Vertical drains, vacuum consolidation & preloading
Buddhima Indraratna2012
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Abstract
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This report reviews several contributions to a technical session focused on ground improvement techniques for soft clay consolidation, particularly through the use of vertical drains, vacuum consolidation, and preloading. Various studies summarized methodologies, case histories, and results from field tests, demonstrating advances in understanding soil behavior, consolidation processes, and design optimizations for infrastructure projects in challenging soil conditions.
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Predictions and Observations of Soft Clay Foundations Stabilized with Geosynthetic Drains and Vacuum Surcharge
This chapter starts with an introduction of a revised analytical model of radial drainage with vacuum preloading in both axisymmetric and plane strain conditions. Observed from large-scale radial drainage consolidation tests, the influence of vacuum pressure distribu- tion along the drain length is examined through the dissipation of average excess pore pressure and associated settlement. The details of an appropriate conversion procedure by transforming permeability and vacuum pressure between axisymmetric and equivalent plane strain conditions are described through analytical and numerical schemes. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are investigated on the basis of average excess pore pressure, consolidation settlement, and time analyses. The writers describe a multi-drain plane strain finite element method analy- sis based on permeability conversion, which is employed to study the behavior of embank- ments stabilized at the site of the Second Bangkok International Airport with vacuum-assisted prefabricated vertical drains. In the field, a constant suction head is not always stable because of the occurrence of air leaks; therefore the magnitude of applied vacuum pressure was adjusted accordingly. The theoretical (numerical) predictions are compared with measured field data such as settlements, excess pore pressures, and lateral movements. The case history analysis employing the writers’ model indicates improved accuracy of the predictions in relation to the field observations. The data indicate that the efficiency of the prefabricated vertical drains depends on the magnitude and distribution of vacuum pressure as well as on the extent of air leak protection provided in practice. In this lecture the interpretations of fully instrumented tests embankments and their role in the development of appropriate ground improvement techniques for highways, motorways and airfields on soft clay deposits is illustrated through well documented case studies in Bangkok, Thailand and Muar Flat Site in Kuala Lumpur. For the Bangkok Plain and with sand backfills the perform- ance of embankments with different schemes of vertical drains was evaluated over a period of 25 years. Aspects such as recharging ef- fects due to the drains, inadequate measures in maintaining vacuum during vacuum applications and possible hydraulic connections with large diameter drains are discussed. For the Muar test embankments, the role of fill strength in residual soil embankment and the field deformation analysis in separating consolidation settlement from immediate settlement and creep settlements is presented. Novel interpretations of settlement from pore pressure dissipations, secondary settlement from field measurements and decay of lateral de- formation rate with time were also made.
Soft ground improvement via vertical drains and vacuum assisted preloading
Application of vacuum assisted preloading is an imperative method when a considerable load is required to meet the desired rate of settlement and an increase in the undrained shear strength upon consolidation. Moreover, where lateral displacements at the edge of a coastal embankment need to be controlled, application of vacuum pressure with a cut off offers the optimum solution. To facilitate vacuum propagation, vertical drains are usually employed in conjunction. The installation of vertical drains using a steel mandrel creates significant remoulding of the subsoil surrounding the drains thereby, reducing soil permeability and adversely affecting the soil consolidation process. In this paper, the simulation of vacuum assisted consolidation using the spectral method and finite element analysis is carried out. Subsequently, the 2D and 3D numerical multi-drain analyses are conducted to predict the excess pore pressures, lateral and vertical displacements. The performance of two selected case histories at the sites of Suvarnabhumi Airport, Thailand and Tianjin Port, China are discussed and analysed. The numerical predictions are then compared with the available field data. Finally, a procedure for the design of vertical drains is presented with a worked-out example.
