Papers by Cong-Oanh Nguyen
Proceedings of the Institution of Civil Engineers - Geotechnical Engineering, 2022
Finite-difference analysis of soft ground treatment with prefabricated vertical drains (PVDs) was... more Finite-difference analysis of soft ground treatment with prefabricated vertical drains (PVDs) was performed using a newly developed consolidation model. The model uses the concept of multi-compression indices and coefficients of consolidation (CoCs) to investigate the behaviour of a soft ground deposit (pore water pressure dissipation, surface settlement and sub-layer ground settlement) and to validate the newly developed Consopro finite-difference procedure. The pre-consolidation pressures of the soft ground deposit were characterised using a combination of piezocone penetration profiling and constant rate-of-strain (CRS) consolidation tests (0.02%/min) on undisturbed samples that were retrieved from the Saigon premier container terminal in the south of Vietnam. After comparison of the back-calculated results with field-observed data, a correlation between the CoCs determined by CRS consolidation tests and the CoCs from piezocone dissipation tests, which were carried out after soil...
Lecture Notes in Civil Engineering, 2019
Recent development of infrastructures in urban area, especially Ho Chi Minh City, has generated m... more Recent development of infrastructures in urban area, especially Ho Chi Minh City, has generated massive construction in waste water treatment and connecting pipelines. Construction of these treatment facilities in soft ground condition, therefore, is such a challenge for practical Engineer who carries design task. The paper presents a case study of difficulty in driving Pre-stressed Concrete (PC) piles with the assistance of pre-boring method by continuous flight auger. A series of static load test with a range from 200% to 300% of working load were performed on D450mm PC piles. This was to investigate the bearing capacity of the piles which were not possibly driven to the initial design level due to the variation of the top of dense sandy soil layer. Adoption of Chin’s method was done accordingly to determine the ultimate bearing capacity of those tested piles. A correlation between the pressing force and ultimate/allowable bearing capacity is eventually established in order to confirm the bearing of the pile group under the waste water treatment reservoir for permanent operation.
Geotechnical and Geological Engineering, 2018
Geotechnical and Geological Engineering, 2019
Recent development of infrastructures in urban area, especially Ho Chi Minh City, has generated m... more Recent development of infrastructures in urban area, especially Ho Chi Minh City, has generated massive construction in waste water treatment and connecting pipelines. Construction of these treatment facilities in soft ground condition, therefore, is such a challenge for practical Engineer who carries design task. The paper presents a case study of difficulty in driving Pre-stressed Concrete (PC) piles with the assistance of pre-boring method by continuous flight auger. A series of static load test with a range from 200% to 300% of working load were performed on D450mm PC piles. This was to investigate the bearing capacity of the piles which were not possibly driven to the initial design level due to the variation of the top of dense sandy soil layer. Adoption of Chin’s method was done accordingly to determine the ultimate bearing capacity of those tested piles. A correlation between the pressing force and ultimate/allowable bearing capacity is eventually established in order to con...
Geotechnical and Geological Engineering
Band-shaped prefabricated vertical drains in the combination of surcharge preloading have been wi... more 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.
ABSTRACT: Constant rate of strain (CRS) consolidation testing has specific advantages over the st... more ABSTRACT: Constant rate of strain (CRS) consolidation testing has specific advantages over the standard incremental loading (IL) consolidation testing. The advantages are 1) shortening testing period of 1 to 2 days compared to 7 days for the standard (IL); 2) testing data are recorded automatically and continuously. Moreover, pre-consolidation pressure from (CRS) is larger than that resulted from (IL), so many researchers have not recognized the application of CRS into the routine design of consolidation problems. Therefore, there has been little effort and application of CRS in the consolidation analysis into the soft ground improvement especially in Vietnam. There has been no standard for CRS applicable in the soft ground improvement design in Vietnam. The paper is focused on characterization of Vietnam soft clay deposit with application of the results of constant rate of strain consolidation tests in order to achieve the input parameters for consolidation analyses. Total 4 PVD and surcharge construction sites with the undisturbed samples taken by stationary hydraulic piston sampler are investigated in this study. The characterized input data are later used in consolidation analyses by finite difference method (FDM) in order to determine the behavior of Vietnam soft clay deposit with comparison the monitored data to the calculated data from a finite difference code which has been developed by the authors.
