Experimental Study on Vacuum Preloading with Combination of Fill Surcharge

Effect of vacuum removal on consolidation settlement under a combined vacuum and surcharge preloading

Combined vacuum and surcharge preloading has gradually been widely used because of its advantages of low cost, green environmental protection, and good treatment effect. The conventional prefabricated vertical drain presents obvious defects in vacuum preloading treatment, such as obvious silting, serious bending of the drainage board, large attenuation of vacuum degree of drainage board along the depth, long construction period, and so on, which affect the final reinforcement effect. In this paper, the MIDAS finite element simulation of combined vacuum and surcharge preloading of prefabricated vertical drains (PVDs) and prefabricated horizontal drains (PHDs) is established through the comparative experiment of the engineering field. The comparative experimental study is carried out from the aspects of the vertical settlement, horizontal displacement, and pore water pressure. The results show that under combined vacuum and surcharge preloading, the consolidation effect of soft soil with PHDs is better than that with PVDs. When PHDs are used, the vertical settlement increases by 7.2% compared with PVDs; the horizontal displacement is larger; and the pore water pressure dissipates faster. This is because when the PHDs are adopted, the consolidation direction of the soil is consistent with the direction of the vacuum suction, which is mainly caused by vertical settlement. With the consolidation, the spacing between PHDs is gradually shortened, and the drainage distance is reduced, which can effectively reduce the consolidation time and improve the reinforcement effect of the soil. In addition, the PHDs can move downward uniformly with the soil during the consolidation process and have almost no bending deformation, which makes the vacuum transfer more uniform and effective.

Rapid settlement assessment in new reclaimed land is important to follow-up engineering construction. Vacuum-surcharge preloading combined with prefabricated vertical drains (PVDs) is a high-efficient and widely used reclaimed soil-reinforced method. This work focuses on micro-macro mechanism of vacuum and surcharge preloading, and the settlement characteristics of subsoil subjected to vacuum-surcharge pressure are discussed. The surcharge preloading is applied to soil particles directly by vertical stress, and the effective stress increases directly by direct contact among particles. However, vacuum pressure increases the effective stress and decreases the pore pressure at the same time. In new reclaimed land, the proposed method is working on the assumption that the new reclaimed land full of water with particles arranging in a loose state of simple cube (SC) way before treatment, while a dense state of terse tetrahedron (TH) way, a stable structure of soil, after treatment. An approach for calculating the final settlement of new reclamation land is derived in two-dimensional (2D) and three-dimensional (3D) perspective, respectively. Meanwhile, this method has been preliminarily applied to two case histories reported in the literature. Results show that 2D estimation method might be more useful for the preliminary assessment of settlement under a combined vacuum and surcharge preloading in new reclaimed land.

ABSTRACT: In this paper, an overview on the mechanisms, techniques and applications of the vacuum preloading method are presented. Some recent developments in the vacuum preloading techniques, including the use of new materials, the expansion of the method, and new analysis or numerical modeling methods are briefly summarized. INTRODUCTION It has been 56 years since the idea of vacuum preloading was proposed by Kjellman (1952). Since then, the vacuum preloading method has evolved into a mature and efficient technique for the treatment of soft clay. This method has been successfully used for soil improvement or land reclamation projects in a number of countries (Holtz 1975; Chen and Bao 1983; Bergado et al. 1998; Chu et al. 2000; Indraratna et al. 2005). With the merging of new materials and new technologies, this method has been further improved in recent years. A brief overview of this technique and the recent developments and applications is given in this paper. In adopting this technique, sand drains and recently prefabricated vertical drains (PVDs) have often been used to distribute the vacuum pressure and discharge pore water. A nominal vacuum load of 80 kPa is normally used in design although a higher vacuum pressure of up to 90 kPa may be achieved sometimes. When a surcharge load higher than 80 kPa is required, a combined vacuum and fill surcharge can be applied. For the treatment of very soft ground, the vacuum preloading method is faster than the fill surcharge method, as the 80 kPa vacuum pressure can be applied almost instantly, without causing stability problem. The vacuum preloading method is also cheaper when compared with the fill surcharge method for an equivalent load (Chu et al. 2000). The vacuum preloading method has also been incorporated in the land reclamation process when clay slurry dredged from seabed is used as fill material for land reclamation. As the clay slurry fill is too soft for fill surcharge to be applied, the vacuum preloading method is ideally used for the consolidation of the clay slurry. Thousands of hectares of land have been

