scholarly journals Recent Developments and Limitations of the SFG Model

2016 ◽  
pp. 41-49
Author(s):  
Daichao Sheng
Keyword(s):  
Shear Strength ◽  
Unsaturated Soils ◽  
Water Retention ◽  
Compacted Soils ◽  
Consistent Description ◽  
Strain Behaviour ◽  
Recent Developments ◽  
Density Dependency ◽  
Significant Attention ◽  
Water Retention Behaviour

The SFG model was proposed in attempt to provide a consistent description of the stress-strain behaviour of unsaturated soils, including compacted soils and soils dried from slurry. It differs from existing models mainly through two aspects: a consistent description of volume change, yield stress and shear strength behaviour of unsaturated soils and a smooth and natural transition between saturated and unsaturated states. The model has attracted significant attention since it was first proposed and has been extended to cover coupled hydro-mechanical behaviour, hysteretic water retention behaviour and density-dependency. This paper presents a summary of the latest developments of the model, as well as the aspects that require further refinements.

Capillary-based effective stress formulation for predicting shear strength of unsaturated soils

2015 ◽  
Vol 52 (12) ◽  
pp. 2067-2076 ◽  
Author(s):  
Jean-Marie Konrad ◽  
Marc Lebeau
Keyword(s):  
Shear Strength ◽  
Effective Stress ◽  
Unsaturated Soils ◽  
Water Retention ◽  
Adsorbed Water ◽  
Soil Strength ◽  
Liquid Films ◽  
Degree Of Saturation ◽  
Stress Parameter ◽  
Effective Stress Parameter

A number of investigations have shown that the shear strength of unsaturated soils can be defined in terms of effective stress. The difficulty in this approach lies in quantifying the effective stress parameter, or Bishop’s parameter. Although often set equal to the degree of saturation, it has recently been suggested that the effective stress parameter should be related to an effective degree of saturation, which defines the fraction of water that contributes to soil strength. A problematic element in this approach resides in differentiating the water that contributes to soil strength from that which does not contribute to soil strength. To address this difficulty, the paper uses theoretical considerations and experimental observations to partition the water retention function into capillary and adsorptive components. Given that the thin liquid films of adsorbed water should not contribute to effective stress, the effective stress parameter is solely related to the capillary component of water retention. In sample calculations, this alternative effective stress parameter provided very good agreement with experimental data of shear strength for a variety of soil types.

scholarly journals Modelling shear strength of compacted soils

2019 ◽  
Vol 92 ◽  
pp. 15007
Author(s):  
Sam Bulolo ◽  
Eng Choon Leong
Keyword(s):  
Shear Strength ◽  
Unsaturated Soils ◽  
Soil Mechanics ◽  
Published Data ◽  
State Variables ◽  
Compacted Soils ◽  
Earth Structures ◽  
Unsaturated Soil Mechanics ◽  
Coulomb Failure Criterion ◽  
Direct Shear Apparatus

Compacted soils constitute most engineering projects such as earth dams, embankments, pavements, and engineered slopes because of their high shear strength and low compressibility. The shear strength of compacted soils is a key soil parameter in the design of earth structures but it is seldom determined correctly due to their unsaturated state. The shear strength of compacted soils can be better evaluated under the framework of unsaturated soil mechanics. Saturated and unsaturated tests were conducted on compacted specimens using conventional direct shear apparatus under constant water content condition. Tests were conducted at different water contents and net normal stresses. The main objective of this study is to develop a shear strength model for compacted soils. Initial matric suction was measured before the test using the filter paper method. The two-stress state variables together with the extended Mohr-Coulomb failure criterion for unsaturated soils were used to obtain a lower bound model of the shear strength. The model was demonstrated using published data.

scholarly journals Relative performance of two unsaturated soil models using different constitutive variables

2014 ◽  
Vol 51 (12) ◽  
pp. 1423-1437 ◽  
Author(s):  
Martí Lloret-Cabot ◽  
Simon J. Wheeler ◽  
Jubert A. Pineda ◽  
Daichao Sheng ◽  
Antonio Gens
Keyword(s):  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Water Retention ◽  
Experimental Tests ◽  
Experimental Results ◽  
Degree Of Saturation ◽  
State Variables ◽  
Retention Behaviour ◽  
Model Predictions ◽  
Water Retention Behaviour