Predictions and Performances of Prefabricated Vertical Drain Stabilised Soft Clay Foundations
In this paper, the analytical solution for radial consolidation of soft soils is proposed considering the impacts of the variation of volume compressibility and permeability. The Cavity Expansion Theory is employed to predict the smear zone caused by the installation of mandrel driven vertical drains in soft clay. The smear zone prediction is then compared to the data obtained from the large-scale radial consolidation tests. Furthermore, the advantages and limitations of vacuum application through vertical drains in the absence of high surcharge embankments are discussed using the proposed solutions. The applied vacuum pressure generates negative pore water pressure, resulting in an increase in the effective stress, which leads to accelerated consolidation. Analytical and Numerical analysis incorporating the equivalent plane strain solution are conducted to predict the excess pore pressures, lateral and vertical displacements. The equivalent plane strain solution can be used as a predictive tool with acceptable accuracy due to the significant progress that has been made in the past few years through rigorous mathematical modelling and numerical analysis developed by the primary author and co-workers (Indraratna et al., 1992-2005). Several case histories are discussed and analysed, including the site of the 2nd Bangkok International Airport. The predictions are compared with the available field data, confirming that the equivalent plane strain model can be used confidently to predict the performance with acceptable accuracy. Difficulties in assuring good performance can also be analysed and interpreted through mathematical modelling, thereby enabling due caution in the design and construction stages. The research findings verify that the role of smear, drain unsaturation, and vacuum distribution can significantly affect soil consolidation, hence, these aspects need to be modelled appropriately in any numerical analysis to obtain reliable predictions.
A Review of the Use of the Pre-loading Technique and Vertical Drains for Soil Consolidation
Previously, because of availability of more sites, people used to choose the best one having geotechnical properties of soil favourable for construction of a project. Accordingly, people would choose the site requiring no ground improvement technique for construction of a project. In view of rapid urbanisation /industrialisation, people are getting limited options/alternatives to choose sites for new projects. In fact, many a time, it happens that people have no choice but to accept the only available site for construction, no matter how poor the soil properties it has. These types of sites with unfavourable geotechnical properties for projects need improvement of soil properties by any suitable and economically viable method. Preloading is a technique by which consolidation of soil can be achieved to a substantial amount before imposition of actual construction load. However, the preloading technique alone may not be found to be satisfactory in reduction of consolidation time to the desired extent. Installation of vertical drains, particularly prefabricated vertical drains, followed by preloading is one of the most acceptable and economical technique to consolidate poor soils within a limited period of desire, and are being used widely in our country now.
Recent advancements in the use of prefabricated vertical drains in soft soils
Faculty of Engineering-Papers, 2008
A system of prefabricated vertical drains with surcharge load to accelerate consolidation by shortening the drainage path is one of the most popular methods of soft ground improvement. An analytical solution is proposed based on radial (lateral) soil permeability while considering variations in vacuum pressure. The predicted smear zone and effects of drain unsaturation are compared with laboratory data obtained from large-scale radial consolidation tests. When a higher load is required to meet the desired rate of settlement and the cost of raising a surcharge embankment is also significant, the application of vacuum pressure with a reduced surcharge load can be used. In this method, the vacuum creates a suction head that increases the effective stress. Analytical and numerical analyses were conducted for several case histories using the equivalent plane strain solution for Darcian and non-Darcian flows. The effectiveness of vertical drains on cyclic loading was also investigated based on a laboratory study.. This paper shows that vertical drains can dissipate the built up excess pore pressure under repeated loading, and that short drains can be sufficient in certain cases rather than driving the drains to cover the entire depth of soft clay deposits. The research findings verify that the effects of soil disturbance and vacuum pressure can affect soil consolidation considerably, which means that these aspects need to be modelled correctly in any numerical approaches.
