Papers by Cholachat Rujikiatkamjorn
Ground Modification and Seismic Mitigation, 2006
A system of prefabricated vertical drains (PVDs) with a surcharge load is an effective method for... more A system of prefabricated vertical drains (PVDs) with a surcharge load is an effective method for accelerating soil consolidation by promoting radial flow. This study presents a three-dimensional (3D) finite element model for soft soil improved by a single drain. In the finite element analysis, the actual rectangular PVD cross-section is considered, and the equivalent drain diameter is then back calculated based on the consolidation curves. The numerical predictions are then compared to previous studies. The settlements observed in the laboratory are subsequently compared with the three-dimensional finite element analysis incorporating the modified Cam-Clay model and Hansbo's theory.
Journal of Geotechnical and Geoenvironmental Engineering, 2019
The paper is an analytical solution for radial consolidation that captures isotaches with a strai... more The paper is an analytical solution for radial consolidation that captures isotaches with a strain rate dependency of pre-consolidation pressure. These relationships are obtained based on Constant Rate of Strain (CRS) and long-term consolidation (LTC) tests, and then used in the radial consolidation model incorporating the field strain rate which is generally much lower compared to the typical laboratory environment. In this study the calculated settlement and associated excess pore water pressure are obtained using the equivalent pre-consolidation pressure from the reference isotache, within the (′ 0 ′)-(̇) domain. Moreover, the change in ratio (i.e. secondary compression index / compression index) with decreasing strain rate is used to calculate the long term settlement. This method is then validated using various case histories in Australia and Southeast Asia, where excess pore water pressure is dissipated at a slower rate in relation to the observed settlement.
In this study the characteristics and extent of the smear zone were investigated using a undistur... more In this study the characteristics and extent of the smear zone were investigated using a undisturbed sample. The aim was to capture the realistic characteristics of the smear zone in relation to the in-situ structure of soil during the installation of prefabricated vertical drain (PVD) using a steel mandrel. The extent of the smear zone for Bulli clay was determined on the basis of normalised permeability (kh/khu) and a reduction in the water content upon consolidation. The permeability and compressibility of the soil were investigated to determine the extent to which the soil surrounding the PVD had become disturbed.
I, Cholachat Rujikiatkamjorn, declare that this thesis, submitted in fulfillment of the requireme... more 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.
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 ... more 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.
The behaviour of saturated soft clays subjected to cyclic loading is of considerable importance i... more The behaviour of saturated soft clays subjected to cyclic loading is of considerable importance in the design of railway subgrades. Soft clays can be extensively found in many coastal regions of Australia up to significant depths, including the coastal belt and the central part of NSW. These soft clay deposits are characterised by very low bearing capacity and excessive settlement. The increase in generated excess pore pressures due to heavy freight trains significantly reduces the bearing capacity and causes serious damage to the rail infrastructure such as clay pumping underneath railway tracks and excessive subsidence. The use of prefabricated vertical drains (PVDs) is one of the popular methods for soft ground improvement. In this paper, the behaviour of soft clay subjected to cyclic loads is investigated using large-scale triaxial tests. Cyclic triaxial tests on remoulded soft clay samples with and without vertical drains have been carried out using a large-scale triaxial apparatus designed and built at the University of Wollongong. It was found that the PVD under cyclic loading allows the generated cyclic excess pore pressures to dissipate during and after the cyclic load application. The extent of smear zone was approximately three times the equivalent diameter of the mandrel used during installation and is comparable to the generally accepted relationship between mandrel and smear produced under static loading. The case history at Sandgate Rail Grade Separation Project is also presented with a Class A numerical prediction. Design methodology and finite element analysis are discussed through associated settlement and lateral displacement.
