Vacuum Preloading Techniques — Recent Developments and Applications

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

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.

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.

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.

This paper presents a case study of using the combined vacuum and fill surcharge preloading method to improve the foundation soil for a storage yard at Tianjin Port, China. A vacuum load of 80 kPa was applied for 4-8 weeks before fill surcharge was added in stages. The site to be improved was recently reclaimed using dredged clay slurry. Both the consolidated clay slurry fill and the seabed soils were very soft. After the combined vacuum and fill surcharge preloading, the ground settled more than 1 m. The average degree of consolidation achieved was more than 80%. The undrained shear strength of the soil increased twofold. The water content of the soil also decreased substantially. The soil improvement procedure using the combined vacuum and fill surcharge preloading method and the instrumentation scheme are described, the field monitoring data are presented and analyzed, and methods to estimate the degree of consolidation and several other issues related to the combined preloading method are discussed.Key words: case history, consolidation, ground improvement.

An Analytical Model of PVD-assisted Soft Ground Consolidation

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.

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: 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.

In the wide array of existing ground improvement schemes, the use of vertical drains with vacuum preloading is considered as the most effective and economical method for improving soft clays (normally consolidated to lightly over-consolidated) prior to construction of infrastructure. Vertical drains installed to significant depths promote radial flow inducing consolidation rapidly enhancing the shear strength of the compressed ground. In this paper, the analytical solutions based on lateral soil permeability (parabolic variation) are discussed considering the variation of vacuum pressure with depth along the prefabricated vertical drains (PVD). Using the Cavity Expansion Theory (CET), the smear zone caused by the installation of PVD by steal mandrel was predicted and compared with laboratory measurements obtained from large-scale radial consolidation tests. The effects of drain unsaturation and vacuum pressure along the drain length are also discussed. The numerical analyses incorporating equivalent plane strain solutions were performed to predict the soil responses based on two selected case histories in Thailand. The research findings provided insight as to which of the above aspects needed to be simulated accurately in numerical modelling. The application of cyclic loading on PVD stabilized ground was also examined using a finite element approach under railway embankment. It is demonstrated that short drains less than 8 m installed beneath tracks are still useful for effective dissipation of cyclic pore pressures and curtailing unacceptable lateral movement immediately below the track level, at the same time avoiding excessive settlement of the track in the short-term.

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.

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.

The quality of pre-investigation and other reasons would lead to the disagreement between the depth of prefabricated vertical drain(PVD) and actual soft ground foundation. All these factors would have a seriously bad effect on the subsidence and the stability of soft ground foundation. The author combines the Suining-Ziyang-Meishan expressway project, detailedly analyses the correlation between different depths of PVD and actual soft ground foundation based on observation settlement data. The results shows: Before the limit fill height(LFH), with load increases, the depth of PVD has little effect on subsidence rate, the curve of PVD depth-subsidence rate is approximately linear; when backfilling height exceeds the LFH, the closer depth of PVD is to actual depth, the more obvious soft foundation reinforcement effect is, the larger rate and value of subsidence are. However, when PVD depth is 1m deeper than the actual, subsidence rate decreases. In addition, according to the difference of observation total settlement and theoretical calculation settlement due to the difference between PVD depth and actual, adjust preloading period to meet the requirements of pavement construction and post-construction settlement.