Mechanical and water retention behaviour of unsaturated soils is investigated in the context of two well established coupled constitutive models, each of which is formulated in terms of a different set of stress state variables or constitutive variables. Incremental relationships describing the volume change and variation of the degree of saturation are derived for each model. These incremental relationships are used to simulate a set of experimental tests on compacted Speswhite kaolin previously reported in the literature. Six individual tests, involving isotropic compression and various forms of shearing, are analyzed in the context of the incremental forms developed, and the model predictions are then compared against experimental results. The results show that, although each constitutive model uses a different set of constitutive variables and a different scheme for coupling mechanical and water retention behaviour, the two sets of model predictions are similar and both sets provide a reasonable match to the experimental results, suggesting that both models are able to capture the relevant features of unsaturated soil behaviour, despite expressing the constitutive laws in different ways.

A method for determination of ϕb for statically compacted soils

1996 ◽  
Vol 33 (2) ◽  
pp. 272-280 ◽  
Author(s):  
S Y Oloo ◽  
D G Fredlund
Keyword(s):  
Shear Strength ◽  
Unsaturated Soils ◽  
Direct Shear Test ◽  
Shear Test ◽  
Strength Parameter ◽  
Direct Shear ◽  
Compacted Soils ◽  
Shear Strength Parameter ◽  
Direct Shear Apparatus

The unsaturated shear strength parameter, ϕb, is usually determined using triaxial of direct shear apparatus that have been modified to allow for the control and (or) measurement of pore-air and pore-water pressures. A fairly high level of expertise is required for the characterization of ϕb using these modified apparatus. A simple procedure for determining ϕb for statically compacted soils at different water contents is presented along with a method of analysis. The tests can be performed on a conventional direct shear apparatus. The unsaturated shear strength parameter, ϕb, obtained using the proposed procedure is shown to be comparable to that obtained using the modified direct shear test. Since the proposed procedure utilizes standard laboratory direct shear equipment and takes a relatively short time to complete, it offers an easy and convenient alternative for the determination of ϕb for statically compacted soils. Key words: shear strength, matric suction, unsaturated soils, statically compacted soils, direct shear test.

scholarly journals Unification of plastic compression in a coupled mechanical and water retention model for unsaturated soils

2014 ◽  
Vol 51 (12) ◽  
pp. 1488-1493 ◽  
Author(s):  
Martí Lloret-Cabot ◽  
Simon J. Wheeler ◽  
Marcelo Sánchez
Keyword(s):  
Unsaturated Soils ◽  
Water Retention ◽  
Yield Curve ◽  
Stress Path ◽  
Degree Of Saturation ◽  
Retention Model ◽  
Plastic Compression ◽  
Single Process ◽  
Elastoplastic Constitutive Model ◽  
Water Retention Behaviour

In 2003, Wheeler, Sharma, and Buisson presented an elastoplastic constitutive model for unsaturated soils that represents both the mechanical behaviour and water retention behaviour, including the coupling between them. A crucial feature of the model is that the occurrence of plastic compression during all types of stress path is unified as a single process, with plastic compression during loading, plastic compression during wetting (collapse compression), and plastic compression during drying (irreversible shrinkage) all represented by yielding on a single loading–collapse yield curve. This paper explains how the model is able to predict the possible occurrence of plastic compression during each type of stress path and, in each case, links this to a physical explanation of the process involved. A simulation of an experimental test demonstrates the capability of the model to accurately predict the variation of both the void ratio and degree of saturation during successive stages of drying, loading, and wetting, where large magnitudes of compression occurred in all three test stages.

scholarly journals Yield stress, volume change, and shear strength behaviour of unsaturated soils: validation of the SFG model

2009 ◽  
Vol 46 (9) ◽  
pp. 1034-1045 ◽  
Author(s):  
Annan Zhou ◽  
Daichao Sheng
Keyword(s):  
Experimental Data ◽  
Shear Strength ◽  
Yield Stress ◽  
Volume Change ◽  
Unsaturated Soils ◽  
Strength Criterion ◽  
Single Equation ◽  
Compacted Soils ◽  
Shear Strength Criterion ◽  
Stress Changes

The model recently presented by Sheng, Fredlund, and Gens, known as the SFG model, provides a consistent explanation of yield stress, shear strength, and volume change behaviour of unsaturated soils as functions of suction. All these functions are based on one single equation that defines the volume change with suction and stress changes. This paper provides a systematic validation of the equation and the derived shear strength criterion against experimental data. The experimental data used include those for samples prepared from slurry soils and compacted soils. It is shown that (i) the method currently used to determine yield stresses of unsaturated soils is incorrect, (ii) volume change behaviour of unsaturated soils can be well predicted by the SFG model, and (iii) shear strength behaviour of unsaturated soils can be represented very well by the criterion in the SFG model.