It has been widely known that load-settlement behaviors of cast-in placed bored piles are determi... more It has been widely known that load-settlement behaviors of cast-in placed bored piles are determined not only by the soil properties alone but also by the construction methods. This paper discusses the performances of (6) instrumented test piles of various diameters (80; 100 and 120 cm) and constructed using different techniques i.e. mud-water, polymer slurry and full-casing under relatively uniform subsurface condition that consisting of very stiff silty clay in West Jakarta. Each test pile was instrumented by vibrating wire strain gauge (VWSGs) and loaded to near ultimate load (2.5 times design load) with objective to obtain the loadtransfer characteristics of the test pile. Variation in pile diameters and construction methods that may affect the concrete modulus, load-settlement and load transfer behaviors are observed and discussed.
A system of vertical drains combined with vacuum preloading to accelerate soil consolidation by p... more A system of vertical drains combined with vacuum preloading to accelerate soil consolidation by promoting radial flow is an effective method. In this paper, the analytical modelling of vertical drains incorporating vacuum preloading in an axisymmetric condition is proposed. The distribution of vacuum pressure along the drain and variations in volume compressibility, and permeability of soil based on large scale laboratory observations are included in the proposed solutions. These analytical predictions are compared with a case history taken from Thailand where the entire embankment was stabilised with vertical drains and subjected to vacuum preloading. This analysis employing the writer's model indicates an improvement in the predictions in relation to the field observations. The data show that the efficiency of the prefabricated vertical drains depends on the magnitude and distribution of vacuum pressure, and the extent to which air is prevented from leaking.
In this paper, the analytical solution for radial consolidation of soft soils is proposed conside... more 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.
In this study, the large-scale apparatus used to examine vacuum preloading in conjunction with co... more In this study, the large-scale apparatus used to examine vacuum preloading in conjunction with conventional surcharge loading. Several tests were performed to examine the effect of a vacuum and determine parameters such as the extent of smear zone and the soil permeability characteristics. The settlement and excess pore pressure associated with a combined vacuum and surcharge load indicates that applying a vacuum has specific benefits. The analytical modeling of one-dimensional consolidation by vertical drains with vacuum preloading considering both variation of soil compressibility and permeability is used to predict the soil consolidation behavior. It is shown that the analytical model can reasonably predict the laboratory behavior. 1.
This study presents an analytical model of radial drainage with vacuum preloading using compressi... more This study presents an analytical model of radial drainage with vacuum preloading using compressibility indices and varying horizontal permeability and considers the influence of linearly distributed vacuum pressure along the drain. An analytical solution based on the Cylindrical Cavity Expansion theory is used to evaluate the extent of the smear zone along the drain length and predict the performance of an embankment stabilized with vacuum-assisted prefabricated vertical drains at the New Bangkok International Airport (NBIA). Here, a significantly reduced height of sand surcharge could be applied because excess pore pressure was reduced by vacuum preloading. The performance of the embankment was evaluated under different vacuum pressure conditions during which the suction head could not be maintained in the field due to probable air leaks. In the analysis, therefore, the magnitude of applied vacuum pressure was based on field measurements and adjusted accordingly. The settlement predictions based on the writers' solutions were compared and agreed with the available field data. The results also indicate that the efficiency of the vertical drains depends on the extent of the smear zone, the magnitude and distribution of vacuum pressure and the extent of air leak protection provided in the field. RÉSUMÉ : Cette étude présente un modèle analytique de système de drainage radial par préchargement par le vide utilisant des indices de compressibilité et une perméabilité horizontale variable et examine l'influence de la distribution linéaire de la pression de vide le long du drain. Une solution analytique basée sur la théorie d'Expansion de Cavité Cylindrique est utilisée pour évaluer l'étendue de la zone de souillure sur toute la longueur du drain et prédit la performance d'un remblai stabilisé à l'aide de drains verticaux préfabriqués sous vide sur le site du Nouvel Aéroport International de Bangkok (NBIA). Ici, on pourrait utiliser une hauteur de surcharge de sable considérablement réduite du fait que le préchargement par le vide a amoindri l'excédent de pression interstitielle. La performance du remblai a été évaluée sous différentes conditions de pression de vide pendant lesquelles la hauteur d'aspiration ne pouvait être maintenue sur le terrain à cause de probables fuites d'air. Dans l'analyse, le niveau de pression de vide appliquée était donc basé sur des mesures de terrain et était ajusté en conséquence. Les prédictions de tassement basées sur les solutions données par les auteurs ont été comparées aux données de terrain à notre disposition et ont été confirmées. Les résultats indiquent aussi que l'efficacité des drains dépend de l'étendue de la zone de souillure, de l'ampleur et de la distribution de la pression de vide, ainsi que de l'importance de la protection contre les fuites d'air disponible sur le site.