Ground Improvement Using Vacuum Loading Together with Vertical Drains
The use of vacuum as a preload together with vertical drains to reduce postconstruction settlement and increase the shear strength of soft ground is increasingly popular. However, conflicting views concerning preloading by vacuum consolidation continue to be disseminated. A review and interpretation of case histories of vacuum loading together with vertical drains indicates that (1) with a novel definition of excess pore-water pressure, existing theories of consolidation, solutions, and associated computer programs that have been developed for fill loading can be applied without any modification to vacuum loading; (2) vacuum that is available in the drainage blanket remains constant with depth within the vertical drains; (3) for vacuum loading as for fill loading, vertical drains may display well resistance; (4) there is no difference in magnitude and rate of settlement for a vacuum load and an equivalent fill load; (5) all empirical concepts of undrained shear strength that have originated from fill loading of soft ground are equally applicable for vacuum loading; (6) a correlation between vertical settlement and horizontal displacement for vacuum loading is expected because both result from consolidation; and (7) preloading by vacuum is accomplished in a shorter period because there is no possibility of undrained failure during vacuum loading, whereas fill loading may require construction in stages to avoid undrained bearing-capacity failure.
Application of Constant Rate of Strain Tests in Consolidation Analysis of the Band-Shaped Prefabricated Vertical Drains for Vietnam Clays: The Validation Analysis
Geotechnical and Geological Engineering, 2020
Band-shaped prefabricated vertical drains in the combination of surcharge preloading have been widely applied in the soft ground improvement cases in Vietnam to eliminate the post-construction settlement of the ground. However, the method of the settlement prediction has remained incorrect on the comparison between the field and calculation. A new procedure for consolidation analysis is proposed to determine the ground settlement more accurately in the routine design of the soft ground improvement. Two case studies are presented to study the ground performance under consolidation with surcharge preloading. The consolidation analysis of the soft ground deposits with the application of the constant rate of strain (CRS) consolidation tests is performed. In addition to that, the input parameters as the coefficient of consolidation and compression indices of the soft ground are characterized based on the CRS tests under the strain rate of 0.02%/min. on undisturbed soil samples that are taken at the two sites by stationary piston sampler. On the comparison of calculated and field monitored data of surface settlement, layered settlement, dissipation of the excess pore water pressure, and undrained strength gain, this analysis is also to confirm the appropriate strain rate for CRS tests, which are utilized in the determination of consolidation parameters.
Observations from Ground Improvement Using Vacuum Consolidation Method
Civil Engineering and Architecture
In the modern world of Civil Engineering, challenges arise during the various phases of construction, starting from the project development phase to the completion of the project. One of the main factors to be considered before commencing any infrastructure project is the foundation of where it is to be built. Without a strong foundation, it's not possible to proceed further to subsequent stages and in some cases, significant improvements may be required before starting the construction process. Although there are several different available methods for soil improvement in Civil Engineering, preloading using vacuum pressure with prefabricated vertical drains (Vacuum Consolidation Method) is one of the commonly applied techniques. Nowadays, it is widely used in countries having soft soil settlement problems. This paper presents the observations made from a soil improvement project using VCM including the site conditions and methodologies adopted during the process. The field data related to parameters such as pore pressure, settlement and shear strength improvement in natural soft clay have been presented and discussed. The degree of consolidation in the field has been back-calculated from settlement data and compared with values obtained from the 1-D consolidation equation. Field investigation tests, such as borehole tests and field vane shear tests, were performed before and after the soil improvement and it was found that the soft soil properties can be enhanced using vacuum consolidation without the use of any surcharge loading as well.
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SOFT CLAY FOUNDATION IMPROVEMENT WITH DRAINAGE AND GEO-INCLUSIONS, WITH SPECIAL REFERENCE TO THE PERFORMANCE OF EMBANKMENTS AND TRANSPORTATION SYSTEMS
In this paper, the geotechnical aspects of soft clay improvement using prefabricated vertical drains (PVDs) with special reference to embankments will be demonstrated. The Cavity Expansion Theory is employed to predict the smear zone caused by the installation of mandrel driven vertical drains. Analytical and Numerical analyses adopting the equivalent plane strain solution are conducted to predict the excess pore pressures, lateral and vertical displacements. The advantages and limitations of vacuum application through vertical drains avoiding the need for high surcharge embankments are discussed using the proposed solutions. A few selected case histories are discussed and analyzed, including the site of the 2nd Bangkok International Airport, the coastal stretch of Muar Clay Plains in Malaysia and the Sunshine embankment, Australia. The predictions are compared with the available field data, verifying that the equivalent plane strain model can be used confidently with acceptable accuracy. Cyclic loading of PVDs is also examined in the laboratory in a manner appropriate for railway environments. It is shown that short PVDs can dissipate excess pore pressure as fast as they are built up under repeated loading conditions. Some selected on-ground experience of the first Author through the Ministry of Science and National Science Foundation during post-tsunami reconstruction efforts is described with specific reference to appropriate ground improvement requirements.