Constant rate of strain (CRS) consolidation test has some advantages over the traditional increme... more Constant rate of strain (CRS) consolidation test has some advantages over the traditional incremental loading (ILT) consolidation test. The advantages consist of shorter testing time (1 to 2 days for each test including the preparation of samples while it normally takes at least 7 days for one incremental loading consolidation test; continuous testing data for CRS consolidation tests. Moreover, preconsolidation stresses from CRS consoldiation tests are higher than those from traditional ILT consolidation tests, researchers have not acknowledged the feasibility of applying CRS consolidation test results into consolidation analyses in engineering practice. That is why there has been less applied research into desgin practice of soft ground improvement method, especially for surcharge and/or vacuum assisted by prefabricated vertical drains (PVDs). This paper is aimed at application of CRS test results on undisturbed samples into the analyses of consolidation with varied PVD length at a case study of soft ground improvement in Vietnam.
One of the commonly challenges for geotechnical engineers is carrying deep excavation in soft and... more One of the commonly challenges for geotechnical engineers is carrying deep excavation in soft and thick marine clayey strata to extend the space underground and to meet the process requirements especially for plant design. A case study of an 11.9 m-deep braced excavation sheet pile wall in marine soft clay in Thi Vai area is presented. A series of finite element analyses have been carried out to evaluate the behavior of the braced sheet pile wall. The field measurements indicate the ratio between the maximum lateral displacement of the braced wall to its maximum excavation depth (delta_h/H_max) is ranging from 1.1 % to 3.4 % and that the ratio between surface settlements to maximum excavation depth (delta_v/H_max) is around 0.9 % under condition of SLS (no- failure, i.e. ULS).
The vacuum preloading method was first developed in 1952, since then it has become a commonly use... more The vacuum preloading method was first developed in 1952, since then it has become a commonly used method all over the world in general and in Vietnam in particular to accelerate the consolidation of the soft ground. However, there is still misunderstanding of the variation of vacuum pressure over the depth of prefabricated vertical drain (PVD) among researchers. A series of Finite Different Analysis (FDA) of the improved ground with multi-layer models on a case study in South of Vietnam are conducted, utilizing of (CRS) consolidation tests on undisturbed samples for input parameters. In comparison with field monitored data, the analyses shows good agreement for magnitude vs. time for both settlement and dissipation of excess pore water pressure as well as strength gain for the first 10-m soft ground while the bottom part shows little or no improvement.
The vacuum preloading method was first developed in 1952, since then it has become a commonly use... more The vacuum preloading method was first developed in 1952, since then it has become a commonly used method all over the world in general and in Vietnam in particular to accelerate the consolidation of the soft ground. However, there is still misunderstanding of the variation of vacuum pressure over the depth of prefabricated vertical drain (PVD) among researchers. The paper presents a case study of soft ground improvement by vacuum preloading method for a land development project in Sothern Vietnam. A series of finite difference analysis (FDA) of the improved ground with multi-layer models to get insight into the effectiveness of the method are conducted. Input parameters are characterized from constant rate of strain (CRS) consolidation tests on undisturbed samples retrieved by stationary piston sampler. Vacuum pressure is considered as equivalent surface load in the analyses. In comparison with field monitored data, the analyses show good agreement for magnitude vs. time for both settlement and dissipation of excess pore water pressure as well as strength gain for the first 10-m soft ground while the bottom part shows little or no improvement.
Cement Deep Mixing (CDM) method has been widely used in soft ground improvement all over the worl... more Cement Deep Mixing (CDM) method has been widely used in soft ground improvement all over the world. The purpose is to improve the soil characteristics of soft soils and strengthen undrained shear strength as well as compressibility of those soils. There are many aspects need thoughroughly understanding within this method. The paper is focused on some aspects of characterizasion of cement deep mixing soils in the design appropriately.
This paper is aimed at presenting an analytical method and ourselves-built software for the desig... more This paper is aimed at presenting an analytical method and ourselves-built software for the design of approaching embankments supported by soil cement columns which are used increasingly all over the world. The method, utilizing the approaching methodology to equally distribute the stress-strain-settlement under small margin over the improved area, has been found to meet the strict requirements on cost efficiency and schedule for such important construction works as airplane runways, highways, high embankments, and container yards constructed in soft ground.
Apparent value of horizontal coe fficient of consolidation, denoted ch(ap) in this paper, is det... more Apparent value of horizontal coe fficient of consolidation, denoted ch(ap) in this paper, is determined from field data for two soft clay deposits in Southern Vietnam improved by vertical drain method. In order to evaluate ch(ap), Asaoka’s observational method is applied to settlement data obtained by extensometers and settlement plates. The values of ch(ap) at the two sites are from 40cm2/d to 80cm2/d with PVD spacing from 0.85m to 1.5m. Then, they are compared with vertical coefficient of consolidation cv given by laboratory consolidation test. The comparison shows that ch(ap) is 2 to 2.5 times as much as cv. Furthermore, ch values are evaluated from ch(ap) by using Barron’s and Hansobo’s approximations. Values of ch are found to be 3 to 7 times as much as cv.
