Finite Element Analysis of Vertical and Horizontal Drainage Structures under Vacuum Combined Surcharge Preloading

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.

Vacuum preloading with combination of fill surcharge has been widely applied in the infrastructure development projects as a common and effective method for soft soil improvement. Complete procedure of soft soil improvement by vacuum preloading method includes not only the application of vacuum pressure but also the removal of vacuum pressure. In this paper, the mechanisms of vacuum preloading and vacuum removal are discussed with the consideration of anisotropy of soils. The analysis shows that the stress-induced anisotropy due to the removal of vacuum pressure may cause soil settlement. A series of triaxial stress path experiments on undisturbed soil samples have been conducted. The experiments were designed to simulate the soil behavior during (a) vacuum preloading and vacuum removal and (b) combined vacuum and surcharge preloading and vacuum removal. Typical test results of axial displacement variations with time are presented and discussed. It is found that the soil deformation is significantly affected by the soil stress state when the vacuum pressure is removed. At low deviator stress level, the vacuum removal will cause upward deformation. When the deviator stress exceeds certain critical value, the vacuum removal will cause vertical deformation or even plastic failure in the soil. It is also found from laboratory tests that the soil settlement increase due to the vacuum removal will only occur in the vacuum and surcharge combined preloading projects.

Vacuum preloading coupled with prefabricated vertical drains (PVDs) has been widely used to improve the property of the dredged slurry. However, the effect of PVD layout on slurry consolidation characteristics remains poorly understood. This study conducted two vacuum preloading model tests—with single and dual PVDs. Particle image velocimetry (PIV) was used to directly observe the slurry displacements and strain paths. The test results revealed that the slurry consolidation characteristics were similar under different PVD layouts in the early stage of vacuum preloading. However, in the mid-stage, the effect of dual PVDs led to a significant reduction and increased dispersion of the slurry horizontal displacement. This, in turn, resulted in a thinner clogging zone and a subsequent decrease in vacuum pressure loss. Consequently, the slurry near the PVDs experienced an extended period of consolidation deformation. During the later stage, the single PVD model test exhibited a distinct zone of both vertical and horizontal compression in the slurry, while under dual PVDs, the slurry primarily underwent vertical compression and horizontal extension. The study also provided the relationship between the clogging zone thickness and time, which provided a reference for theoretical consolidation calculations.

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

The vacuum preloading method has been used in many countries for soil improvement and land reclamation. However, the treatment time is long and the improvement effect is poor for the straight-line vacuum preloading method. To alleviate such problems, a novel combined air booster and straight-line vacuum preloading method for shallow ground treatment is proposed in this study. Two types of traditional vacuum preloading and combined air booster and straight-line vacuum preloading tests were conducted and monitored in the field. In both tests, the depth of prefabricated vertical drains (PVDs) is 4.5m, the distance between PVDs is 0.8m, and the vacuum preloading time is 60 days. The prominent difference between the two methods is when the preloading time is 45 days, the injection pressure of 250 kPa is adopted for combined air booster and straight-line vacuum preloading test to inject air into the ground. Based on the monitoring data, this paper systematically studied the mechanical parameters, hydraulic conductivity, pore water pressure, settlement and subsoil bearing capacity, as determined by the vane shear strength, to demonstrate that the air-pressurizing system can improve the consolidation. The consolidation time decreased by 15 days, the pore water pressure decreased to 60.49%, and the settlement and vane shear strengths increased by 45.31% and 6.29%, respectively, at the surface. These results demonstrate the validity of the combined air booster and straight-line vacuum preloading method. Compared with the traditional vacuum preloading, the combined air booster and straight-line vacuum preloading method has better reinforcement effect. In addition, an estimation method for evaluating the average degree of consolidation and an empirical formula for evaluating the subsoil bearing capacity are proposed to assist in engineering decision making.

ABSTRACT: This paper focuses on increasing the efficiency in accelerating the rate of consolidation of prefabricated vertical drains (PVDs) in improved soft Bangkok clay using vacuum and heat preloading. Laboratory model tests were performed in new, large-scale consolidometers with PVD-improved reconstituted specimens using the following four schemes: surcharge preloading (PVD only), surcharge combined with vacuum pressure preloading (vacuum-PVD), surcharge combined with heat (thermo-PVD), and surcharge combined with heat and vacuum pressure (thermo-vacuum-PVD). Subsequently, the flow parameters in terms of the horizontal coefficient of consolidation (Ch) and the ratio between the horizontal hydraulic conductivity in the undisturbed zone (kh) to the horizontal hydraulic conductivity in the smear zone (ks), based on Hansbo's method. were investigated by back-calculation. The results show that the values of Ch were 1.93 m2/yr, 2.23 m2/yr, 4.17 m2/yr and 4.37 m2/yr for the PVD, vacuum-PVD, thermo-PVD and thermo-vacuum-PVD, respectively, with corresponding kh/ks values of 3.0, 2.7, 1.4 and 1.1, respectively. On the other hand, the increase in Ch of reconstituted specimens using the vacuum-PVD, thermo-PVD and thermo-vacuum-PVD were 15.54%, 116.06% and 126.42%, and the increases in kh/ks were 10.00%, 53.33% and 63.33%, respectively. Thus the vacuum-PVD can increase the horizontal coefficient of consolidation, Ch, resulting in a faster rate of settlement at the same magnitudes of settlement, compared with the PVD. The thermo-PVD and thermo-vacuum-PVD can further increase the coefficient of horizontal consolidation, Ch, with associated reduction of kh/ks values, by reducing the drainage retardation effects in the smear zone around the PVD, and consequently faster rates of consolidation were obtained. However, higher magnitudes of settlement were obtained upon application of heat owing to the associated collapse of the clay structure, similar to the results of previous investigations.

