The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform
Theodoros Triantafyllidis - One of the best experts on this subject based on the ideXlab platform.
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AVISA: anisotropic visco-ISA model and its performance at cyclic loading
Acta Geotechnica, 2020Co-Authors: Merita Tafili, Theodoros TriantafyllidisAbstract:In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the Overconsolidation Ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235–1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) Overconsolidation Ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes ( $$
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AVISA: anisotropic visco-ISA model and its performance at cyclic loading [in press]
Acta Geotechnica, 2020Co-Authors: Merita Tafili, Theodoros TriantafyllidisAbstract:In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the Overconsolidation Ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235–1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) Overconsolidation Ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes (
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Monotonic and cyclic tests on kaolin: a database for the development, calibRation and verification of constitutive models for cohesive soils with focus to cyclic loading
Acta Geotechnica, 2018Co-Authors: Torsten Wichtmann, Theodoros TriantafyllidisAbstract:A database with about 60 undrained monotonic and cyclic triaxial tests on kaolin is presented. In the monotonic tests, the influences of consolidation pressure, Overconsolidation Ratio, displacement rate and sample cutting direction have been studied. In the cyclic tests, the stress amplitude, the initial stress Ratio and the control (stress vs. strain cycles) have been additionally varied. Isotropic consolidation leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the cyclic tests, while a failure due to an excessive accumulation of axial strain and lens-shaped effective stress paths was observed in the case of anisotropic consolidation with $$q^{\text{ ampl }}< |q^{\text{ av }}|$$ q ampl < | q av | . The rate of pore pressure accumulation grew with increasing amplitude and void Ratio (i.e. decreasing consolidation pressure and Overconsolidation Ratio). The “cyclic flow rule” well known for sand has been confirmed also for kaolin: With increasing value of the average stress Ratio $$|\eta ^{\text{ av }}| = |q^{\text{ av }}|/p^{\text{ av }}, $$ | η av | = | q av | / p av , the accumulation of deviatoric strain becomes predominant over the accumulation of pore water pressure. The tests on the samples cut out either horizontally or vertically revealed a significant effect of anisotropy. In the cyclic tests, the two kinds of samples exhibited an opposite inclination of the effective stress path. Furthermore, the horizontal samples showed a higher stiffness and could sustain a much larger number of cycles to failure. All data of the present study are available from the homepage of the first author. They may serve for the examination, calibRation or improvement in constitutive models dedicated to cohesive soils under cyclic loading, or for the development of new models.
George A. Athanasopoulos - One of the best experts on this subject based on the ideXlab platform.
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PRECONSOLIDATION VERSUS AGING BEHAVIOR OF KAOLINITE CLAY. DISCUSSION AND CLOSURE
Journal of Geotechnical Engineering, 1994Co-Authors: George A. AthanasopoulosAbstract:A discussion of a paper with the aforementioned title by G.A. Athanasopoulos, published by this journal (Volume 119, Number 6, June 1993), is presented. The discussion focuses on the alleged relationship between aging-induced Overconsolidation Ratio and age of the clay. Mesri and Shahien question this relationship as it is inconsistent with existing laboratory and in situ aging information on soft clay deposits. The discussers offer citations. Discussion is followed by closure from the author.
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Preconsolidation versus Aging Behavior of Kaolinite Clay
Journal of Geotechnical Engineering, 1993Co-Authors: George A. AthanasopoulosAbstract:Resonant column test results were used to determine the effects of the Overconsolidation Ratio (OCR) and of aging on the normalized rate of secondary increase of low amplitude shear modulus, N sub G, of a remolded kaolinite clay. It was found that N sub G decreases linearly with the logarithm of OCR. Comparing this behavior with the similar effect of aging on the value of N sub G, a correlation was developed between duRation of aging and equivalent Overconsolidation Ratio, OCR sub eq. The details of the study are described, and the results are discussed.
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Effects of ageing and Overconsolidation on the elastic stiffness of a remoulded clay
Geotechnical & Geological Engineering, 1993Co-Authors: George A. AthanasopoulosAbstract:Results of resonant column tests were used to determine values of low amplitude shear modulus ( G _0) of a remoulded kaolinite clay for different duRations of ageing and for different values of consolidation stress ( σ ′_0) and Overconsolidation Ratio (OCR). It was found that after completion of primary consolidation the values of G _0 increased linearly with the logarithm of time and after a week of confinement the value of normalized rate of secondary increase of shear modulus ( N _ G ) could be reliably estimated. Values of N _ G were found to derease linearly with the logarithm of OCR and with the logarithm of ageing duRation. This similarity of behaviour provided a basis for establishing an equivalency between age and equivalent Overconsolidation Ratio (OCR)_eq. The effects of σ ′_0 and OCR on the value of G _0 were also established in a functional form that indicated a stronger influence compared to that predicted by the Hardin Equation.
Mostefa Belkhatir - One of the best experts on this subject based on the ideXlab platform.