Geotechnical Engineering
In this paper the analytical solutions for radial consolidation that include time dependent surch... more In this paper the analytical solutions for radial consolidation that include time dependent surcharge loading and vacuum pressure are proposed, whilst also considering the impact of the parabolic variation of permeability in the smear zone. The use of the spectral method for multilayered soil consolidation is introduced and verified. The Elliptical Cavity Expansion Theory is used to predict the extent of soil disturbance (smear zone) caused by the installation of mandrel driven vertical drains. The predicted smear zone is then compared to the data obtained from large-scale radial consolidation tests. Furthermore, the advantages and limitations of applying a vacuum through vertical drains are discussed using the proposed solutions. The vacuum pressure applied generates a negative pore water pressure that increases the effective stress within the soil, which leads to an accelerated consolidation. Vacuum pressure is modelled as a distributed negative pressure (suction) along the length...
Proceedings of the Institution of Civil Engineers - Geotechnical Engineering, 2015
The installation of drains creates a disturbed region known as a smear zone where the change in t... more The installation of drains creates a disturbed region known as a smear zone where the change in the clay structure affects the horizontal permeability and compressibility. The parameters required to characterise the smear effect are the extent of the smear zone and the ratio of the horizontal coefficient of permeability in the undisturbed zone and in the smear zone. Only limited studies have been carried out on different aspects of soil disturbance due to driving vertical drains and its effects on the subsequent consolidation. In this paper the disturbed zone around a rectangular mandrel was characterised using soil samples obtained from the soft clay layer at various locations beneath an embankment built at Ballina, Australia, where vertical drains were installed. By determining the change in the coefficient of permeability, the water content and volume compressibility across the smear zone, the effects of soil disturbance on consolidation due to the installation of drains can be q...
GeoCongress 2006, 2006
In this study, the analytical modeling of one-dimensional consolidation by vertical drains with v... more In this study, the analytical modeling of one-dimensional consolidation by vertical drains with vacuum preloading considering both vertical and horizontal drainage is presented. In this method, the total degree of consolidation based on excess pore water pressure dissipation is related to the time factor (T h), drain configuration and anisotropic soil permeability. The analytical predictions are compared with the observed data from consolidation testing. Three different test series were conducted in a large-scale consolidation apparatus designed and installed at the University of Wollongong. It is shown that the authors' analytical model can accurately predict the laboratory behavior. Finally, the analysis of a selected case history employing the authors' model demonstrates the applicability of this approach for typical field conditions.
Proceedings of the Institution of Civil Engineers - Ground Improvement, 2014
A review of the recent developments in soft soil improvement through consolidation or preloading ... more A review of the recent developments in soft soil improvement through consolidation or preloading is presented in this paper. The topics covered range from fundamental analysis to methods of implementation. Various methods and processes related to vertical drains, vacuum preloading or combined vacuum and fill surcharge, and dynamic consolidation with enhanced drainage or vacuum are compared and discussed. Factors affecting the design and analyses for the methods discussed are also elaborated.