Zhu, W.; Zhang, M.; Yu, G.; Petukhov, V.; Mymrin, V., and Gidarakos, E., 2020. Flow resistance in dredged soil under a vacuum preloading system with vertical drainage boards. Journal of Coastal Research, 36(2), 327–338. Coconut Creek (Florida), ISSN 0749-0208.Vacuum preloading combined with prefabricated vertical drains is one of the most widely applied methods to improve soft-ground loading capacity in coastal engineering projects. The resistance of solid particles to the pore-water flow in vacuum-preloaded soil plays an important role in the soil consolidation and pore-water dissipation. However, the flow resistance is still not clearly known during the vacuum drainage process. It is not clear whether the formulae for calculating the flow resistance factor for flow in uniform non-deformational porous media are applicable for flow in vacuum-preloaded soils or not. Because of a lack of knowledge on flow resistance in the vacuum-preloaded soil, coastal engineers only rely on their experience to select vacuum pumps. In this paper, the hydraulic resistance to flow in vacuum-preloaded soils was investigated. Energy conservation in the vacuum preloading system during the drainage process was analyzed. Sixteen laboratory tests were conducted, in which the vacuum pressure, flow rate, seepage velocity, and particle Reynolds number were observed or calculated. Results showed that particle size, uniformity coefficient, and porosity of the soil have important effects on resistance in dredged soils under vacuum preloading condition. The flow resistance in the vacuum-preloaded soil behaved quite differently from that in uniform porous medium. The formulae for calculating the flow resistance factor for flow in uniform packed beds were not applicable for flow in vacuum-preloaded soils. A new formula was proposed to calculate the flow resistance factor in vacuum-preloaded soils with acceptable accuracy. Because the vacuum preloading system with fixed pump power consumes much energy, new vacuum preloading systems with low energy consumption are necessarily developed in future work.

A model of consolidation for a single-drain well under vacuum preloading, based on Biot’s axisymmetric theory and considering the “real strain” hypothesis, was established using the finite element method. Degenerating the consolidation equation of real strain yielded the Barron’s classical free strain and equal strain equations. The free strain and equal strain finite element models were derived by imposing boundary and constraint conditions on the real strain FEM model. The validation of the simulation process confirmed that the numerical model achieved consistent outcomes, with the theoretical values postulating its effectiveness. The real strain model revealed the Mandel–Cryer effect of soil near the vertical drain during the initial vacuum preloading consolidation process, causing the effective stress to increase more than the effective stress generated by the vacuum pressure and leading to a greater reduction in the void ratio. This phenomenon is one of the reasons for clogging during the vacuum preloading process. However, the free strain and equal strain models cannot produce this effect because they cannot describe the coupling between soil deformation and fluid flow. The parameter analysis of the real strain model showed that as Poisson’s ratio for soil decreased, the consolidation rate of soil also decreased, while the Mandel–Cryer effect of soil increased.

Measured and predicted performance of prefabricated vertical drains (PVDs) with and without vacuum preloading

This study describes the effect of unsaturation at a drain boundary on the behaviour of a single prefabricated vertical drain subjected to vacuum preloading based on a two-dimensional plane strain finite element model incorporating the modified Cam–Clay model. Unsaturation of soil adjacent to the drain can occur due to mandrel withdrawal or application of vacuum pressure through prefabricated vertical band drains (PVDs). Following initial laboratory simulation in a large-scale radial drainage consolidometer, the authors have attempted to explain the observed retardation of pore pressure dissipation through a series of models, which consider the effect of unsaturation at the drain-soil interface. The results indicate that the introduction of an unsaturated soil layer adjacent to a PVD improves the accuracy of numerical predictions. Lastly, a multidrain analysis is conducted to study the behaviour of an embankment stabilized with vertical drains subjected to vacuum preloading, and the field measurements are compared with a series of numerical model predictions.Key words: consolidation, embankment, finite element method, settlement, soft clay, vacuum preloading, vertical drains.

Stress state of soft soil foundation under vacuum preloading is completely different from its counterpart under surcharge preloading, and the method of calculating the final settlement of soil mass by means of surcharge preloading (positive pressure) is unreasonable in present design. According to a case of soft foundation stabilized with vacuum preloading and through consolidated-undrained triaxial (CU) test on the specimens sampled in situ, the stress paths under different confining pressure are obtained. The settlement of soft foundation treated with vacuum preloading is calculated by strain isoline method. Compared with the practically measured field data, the errors are analyzed and discussed.

The distributions of deformation of soft soil subjected to vacuum preloading have been presented in many case studies; however, the explanations for that deformation behavior have not yet been investigated thoroughly. The influences by the scale are depicted in this paper based on simple numerical analyses and statistical presentation of case studies. Both the width and the depth of treated zone affect the deformation of soft soil in treated zone, which the reason is the development of the effective stresses and stress path. Due to boundary constraints, the increase in effective stresses may not be isotropic under vacuum preloading in the center of large area treated zone, and the maximum settlements and inward lateral displacements have been significant affected by the treated width and depth. K0 consolidation condition under vacuum preloading may be estimated of 2.5 times treated depth away from the treated edge.