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packing density and Overconsolidation Ratio effects on the mechanical response of granular soils
Geotechnical and Geological Engineering, 2020Co-Authors: Youcef Mahmoudi, Mostefa Belkhatir, Abdellah Cherif Taiba, Leila Hazout, Wiebke BailleAbstract:Liquefaction of sandy soils is associated with a sudden loss of the shear strength due to a dramatic increase of the excess pore water pressure in undrained conditions. Its occurrence usually causes significant damages to buildings and infrastructures during earthquakes. Therefore, the proposed laboratory research investigation aims to study the effects of the combined effects of sample depositional methods, initial relative density and Overconsolidation Ratio on the undrained shear strength of Chlef sand samples under static loading conditions. For this purpose, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated Chlef sand samples and subjected to different Overconsolidation Ratios (OCR = 1, 2, 4 and 8). The initial confining pressure for all tests was kept constant (σ′3 = 100 kPa). The samples were prepared using two depositional methods named: dry funnel pluviation (DFP) and wet deposition (WD) at three selected relative densities (Dr = 18%, 52% and 88%). The obtained data confirm the existence of a linear relationship between the undrained peak shear strength of overconsolidated Chlef sand samples (qpeak_OC) and undrained shear strength of normally consolidated Chlef sand (qpeak_NC) for the two sample prepaRation techniques (DFP and WD) and range of the initial relative density (18% ≤ Dr ≤ 88%) under consideRation. The Overconsolidation (OCR) and initial relative density (Dr) increase the dilativeness of the normally consolidated and overconsolidated sandy samples for the two sample reconstitution techniques. The dry funnel pluviated sandy samples exhibit higher undrained shear strength (liquefaction resistance) than the wet deposited sandy samples. Moreover, the obtained data reveal that the wet deposited sandy samples were more vulnerable to liquefaction resistance than dry funnel pluviated sandy samples for the range of the initial relative density (18% ≤ Dr ≤ 88%) and lower Overconsolidation Ratios (1 ≤ OCR ≤ 4).
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Experimental Investigation on Undrained Shear Behavior of Overconsolidated Sand-Silt Mixtures: Effect of Sample Reconstitution
Geotechnical Testing Journal, 2016Co-Authors: Youcef Mahmoudi, Mostefa Belkhatir, Abdellah Cherif Taiba, Tom SchanzAbstract:Liquefaction stability analysis using the undrained shear strength characteristics of sandy soils that are prone to liquefaction is a major challenge in geotechnical earthquake engineering. The objective of this laboratory research work was to study the combined effect of the sample reconstitution (dry funnel pluviation and wet deposition) and Overconsolidation Ratio (OCR = 1, 2, 4, and 8) on the undrained shear strength of medium dense (D_r = 52 %) sand-silt mixtures under undrained monotonic loading conditions. For this purpose, a series of triaxial tests were carried out on reconstituted saturated silty sand samples with fines content ranging from 0 to 40 %. The confining pressure was kept constant to 100 kPa in all tests. The obtained data showed that the dry funnel pluviated samples were more resistant than the wet deposited samples and complete static liquefaction of samples reconstituted with wet deposition method was observed for the lower Overconsolidation Ratios (OCR = 1, 2, and 4). The undrained shear strength decreases with the increase of fines content for dry funnel pluviation (DFP) and the inverse tendency was observed in the case of wet deposition (WD) for the range of the Overconsolidation Ratio under consideRation (1 ≤ OCR ≤ 8).
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Undrained behavior of silty sand: effect of the Overconsolidation Ratio
Arabian Journal of Geosciences, 2013Co-Authors: Ahmed Djafar Henni, Ahmed Arab, Mostefa Belkhatir, A. Saaed Hamoudi, Hamid KhelafiAbstract:This experimental study deals with the effect of the Overconsolidation Ratio on the monotonic undrained shear behavior of silty sand. The study is based on the undrained monotonic triaxial tests for the Overconsolidation Ratios (OCR = 1, 2, 4, and 8), with different silt contents ranging from 0% to 40%. The laboratory tests were carried out at an initial relative density of Dr = 50%. The paper is composed of two parts. The first one presents the tested soils; the second one gives an analysis of the test results and discusses the influence of the Overconsolidation Ratio on the shear strength of the soil. The test results indicate that the shear strength of the soil increases with the increase of the Overconsolidation Ratio resulting in an increase of soil dilatancy. The increase in the amount of fines from 0% to 40% increases the phase of the contractancy and consequently reducing the phase of dilatancy of the tested material
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Laboratory Investigation on the Effects of Overconsolidation and SatuRation on Undrained Monotonic Shear Behavior of Granular Material
Marine Georesources & Geotechnology, 2011Co-Authors: Noureddine Della, Ahmed Arab, Mostefa BelkhatirAbstract:In order to study pore water response and static liquefaction characteristics of silty sand, which has previously experienced liquefaction, two series of monotonic triaxial tests were run on medium dense sand specimens (RD = 50%) at confining pressure of 100 kPa. In the first test series, the influence of the soil satuRation under undrained static loading has been studied. It summarizes results of monotonic tests performed on Chlef sand at various values of the Skempton's pore pressure coefficient. Analysis of experimental results gives valuable insights on the effect of soil satuRation on sand response to undrained monotonic paths. In the second series of tests, the Overconsolidation influence on the resistance to the sands liquefaction has been realized on samples at various values of Overconsolidation Ratios (OCR). It was found that the increase of Overconsolidation Ratio (OCR) increases the resistance of sands to liquefaction.