Analytical and numerical modelling of soft clay foundation improvement via prefabricated vertical drains and vacuum preloading
2005
I, Cholachat Rujikiatkamjorn, declare that this thesis, submitted in fulfillment of the requirements for the award of Doctor of Philosophy, in the Department of Civil Engineering, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution.
Soft Clay Stabilization with Geosynthetic Vertical Drains beneath Road and Railway Embankments: A Critical Review of Analytical Solutions and Numerical Analysis
Advances in Measurement and Modeling of Soil Behavior, 2007
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.
Analytical and Numerical Modeling of Soft Soil Stabilized by Prefabricated Vertical Drains Incorporating Vacuum Preloading
This paper describes the analytical formulation of a modified consolidation theory incorporating vacuum pressure, and numerical modeling of soft clay stabilized by prefabricated vertical drains, with a linearly distributed ͑trapezoidal͒ vacuum pressure for both axisymmetric and plane strain conditions. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average time-dependent excess pore pressure and consolidation settlement analyses. The plane strain analysis was executed by transforming the actual vertical drains into a system of equivalent parallel drain walls by adjusting the coefficient of permeability of the soil and the applied vacuum pressure. The converted parameters are incorporated in the finite element code ABAQUS, employing the modified Cam-clay theory. Numerical analysis is conducted to study the performance of a full-scale test embankment constructed on soft Bangkok clay. The performance of this selected embankment is predicted on the basis of four different vacuum pressure distributions. The predictions are compared with the available field data. The assumption of distributing the vacuum pressure as a constant over the soil surface and varying it linearly along the drains seems justified in relation to the field data.
A New Approach to Analyse the Consolidation of Soft Soils Improved by Vertical Drains and Submitted to Progressive Loading
KSCE Journal of Civil Engineering, 2020
Prefabricated vertical drains combined with preloading and staged construction is a frequently used technique to accelerate consolidation in soft soils. Nevertheless, an installed vertical drain discharge capacity rapidly reduces over consolidation. Most reasons are the lateral confining pressure, the drain deformation and filter clogging. This greatly affects the rate of consolidation by increasing well resistance. This article presents an exact solution under equalstrain consolidation case of progressive loading. Total vertical stress increase caused by loading is principally modelled as a linear curve depending on time and depth. The discharge capacity of the drain is supposed to lower linearly with depth and inversely with time. The smear effect is also considered by assuming constant permeabilities of the soil in both smear zone and undisturbed zone. Excess pore-water pressure at any arbitrary point in soil is assessed, and the global average degree of consolidation is found. Comparisons are made with an available solution from the literature. The suggested solution is then applied to three pre-loading embankments of the Moroccan high-speed railway project. A good agreement is found between the solution and measured data from field monitoring. Soft soil Consolidation Discharge capacity Vertical drain Settlement CORRESPONDENCE Fouzia Kassou
Vacuum Preloading Versus Conventional Embankment Preloading for Accelerating Consolidation Process: A Comparison Study from Analysis of Full Scale Test
This paper discusses the comparison of the effectiveness of using vacuum preloading to embankment preloading in accelerating consolidation process with prefabricated vertical drain (PVD). Sources for analysis were obtained from a full scale vacuum consolidation test data of Full Scale Test Geostructure Vacuum System Project in Pantai Indah Kapuk, Jakarta, conducted by PT. Geostructure Dynamics. The paper covers assessment of degree of consolidation and prediction of gain strength of soil due to dissipation of excess pore water pressure under vacuum preloading. The degree of consolidation and gain strength are calculated based on settlement data and pore water pressure data. The result from the analysis of consolidation with PVD-vacuum next is compared with the result of consolidation with PVD-embankment using PLAXIS 2D. In PLAXIS 2D, PVD is modeled in respect to geometric and permeability matching. Post test soil investigation data is also used to verify the analysis result. Comparison result for this location shows that with the same equivalent load and same spacing of PVD, vacuum method can reduce consolidation time up to 43% or accelerate consolidation process up to 1.7 times faster than conventional preloading method. Keywords: Consolidation, Prefabricated Vertical Drain (PVD), Vacuum Preloading