A Øb Model for Predicting Shear Strength of Unsaturated Soils

2012 ◽  
Vol 594-597 ◽  
pp. 425-429
Author(s):  
Jing Yu Chen ◽  
Ying Hai
Keyword(s):  
Shear Strength ◽  
Unsaturated Soils ◽  
Arid Regions ◽  
Water Retention ◽  
Shear Strength Parameters ◽  
Soil Water Retention ◽  
Strength Parameters ◽  
The Past ◽  
The Mean ◽  
Good Agreement

Generally the shear strength of saturated soils has been predicted by the Mohr-Coulomb theory. However, field data accumulated in the past have evidently shown that saturated conditions are actually never reached in arid and semi-arid regions. An analytical model is developed to predict shear strength of an unsaturated soil with respect with matric suction. The formulation takes into account the influence of the mean stress on soil water retention curves. The results of the model developed for predicting the shear strength are compared with available experimental results. The good agreement between the estimates and the experimental data indicates that the proposed equation is promising and reliable for estimating preliminary unsaturated shear strength parameters.

Evaluation of capillary water retention effects on the development of the suction stress characteristic curve

2020 ◽  
Vol 57 (10) ◽  
pp. 1439-1452 ◽  
Author(s):  
Emad Maleksaeedi ◽  
Mathieu Nuth
Keyword(s):  
Shear Strength ◽  
Soil Water ◽  
Unsaturated Soils ◽  
Water Retention ◽  
Characteristic Curve ◽  
Degree Of Saturation ◽  
Soil Water Retention ◽  
Retention Model ◽  
Suction Stress ◽  
Effective Degree

The suction stress characteristic framework is a practical approach for relating the suction and the water-filled pore volume to the stress state of unsaturated soils. It predicts the effective stress by developing the suction stress characteristic curve from the soil-water retention curve. In this framework, the effective degree of saturation is usually calculated by the empirical water retention model of van Genuchten (published in 1980). In this paper, the use of a generalized soil-water retention model proposed by Lu in 2016, which differentiates the role of capillary and adsorption mechanisms, in the suction stress characteristic framework is studied. A redefinition of the effective degree of saturation is suggested, by choosing the retention state where capillarity approaches zero instead of the residual retention state. The validity of this assumption is examined using experimental data obtained by unsaturated shear strength and retention tests and datasets collected from the literature. The proposed definition is applicable for a variety of soils where capillarity is the dominant mechanism in producing suction stress within the range of suction 0–1500 kPa. In addition, it is observed that the generalized soil-water retention model presents a more realistic prediction of unsaturated shear strength compared with empirical water retention models.

Capillary water retention curve and shear strength of unsaturated soils

2016 ◽  
Vol 53 (6) ◽  
pp. 974-987 ◽  
Author(s):  
Annan Zhou ◽  
Ruiqiu Huang ◽  
Daichao Sheng
Keyword(s):  
Shear Strength ◽  
Unsaturated Soils ◽  
Water Retention ◽  
Adsorbed Water ◽  
Strength Criterion ◽  
Degree Of Saturation ◽  
Water Retention Curve ◽  
Capillary Water ◽  
Shear Strength Criterion ◽  
Wide Range

This paper proposes a new water retention model for unsaturated soils, which takes into account capillary condensation of adsorbed water. In this model, the degree of saturation of a soil is separated into that based on capillary water and that based on adsorbed water. Through analysis of a partially saturated two-cylinder system, a new shear strength criterion for unsaturated soils is proposed, in which only the degree of saturation based on capillary water contributes to the variation of shear strength with suction. The proposed shear strength criterion is justified against thermodynamic principles and is compared against existing criteria in the literature, which shows that it provides a much improved prediction of the experimental data for a wide range of suction values.

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