It is difficult to operate on the hydraulic-filled mucky soil foundation which the water content is high, so the vacuum preloading method with no sand cushion is used to deal with the foundation in some areas in China. Although there are still many problems in it’s application, such as, the effective range of the prefabricated vertical drains(PVDs) is small, the surrounding soil is easy to be accumulated and the soil strength increases little. For these problems, a laboratory model test on dredger fills made from the 4th layer of the Shanghai mucky soil by vacuum preloading with no sand cushion is performed. The changes and distributions in the settlement, water contents, permeability, plasticity index and grain composition of the dredger fill are studied in the process of the vacuum preloading. It is found that the water contents and permeability of the soft clay near the PVD (the diameter is about 40 to 50cm) are much lower than the outside, while the content of fine particles, plasticity index and soil strength near the PVD is much higher than the others. The measured data shows that the fine particles gather to a soil column at the center of the PVD under the action of vacuum negative pressure, then the permeability and the grain composition of the soil around the PVD is changed. As a result, soil parameters are not evenly distributed. Finally, some suggestions about how to reduce the non-uniformity are proposed.

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

Pilot tests on the usage of vacuum preloading combined with short prefabricated vertical drains (PVDs) to form a working platform for future soil improvement work were conducted at a land reclamation site in Tianjin, China. The short PVDs were connected using three methods – the conventional vacuum preloading method, fish-bone connectors and embedded horizontal pipes. To investigate the influence of PVD spacing, short PVDs were installed in square grids with spacings of 0·4 m and 0·6 m. The ground surface settlement, pore water pressures, water content and undrained shear strength of the soil were measured during the specific period of vacuum preloading. The pilot tests indicated that short PVDs connected using embedded vacuum pipes installed at 0·4 m spacing were the most suitable method to form a working platform on the surface of the dredged marine clay. This method yields a substantial saving on construction cost and time while exhibiting similar efficiency to the conventional vacuum preloading method. The undrained shear strength and degree of consolidation of the soil after 60 d of vacuum preloading were 23·6 kPa and 85·1%, respectively, which met the requirements for the working platform.

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

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.

To address land demands from coastal urbanization, dredged sediments are increasingly utilized as landfill materials, consolidated by vacuum preloading (VP) with prefabricated vertical drains (PVD). While VP is well-established for conventional soft soils, the performance differential between PVD types in ultra-soft hydraulic fills remains poorly quantified which is characterized by high water content and fine-grained sediments (d50=0.005mm). This study evaluates VP efficiency usingPVD-A and PVD-B in 3,000 m² reclaimed plots over 160 days. Comprehensive monitoring tracked pore water pressure, surface settlement(SS), layered settlement, water content, and vane shear strength(VSS). Results demonstrated PVD-A’s superior performance, with greater SS, higher VSS, larger PWP reduction, lower consolidation degree, and higher water content. This improved performance may be attributed to PVD-A’s lower longitudinal discharge capacity, which reduced fine particle migration and clogging during early consolidation stages, thereby sustaining drainage efficiency long-term.These findings of PVD-A’s advantage are valuable for optimizing VP design in reclamation projects.

— This study evaluates vacuum-assisted soft-clay improvement with Prefabricated Vertical Drains (PVD) and its implications for an in-service buried pipeline along a planned toll-road corridor. A descriptive–quantitative approach with PLAXIS 3D couples ground consolidation and soil–pipeline interaction: PVDs are modeled as line drains on a 1.5 m grid to 22 m depth beneath a permeable sand blanket; a 50 kPa vacuum is applied, and embankment loading is staged in four 1 m lifts (design surcharge 123.6 kPa, vacuum-reduced equivalent ≈ 73.6 kPa, fill height ≈ 4 m). Results show rapid dissipation of excess pore pressure and attainment of ~90% consolidation, with total settlement ≈ 3.10 m by the end of the final dwell (~30 days) and a low residual settlement rate. The pipeline’s peak actions are a bending moment 12.86 kN·m/m, shear 115.4 kN/m, and axial force 2,884 kN/m, producing a governing stress of 381 MPa, safely below the 435 MPa allowable (minimum FS ≈ 1.1). Differential settlement arises from non-uniform loading, with a maximum observed contrast of ~100 mm between loaded and lightly loaded segments.Â

A new technology of vacuum preloading with prefabricated radiant drain (PRD), including prefabricated vertical drain (PVD) and prefabricated horizontal drains (PHD), was proposed in this paper. The validity of numerical model parameter was verified by comparative analysis of deformation development of numerical simulation results and field tests. In addition, two influence factors of PHD spacing and PHD length for vacuum preloading with PRD technique were systematically studied by finite element method. The results show that vacuum preloading with PRD can obviously improve the reinforcement effect of dredged fill ground. Compared with the conventional vacuum preloading, the maximum settlement of vacuum preloading with PRD respectively is increased by 88.94 % and the maximum lateral displacement is 110.4 % at the PHD spacing of 0.5 m. Moreover, reducing the PHD spacing can improve the ground treatment effect. Compared with the PHD spacing of 1.5 m, the maximum settlement respectively is enlarged by 39.93 % and the maximum lateral displacement is increased by 49.4 % for the PHD spacing of 0.5 m. Increasing the PHD length can boost settlement and lateral displacement for the PHD length range of 0.2 m~0.4 m. The settlement and lateral displacement are 38.98 % and 50.3 % higher for the PHD length of 0.4 m than those for a PHD length of 0.2 m.

An Analytical Model of PVD-assisted Soft Ground Consolidation