Kaare Hoeg - One of the best experts on this subject based on the ideXlab platform.
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Normalized Undrained Shear Strength of Clay Shales
Engineering Geology, 2008Co-Authors: Marte Gutierrez, Runar Nygard, Kaare Hoeg, Toralv BerreAbstract:This paper examines the possibility of normalizing the undrained shear strength behavior of clay shales. Shales are formed in sedimentary basins by diagenetic processes, which turn young clay sediments into compacted and lithified shales. Cementation at grain contacts affects many properties of shales including the apparent preconsolidation and shear strength. Based on a study of 25 different types of materials, the normalized undrained shear strength of clay shales was found to correlate satisfactorily with Overconsolidation Ratio following the SHANSEP (Stress History and Normalized Soil Engineering Properties) normalization procedure for clays. The normalized undrained shear strength correlation appears valid regardless of the cause of the apparent preconsolidation. Correlations of the apparent preconsolidation stress with unconfined compressive strength and compressional wave velocity are proposed to predict the normalized undrained shear strength of clay shales.
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brittle ductile transition shear failure and leakage in shales and mudrocks
Marine and Petroleum Geology, 2006Co-Authors: Runar Nygard, Marte Gutierrez, Rolf K Bratli, Kaare HoegAbstract:Abstract This paper presents an experimental study on brittle–ductile transition, shear behaviour, the formation of shear fractures in shales and mudrocks, and the hydraulic properties of shales and mudrocks with shear induced fractures. The experimental study is based on triaxial tests on different mudrocks and shales from North Sea reservoirs and adjacent areas sheared at different effective confining stresses. It is shown that during burial, mudrocks behave as normally consolidated materials and exhibit ductile response to increased load. However, several processes like uplift, chemical diagenesis and overpressure build-up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading. Triaxial tests with different loading conditions have been performed to establish the brittle-to-ductile transition of various mudrocks. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the Overconsolidation Ratio. A relationship between normalized undrained shear strength and Overconsolidation Ratio is established. In combination with this relationship, a correlation between compressional wave velocity and apparent pre-consolidation stress, which accounts for both mechanical and chemical diagenesis, may be used as a tool to evaluate possible leakage of hydrocarbon seals.
Runar Nygard - One of the best experts on this subject based on the ideXlab platform.
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Normalized Undrained Shear Strength of Clay Shales
Engineering Geology, 2008Co-Authors: Marte Gutierrez, Runar Nygard, Kaare Hoeg, Toralv BerreAbstract:This paper examines the possibility of normalizing the undrained shear strength behavior of clay shales. Shales are formed in sedimentary basins by diagenetic processes, which turn young clay sediments into compacted and lithified shales. Cementation at grain contacts affects many properties of shales including the apparent preconsolidation and shear strength. Based on a study of 25 different types of materials, the normalized undrained shear strength of clay shales was found to correlate satisfactorily with Overconsolidation Ratio following the SHANSEP (Stress History and Normalized Soil Engineering Properties) normalization procedure for clays. The normalized undrained shear strength correlation appears valid regardless of the cause of the apparent preconsolidation. Correlations of the apparent preconsolidation stress with unconfined compressive strength and compressional wave velocity are proposed to predict the normalized undrained shear strength of clay shales.
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brittle ductile transition shear failure and leakage in shales and mudrocks
Marine and Petroleum Geology, 2006Co-Authors: Runar Nygard, Marte Gutierrez, Rolf K Bratli, Kaare HoegAbstract:Abstract This paper presents an experimental study on brittle–ductile transition, shear behaviour, the formation of shear fractures in shales and mudrocks, and the hydraulic properties of shales and mudrocks with shear induced fractures. The experimental study is based on triaxial tests on different mudrocks and shales from North Sea reservoirs and adjacent areas sheared at different effective confining stresses. It is shown that during burial, mudrocks behave as normally consolidated materials and exhibit ductile response to increased load. However, several processes like uplift, chemical diagenesis and overpressure build-up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading. Triaxial tests with different loading conditions have been performed to establish the brittle-to-ductile transition of various mudrocks. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the Overconsolidation Ratio. A relationship between normalized undrained shear strength and Overconsolidation Ratio is established. In combination with this relationship, a correlation between compressional wave velocity and apparent pre-consolidation stress, which accounts for both mechanical and chemical diagenesis, may be used as a tool to evaluate possible leakage of hydrocarbon seals.