Analytical model for vacuum consolidation incorporating soil disturbance caused by mandrel-driven drains
Canadian Geotechnical Journal, 2017
When vacuum preloading is applied with vertical drains, the rate of consolidation can be increased, and the stability of an embankment is enhanced due to the inward lateral movement. The aim of this study is to develop an analytical solution for vacuum preloading that accurately captures the more realistic variations in compressibility and permeability in actual ground conditions as a result of drain installation. The soil samples were obtained from various locations after drain installation to determine the characteristics of soil surrounding the vertical drain in terms of compressibility and permeability. The main differences between the proposed and conventional models are described by considering the stress history and preloading pressure. The effect of pre-consolidation pressure and the magnitude of applied preloading are examined through the dissipation of average excess pore pressure and associated settlement. The analysis of a selected case history employing the writers’ sol...
Verification of the preloading design of soft clay with prefabricated vertical drains by full-scale field instrumentation and numerical modelling
International Journal of Geotechnical Engineering, 2018
Preloading with surcharge and prefabricated vertical drains (PVDs) was used to construct road and rail embankments on a 26-m-thick soft clay layer across Boubyan Island, Kuwait. Given the variations in the horizontal and vertical coefficients of consolidation (Ch and Cv, respectively) measured in the laboratory, the preliminary analysis predicted that the time to reach 90% consolidation may vary from three months to one year. To validate these predictions, a full-scale pilot project that included an instrumented section under a surcharge load with PVDs was carried out. Back analysis of the consolidation parameters was carried out by an observational method and numerically by finite element modelling. The results indicated that 98% consolidation was achieved in 367 days. The numerical model of the heave due to surcharge removal and settlement due to reloading predicted 70 mm of heave and minimum primary and secondary consolidation for a service period of 20 years.
Improvement of Soft Clay by Prefabricated Vertical Drains
One of the most effective methods to improve soft clay is by the use of preloading with or without vertical drains. From the first cardboard wick drain used by to the modern day band drains made of synthetic geocomposites, the evolution process has seen a sea change over the years. This paper reports the method adopted to improve thick deposits of marine clay at the site of a steel plant using commercially available geodrains without resorting to conventional preloading. Feasibility studies have indicated that the method adopted is best suited for the existing site conditions and is also the most cost effective. The calculated degrees of consolidation and consequent enhancement of shear strength indicate sufficient improvement to render the sub-soil suitable to support superimposed loading.
Analytical and numerical solutions for soft clay consolidation using geosynthetic vertical drains with special reference to embankments
2005
Good quality geologic materials for construction are also becoming scarce. Due to these reasons and because of the environmental restrictions on certain public works, ground improvement is becoming an essential part of infrastructure development. As a result, Civil Engineers are forced to utilise even the poorest soft clay foundations for buildings, highways and railway tracks. Therefore, the application of prefabricated vertical drains with preloading has now become common practice and one of the most effective ground improvement techniques. The classical solution for vertical drains (single drain analysis) has been well documented in the literature, where there are many vertical drains, a true 3-D analysis of the site becomes very difficult. Therefore, equivalent 2-D plane strain models have been employed, using the methods of geometric and permeability matching concepts. The equivalent plane strain solution can now be used as a predictive tool with acceptable accuracy as a result of the significant process that has been made in the past few years through rigorous mathematical modelling and numerical analyses.
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