Due to the inaccuracy and incapability of the conventional methods of monitoring the groundwater level in vacuum preloading, a visual apparatus of the measuring groundwater level has been designed. The proposed visual apparatus was efficiently developed based on the video camera technology. Utilizing a video camera system, it could be transmitted to the location of the groundwater level in real time to the image receiver. This paper proposes a novel approach using the visual measurement device and assess the conventional methods for performing ground improvement in an area located in Lianyungang, China. The groundwater level, vacuum pressure and the shear strength of soil were recorded during the vacuum consolidation process. Since groundwater level varied in vacuum consolidation, it would be necessary to analyze and assess the variation of shear strength of soil caused by changing the groundwater level. The results showed that the proposed approach is more reliable and intuitive than conventional methods in order to measure the groundwater level under vacuum preloading. Eventually, it is worth mentioning that, the shear strength of soft soil was different at above and below the groundwater level.

Prefabricated horizontal drain (PHD) assisted by vacuum pressure has been used in practice to accelerate the consolidation process of dredged soft mud. However, very few studies have investigated the large-strain nonlinear consolidation caused by vacuum-assisted PHD. Therefore, based on the piecewise-linear method, this study proposes a one-dimensional consolidation model (called VCS herein) for the soft soil consolidation induced by vacuum-assisted PHD. The proposed VCS model has the capability to consider the large strain, PHD assisted by time-dependent vacuum pressure, soil self-weight, time-dependent surcharge loading and nonlinear changes of the geotechnical parameters and is thus superior to the existing models in the literature. The VCS was validated by comparing with the analytical and semi-analytical solutions reported in the literature. In addition, model test of PHD-vacuum preloading consolidation was designed and performed to further verify the proposed model, and excellent agreement was observed. Based on this model, a parametric study was performed to investigate the effect of several relevant parameters on the vacuum preloading consolidation. Moreover, the differences between PHD-vacuum consolidation and PVD-vacuum radial consolidation were investigated. The results indicate that the initial soil height, vacuum pressure and boundary condition considerably influence the performance of vacuum preloading (e.g., time rate of consolidation and final settlement), whereas the time-dependent vacuum preloading affects only the time rate of the vacuum preloading consolidation and not the final settlement magnitude. For the same soil layer, the final settlement of PHD-vacuum consolidation and PVD-vacuum radial consolidation is relatively close. However, the consolidation rate of PHD-vacuum consolidation is obviously faster than that of PVD-vacuum consolidation.

In this study, 2D and 3D numerical analyses were conducted to predict embankment behaviour on soft clay foundations stabilised by Combined Vacuum and Surcharge Preloading. To determine the equivalent plane strain permeability, a revised conversion procedure for plane strain condition considering linear permeability variation in the smear zone is proposed. The equivalent permeability coefficients are then incorporated in finite element codes, employing the modified Cam-clay theory. The advantages and limitations of vacuum application through vertical drains are discussed through two selected case histories from China and Australia and the predictions are compared with the available field data. Apart from realistic 3D numerical modelling, it is demonstrated that the equivalent plane strain analysis can now be used as a predictive tool with acceptable accuracy.

In Indonesia, the construction of the road has challenges because the road was built on soft clay soil. The vacuum preloading method was used to improve the shear strength and compressibility properties of soft soil in this project. Moreover, what needs to be a concern for practitioners is the issue of increasing simulation accuracy in predicting soil settlement in a vacuum preloading system. The research objective of this study was to determine changes in soil settlement behavior that occurred from the vacuum preloading system using a numerical simulations Geostudio with the 2D Multi Drain-Plane Strain approach and the settlement result of the simulation will be compare with instrumentation data. In this study the vacuum pressure distribution is modeled using water total head negative pore water pressure and the pressure value used following vacuum gauge data in the field with distribution approach is 100% at the surface of the sand platform, 85% to a depth of 5 m, then 60% to the end of the PVD. Based on the simulations, the conlusion is the vacuum pressure applied along the vertical drainage is not modeled constant, but changes with depth, the value of 60% at the bottom of the vertical drainage is quite representative of the conditions in the field and the settlement from the simulation is quite good at approaching the field observation with a prediction of the settlement due to vacuum preloading of ±0.93 m, when compared to the field observation data there is a difference of about 1.6%.

Vacuum preloading combined with prefabricated vertical drains (PVDs) has been widely used to improve soft clayey soil deposits. However, the consolidation deformation of high-water-content slurry under vacuum pressure is still not fully understood. In this study, the displacement field of the slurry during vacuum preloading was directly observed using the particle image velocimetry (PIV) technique. The test results showed that the displacement field of the slurry could be divided into three different zones that reflect distinct consolidation patterns. In zones I and III, the slurry was predominantly compressed along the horizontal and vertical directions, respectively, whereas in zone II, compression occurred in both vertical and horizontal directions. In particular, the slurry elements in zones I and II underwent horizontal extension prior to horizontal compression. The study also visualized the boundary of the soil column and provided an equation for the relation between soil column thickness and consolidation time, which could provide a reference for the theoretical consolidation calculations related to soil columns.