This paper presents a case study of field behavior of braced sheet pile wall in soft ground. The ... more This paper presents a case study of field behavior of braced sheet pile wall in soft ground. The focuses are on stress strain behaviors with field monitoring of lateral displacement of the braced sheet pile wall and settlement of the surrounding area during the 11m deep excavation of the pump pit. The relationship between lateral displacement along the sheet pile wall and the settlement of the soft ground is found afterward. Furthermore, comparison of calculated and monitored data is shown to get insight into actual behavior of the braced cut wall. Observation procedure was used in control the safety of this underground work.
Determination of undrained shear strength su and consolidation yield stress ’y is the first step... more Determination of undrained shear strength su and consolidation yield stress ’y is the first step of soil improvement works. Various tests in the field and laboratory are practically available to determine these design parameters. Field vane test, unconfined compression test, and triaxial test etc are usually used to investigate su-profile of clay deposits. In order for reliable determination of su-profile, it is inevitable to investigate appropriateness of su as undrained shear strength for design use su(mob). For ’y-profile, constant rate of strain consolidation test is sometimes used for high compressible clay. The values of ’y given by this test is generally higher than that from standard consolidation test. Therefore, it is important for reliability of these design parameters and economical efficiency of soil improvement works to study the relations among su values and among ’y values determined by various tests.
In this regard, the values of su and ’y of high compressible clay deposits found in three construction sites in Vietnam, two in the South and one in the North, are studied in this paper. Direct shear test (DST), unconfined compression test (UCT), field vane test (FVT), cone penetration test (CPT) and torvane test (TVT) were carried out to determine su values and relation among them. An event of sliding failure took place in one site just after 5m sand filling on reclaimed ground was finished. This event was used to discuss the appropriateness of the values of su determined by these tests. While, standard consolidation test (STD) and constant rate of strain consolidation test (CRS) were performed to investigate ’y values and relation between them.
DST in this study was conducted with recompression method, in which specimen is consolidated with an insitu effective overburden stress before shear.
The paper presents a case study of braced excavation using sheet pile wall in Thi Vai soft clay i... more The paper presents a case study of braced excavation using sheet pile wall in Thi Vai soft clay in South East of Ho Chi Minh city, Vietnam. The study is focused on stress-strain-displacement behavior with field monitoring of lateral displacement of the wall and settlement of the surrounding area while the 11m excavation of the pump pit was performed. Furthermore, comparison of calculated and monitored data is shown to get insight into actual behavior of the braced cut wall. It is finally found that the ratio between maximum lateral displacement to maximum excavation depth ranging from 1.12%, 3.08% and 3.35% respectively for inclinometers INC-01 to INC-03 and that between surface settlement to maximum excavation depth 0.93% without any failure of the braced excavation and effective stress analysis can be used for getting reasonable and practically accepted values of both lateral displacement and surface settlement.
Vietnam locates in soft Hollocence ground area with great thickness, so geotechnical engineers al... more Vietnam locates in soft Hollocence ground area with great thickness, so geotechnical engineers always face challenges in dealing with soft ground treatment methods. Among those, preloading method by surcharge and/or vacuum assisted with prefabricated vertical drain (PVD) is often used to accelerate consolidation of the ground, and to reduce post-construction settlement before applying operation load throughout lifetime of the projects. Therefore, the paper is focused on using finite difference method (FDM) and constant rate of strain (CRS) consolidation test results to analyze the consolidation of a case study of soft marine clay found in South of Vietnam under surcharge preloading assisted with PVD. The approach aims at numerical solution of multi-layer model of soft clay a FDM procedure coded by the authors. Moreover, the authors determine stress-strain relation and parameters of the soft marine soil for the analysis with application of constant rate of strain consolidation tests carried out at the strain rate of 0.02%/min. The results of the analyses show good consistency between calculated and monitored data in settlement, dissipation of excess pore water pressure and un-drained strength gain of the investigated soft clay deposit. Therefore, finite difference method (FDM) with multi-layer consideration and CRS consolidation test results are applicable to the estimation of settlement of soft clays improved by PVD method in Vietnam.
