering mechanism of sludge in the geo-tubes. Analytical solutions are given and validated by the experiment. The impact of major variables on tube efficiency is further discussed to reference the practice design. Parametric analyses reveal the critical time corresponding to the optimal efficiency of the geotextile tube, after which its consolidation efficiency decreases significantly. However, the consolidation efficiency of a geomembrane tube increases throughout the duration. Consequently, a switching point corresponding to the iso-efficiency state of the geotextile tube and geomembrane tube can be determined for the tube conversion. [ABSTRACT FROM AUTHOR]

A very soft clay layer is highly compressible and exhibits significant creep under loading. The classical linearly elastic constitutive relationship and small-strain assumption are not suitable for the consolidation analysis of very soft clays. This paper presents a new large-strain consolidation model that incorporates the Yin-Graham elastic viscoplastic (EVP) constitutive equation for use in studying the consolidation of very soft clay layers with vertical drains under preloading. First, the large-strain fluid continuity equation and the EVP constitutive equation are incorporated into a quadratic differential equation of pore-water pressure and its integral terms. Second, the alternating-direction implicit (ADI) method and virtual node method are adopted to obtain the finite difference solution. A computer program named 'BSSDS' is developed for large-strain EVP consolidation analysis of clay layers with vertical drains, taking into account the complicated in situ conditions, such as resistance of vertical drains, smear effects, variation of permeability with void ratio, and multilayered soils. Third, the new large-strain numerical method is applied to the consolidation modeling of very soft clay layers with vertical drains under preloading at a site that is part of the Hong Kong-Shenzhen Western Corridor Link Project. It is found that the foundation settlements of the new large-strain EVP consolidation model have good agreement with the measured data. Finally, three different consolidation models are used to calculate the average degree of consolidation and settlements of the clay layers. The analysis shows that it is essential to consider both large-strain compression and creep effects in the analysis of very soft clay layers with vertical drains under loading. Key words: soft clay, consolidation, elastic viscoplastic model, large strain, vertical drain. Une couche d'argile tres molle est consideree comme tres compressible et presente du fluage significatif lors de sollicitations. La relation lineaire elastique constitutive classique et l'hypothese des faibles deformations ne sont pas appropriees lors de l'analyse de consolidation d'argiles tres molles. Cet article presente un nouveau modele de consolidation a grande deformation qui integre l'equation constitutive elastique viscoplastique (EVP) de Yin-Graham pour l'etude de la consolidation dans des couches d'argile tres molle avec des drains verticaux, soumises a des pre-charges. Premierement, l'equation de continuite des fluides a grande deformation et l'equation constitutive EVP sont integrees dans une equation differentielle quadratique decrivant la pression interstitielle et ses termes integraux. Deuxiemement, la methode implicite de direction alternative (IDA) et la methode du noeud virtuel sont adoptees pour obtenir la solution de la difference finie. Un programme informatique appele BSSDS a est developpe pour l'analyse de consolidation EVP a grande deformation de couches d'argile ayant des drains verticaux, en considerant les conditions in situ complexes, telles que la resistance des drains verticaux, les effets de souillures, la variation de la permeabilite selon l'indice des vides, ainsi que les sols multicouches. Troisiemement, la nouvelle methode numerique a grande deformation est appliquee pour la modelisation de la consolidation de couches d'argile tres molle avec des drains verticaux et soumis a des pre-charges sur un site qui fait partie du projet de Corridor Ouest Hong Kong-Shenzhen. Il est demontre que les tassements de la fondation selon le nouveau modele de consolidation EVP a grande deformation correspondent bien avec les donnees mesurees. Enfin, trois modeles de consolidation differents sont utilises pour calculer le degre de consolidation moyen et les tassements des couches d'argile. L'analyse montre qu'il est essentiel de considerer la compression a grande deformation et les effets de fluage lors de l'analyse de couches d'argile tres molle ayant des drains verticaux et soumises a des solicitations. [Traduit par la Redaction] Mots-cles: argile molle, consolidation, modele elastique viscoplastique, grande deformation, drain vertical., Introduction With rapid economic growth, the demand for infrastructure construction over reclaimed lands and soft marine clays has continuously increased in the coastal areas of many countries and regions. In [...]

Lime treatment combined with vacuum preloading has been used in the treatment of dredging slurry for many years. Previous studies have indicated that particle grading, which is one of the essential properties of slurry, could affect the pore-size distribution and compression characteristics of slurry and thus affect the efficacy of the lime treatment and vacuum preloading. However, the effect of particle grading has not yet been examined quantitatively. This study investigated the compressibility of lime-treated slurries with different particle gradings using a one-dimensional oedometer for compression testing. A series of laboratory model tests were carried out to examine the effect of particle grading on the efficacy of lime-treated slurry with vacuum preloading. The results showed that particle grading significantly influences optimal lime content. For clay or low-plasticity silt, the optimum lime content for lime treatment combined with vacuum preloading was found to be 2%. When slurry contains increased silt and sand grains, more lime is required to achieve effective lime treatment. However, excessive lime would reduce the efficacy of lime treatment on the compressibility and permeability of the soil. [ABSTRACT FROM AUTHOR]

The design of preloading involves loading schedule design and prediction of post-construction settlement. As the in situ measurement data are usually unavailable during the loading schedule design, it is difficult [...]

Summary: The system of vacuum pressure combined with vertical drains to accelerate soil consolidation is one of the most effective ground improvement methods. The consolidation theories of soft soil improved by vertical drains including void ratio–dependent compressibility and permeability have been widely applied in practice to predict the consolidation behavior. In this paper, analytical solutions of the consolidation of vertical drains are derived incorporating the loss and propagating stage of vacuum pressure. In addition, special solutions are obtained for the cases of instantaneous surcharge loading and staged surcharge loading, based on the general solution. The solution is verified by ignoring the propagating stage of vacuum pressure formation and comparing it with an existing solution. The effects of vacuum pressure loss and propagating stage combined with other parameters are investigated through the ratio between excess pore water pressure and surcharge loading. [ABSTRACT FROM AUTHOR]

The sedimentation process of granular materials exists ubiquitously in nature and many fields which involve the solid-liquid separation. This paper employs the coupled computational fluid dynamics and discrete element method (CFD-DEM) to investigate the sedimentation process of non-cohesive particles, including the hindered settling stage and the deposition stage. Firstly, the coupled CFD-DEM model for sedimentation is validated by the hindered settling velocity at different solid volume concentrations of suspension $$\phi _{0} $$ , i.e., $$\phi _0 =$$ 0.05-0.6. Two typical modes of sedimentation are also presented by the concentration profiles and the equal-concentration lines. Then, the comparisons between mono- and poly-dispersed particle system are detailed. In the sedimentation of the poly-dispersed particle system, the segregation phenomenon is simulated. Furthermore, this segregation effect reduces with the increase of the initial solid concentration of suspension. From the simulations, the contact force between every pair of particles can be obtained, hence we demonstrate the 'effective stress principle' from the view of the particle contact force by giving the correspondence between the particle contact force and the 'effective stress', which is a critical concept of soil mechanics. Moreover, the deposition stage can be simulated by CFD-DEM method, therefore the solid concentrations of sediment bed $$\phi _{\mathrm{max}} $$ on different conditions are studied. Based on the simulation results of $$\phi _{\mathrm{max}} $$ and the theory of sedimentation, this paper also discusses a method to calculate the critical time when sedimentation ends of two typical modes of sedimentation. [ABSTRACT FROM AUTHOR]