International Journal of Geomechanics, 2008
This paper presents a three-dimensional (3D) and two-dimensional (2D) numerical analysis of a cas... more This paper presents a three-dimensional (3D) and two-dimensional (2D) numerical analysis of a case study of a combined vacuum and surcharge preloading project for a storage yard at Tianjin Port, China. At this site, a vacuum pressure of 80 kPa and a fill surcharge of 50 kPa was applied on top of the 20m thick soft soil layer through prefabricated vertical drains (PVD) to achieve the desired settlements and to avoid embankment instability. In 3D analysis, the actual shape of PVDs and their installation pattern with the in-situ soil parameters were simulated. In contrast, the validity of 2Dplane strain analysis using equivalent permeability and transformed unit cell geometry was examined. In both cases, the vacuum pressure along the drain length was assumed to be constant as substantiated by the field observations. The finite element code, ABAQUS, using the modified Cam-clay model was used in the numerical analysis. The predictions of settlement, pore water pressure and lateral displacement were compared with the available field data, and an acceptable agreement was achieved for both 2D and 3D numerical analyses. It is found that both 3D and equivalent 2D analyses give similar consolidation responses at the vertical cross section where the lateral strain along the longitudinal axis is zero. The influence of vacuum may extend more than 10m from the embankment toe, where the lateral movement should be monitored carefully during the consolidation period to avoid any damage to adjacent structures.
International Journal of Geomechanics, 2008
In this study, an attempt is made to analyse the extent of the smear zone caused by mandrel drive... more In this study, an attempt is made to analyse the extent of the smear zone caused by mandrel driven vertical drains, employing the cavity expansion theory for soft clay obeying the modified Cam-clay model. The predictions are verified by large-scale laboratory tests, where the extent of the smear zone was estimated based on the indications such as the pore pressure generated during mandrel driving, change in lateral permeability and the water content reduction. This study reveals that the radius of smear zone is about 4-6 times the equivalent vertical drain radius, and the lateral permeability (inside the smear zone) is 61-92% of that of the outer undisturbed zone. Finally, the predicted size of the smear zone using the undrained cavity expansion solution is incorporated in the finite element code PLAXIS to study the performance of a test embankment selected from the Sunshine Motorway, Queensland, Australia. A good agreement between the predicted values and field measurements was found.
Géotechnique, 2013
A modified 150 mm Rowe cell equipped with pore water pressure measurement was used to capture the... more A modified 150 mm Rowe cell equipped with pore water pressure measurement was used to capture the flow relationship during vacuum-assisted radial consolidation. Based on the measured data, a radial consolidation model incorporating the effects of vacuum preloading is proposed, based on a non-linear relationship between the flow velocity and hydraulic gradient. The predictions of the proposed consolidation model are then compared with the predictions based on Hansbo's Darcian and non-Darcian models. The agreement between the proposed model and the measured data is shown, and the advantages of the proposed model compared with the existing models are discussed. An embankment case history taken from the reclamation project at the Port of Brisbane, Australia, was analysed, based on the current solution, and compared with the field measurements.
Géotechnique, 2012
Vertical drains increase the rate of consolidation in soft soils by facilitating faster dissipati... more Vertical drains increase the rate of consolidation in soft soils by facilitating faster dissipation of excess pore water pressure through short, horizontal drainage paths. This paper presents an analytical solution for non-linear radial consolidation under equal-strain conditions incorporating smear but ignoring well resistance. Three aspects of non-linearity are considered: (a) non-Darcian flow, (b) a log-linear void-ratio–stress relationship; and (b) a log-linear void-ratio–permeability relationship. The analytical solution to non-linear radial consolidation can explicitly capture the behaviour of both overconsolidated and normally consolidated soils. For non-linear material properties, consolidation may be faster or slower when compared with the cases with constant material properties. The difference depends on the compressibility/permeability ratios (Cc/Ck and Cr/Ck), the preconsolidation pressure and the stress increase. If Cc/Ck < 1 or Cr/Ck < 1 then the coefficient of c...
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Papers by Cholachat Rujikiatkamjorn