Conference Presentations by Cong-Oanh Nguyen
Full-scale static loading tests on eight single test piles and hydrotests on two 50-m diameter ta... more Full-scale static loading tests on eight single test piles and hydrotests on two 50-m diameter tanks supported on 849 piles intended for storing refrigerated gas were performed at Cai Mep Industrial Park approximately 90 km southeast of Ho Chi Minh City, Vietnam. The test piles were precast concrete piles installed through 21 m of soft clay into dense sand to about 45 m depth below ground surface by driving (four piles) and jacking (four piles). The maximum pile test loads were 3,000 kN, which was well below any ultimate resistance, but showed that the tank-foundation piles, installed by jacking, could expect to shorten about 3 mm for the applied working load. The hydrotests included loading the tanks to the 636-MN maximum service load, and holding it constant for one week. The settlement of benchmarks along the tank perimeters were monitored and the records showed the tank perimeters to settle about 15 mm in addition to pile shortening. Analysis of the tank foundations modeled as flexible equivalent rafts showed that the settlement of the tank center was about three times larger than that of the perimeter. Analysis of long-term settlements indicated that, under service loading, the tank perimeters and centers will settle 100 mm and 300 mm, respectively. Due to a 3 m thick fill placed over the site causing the clay to consolidate, the ground surface is expected to settle more than 1 m over the long-term. Because the pile neutral plane is located in the sand below the clay, downdrag is not an issue for the piled foundation. The drag force will be well below the limit of the pile axial structural strength. RÉSUMÉ Des essais de chargement statique sur huit pieux individuels, et des hydrotests sur deux réservoirs de 50 m de diamètre supporté sur 849 pieux destinés à emmagasiner du gaz réfrigéré, ont été effectués au Parc Industriel Cai Mep à environ 90 km au sud-est de la ville Ho Chi Minh, Vietnam. Les pieux d'essai étaient des pieux en béton préfabriqués et ont été installés à travers 21 m d'argile molle dans du sable dense jusqu'à environ 45 m de profondeur sous la surface du sol par battage (quatre pieux) et par fonçage (quatre pieux). Les charges maximales d'essai de pieux étaient 3.000 kN, ce qui était inférieur à la résistance ultime des pieux, mais a montré que les pieux de fondation du réservoir, installés par fonçage, devraient se raccourcir d'environ 3 mm sous la charge appliquée. Les hydrotests ont inclus le chargement des réservoirs à 636 MN, la charge maximale de service, maintenue constante pendant une semaine. L'affaissement des points de repère le long des périmètres des réservoirs ont été surveillés et les niveaux ont montré que les périmètres des réservoirs sont affaissés d'environ 15 mm en plus que le raccourcissement des pieux. L'analyse des fondations des réservoirs modélisés comme des radiers flexibles a montré que l'affaissement du centre du réservoir était environ trois fois plus grand que celui du périmètre. Analyse des affaissements à long terme indique que, sous le chargement de service des périmètres et des centres des réservoirs s'affaisseront de 100 mm et 300 mm, respectivement. En raison d'un remblayage épais de 3 m placé sur le site provoquant l'argile à consolider, il est prévu que la surface du sol s'affaissera de plus de 1 m à long-terme. Parce que le plan neutre du pieux se trouve dans le sable en dessous de l'argile, le frottement négatif ne posera pas un problème pour la fondation en pieux. La force de traînée sera bien inférieure à la capacité structurale axiale du pieux.
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Papers by Cong-Oanh Nguyen
In this regard, the values of su and ’y of high compressible clay deposits found in three construction sites in Vietnam, two in the South and one in the North, are studied in this paper. Direct shear test (DST), unconfined compression test (UCT), field vane test (FVT), cone penetration test (CPT) and torvane test (TVT) were carried out to determine su values and relation among them. An event of sliding failure took place in one site just after 5m sand filling on reclaimed ground was finished. This event was used to discuss the appropriateness of the values of su determined by these tests. While, standard consolidation test (STD) and constant rate of strain consolidation test (CRS) were performed to investigate ’y values and relation between them.
DST in this study was conducted with recompression method, in which specimen is consolidated with an insitu effective overburden stress before shear.
Conference Presentations by Cong-Oanh Nguyen
In this regard, the values of su and ’y of high compressible clay deposits found in three construction sites in Vietnam, two in the South and one in the North, are studied in this paper. Direct shear test (DST), unconfined compression test (UCT), field vane test (FVT), cone penetration test (CPT) and torvane test (TVT) were carried out to determine su values and relation among them. An event of sliding failure took place in one site just after 5m sand filling on reclaimed ground was finished. This event was used to discuss the appropriateness of the values of su determined by these tests. While, standard consolidation test (STD) and constant rate of strain consolidation test (CRS) were performed to investigate ’y values and relation between them.
DST in this study was conducted with recompression method, in which specimen is consolidated with an insitu effective overburden stress before shear.