Volume–mass unsaturated soil constitutive model for drying–wetting under isotropic loading–unloading conditions

2011 ◽  
Vol 48 (2) ◽  
pp. 280-313 ◽  
Author(s):  
Hung Q. Pham ◽  
Delwyn G. Fredlund
Keyword(s):  
Constitutive Model ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Research Literature ◽  
Degree Of Saturation ◽  
Soil Conditions ◽  
Constitutive Relationships ◽  
Proposed Model ◽  
Wide Range

A rigorous volume–mass constitutive model is proposed for the representation of drying–wetting under isotropic loading–unloading conditions for unsaturated soils. The proposed model utilizes concepts arising from soil physics and geotechnical engineering research and requires readily obtainable soils data for soil properties. The model can be used to predict void ratio and water content constitutive relationships (and therefore degree of saturation) for a wide range of unsaturated soils. Various stress paths (i.e., loading–unloading and drying–wetting) can be simulated, and hysteresis associated with the soil-water characteristic curve is taken into account. Two closed-form equations for the volume–mass constitutive relationships are presented for soils starting from slurry conditions. A number of test results (i.e., from experimental programs reported in the research literature) were used during the verification of the proposed volume–mass constitutive model. The volume–mass constitutive model captures key unsaturated soil conditions such as air-entry value, water-entry value, and residual conditions. The proposed model appears to satisfactorily predict unsaturated soil behavior for soils ranging from low compressible sands to high compressible clays.

scholarly journals Verification of the Fredlund (2019) Unsaturated Shear Strength Function

Geosciences ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 151
Author(s):  
Thi Phuong An Tran ◽  
Delwyn G. Fredlund
Keyword(s):  
Shear Strength ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Strength Function ◽  
Past Research ◽  
Research Literature ◽  
Reference Points ◽  
Degree Of Saturation ◽  
Anchor Points

There has been a proliferation of equations proposed to describe the unsaturated shear strength envelope going back to the 1970s. However, there have been limited studies to verify the suitability of one unsaturated shear strength equation over another. Most proposed shear strength equations have attempted to relate the shear strength of an unsaturated soil to some aspect(s) of the soil–water characteristic curve (SWCC). Estimation procedures have generally focused on using that of air-entry value (AEV) as defined by the drying (or desorption) branch of the degree of saturation SWCC (S-SWCC). This paper studies the suitability of using two “anchor points” (or reference points) along the drying S-SWCC to estimate the unsaturated soil shear strength function. The anchor points referred to are the air-entry value (AEV) of the soil and the “residual suction point” of the soil defined in terms of the S-SWCC. Shear strength conditions associated with both so-called anchor points are used as “boundary conditions” that should be satisfied when estimating the shear strength function for unsaturated soils. Past research laboratory measurements published in the research literature are used as part of the verification process for this study.

Predicting the permeability function for unsaturated soils using the soil-water characteristic curve

1994 ◽  
Vol 31 (4) ◽  
pp. 533-546 ◽  
Author(s):  
D.G. Fredlund ◽  
Anqing Xing ◽  
Shangyan Huang
Keyword(s):  
Water Content ◽  
Soil Water ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Degree Of Saturation ◽  
Residual Water ◽  
Soil Water Characteristic Curve ◽  
Coefficient Of Permeability ◽  
Permeability Function

The coefficient of permeability for an unsaturated soil is primarily determined by the pore-size distribution of the soil and can be predicted from the soil-water characteristic curve. A general equation, which describes the soil-water characteristic curve over the entire suction range (i.e., from 0 to 106 kPa), was proposed by the first two authors in another paper. This equation is used to predict the coefficient of permeability for unsaturated soils. By using this equation, an evaluation of the residual water content is no longer required in the prediction of the coefficient of permeability. The proposed permeability function is an integration form of the suction versus water content relationship. The proposed equation has been best fit with example data from the literature where both the soil-water characteristic curve and the coefficient of permeability were measured. The fit between the data and the theory was excellent. It was found that the integration can be done from zero water content to the saturated water content. Therefore, it is possible to use the normalized water content (volumetric or gravimetric) or the degree of saturation data versus suction in the prediction of the permeability function. Key words : coefficient of permeability, soil-water characteristic curve, unsaturated soil, water content, soil suction.

Soil Strength Reduction Method Induced by Variation of Water Content

2012 ◽  
Vol 170-173 ◽  
pp. 847-852
Author(s):  
Peng Ming Jiang ◽  
Zhong Lei Yan ◽  
Peng Li
Keyword(s):  
Slope Stability ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Strength Reduction ◽  
Actual State ◽  
Soil Slope ◽  
Simple Method ◽  
The Common ◽  
The Stability

As the complexity of unsaturated soil theory, and it must have a long test period when we study the unsaturated soils, so the conventional design analysis software does not provide such analysis, so we can imagine that such a slope stability analysis does not accurately reflect the actual state of the slope. Based on the known soil moisture content，this paper use the soil water characteristic curve and strength theory of unsaturated soil to calculate the strength reduction parameters of soil which can calculate the stability of the soil slope when using the common calculation method. It is noticeable that this method can be extended and applied if we establish regional databases for this simple method, and these databases can improve the accuracy of the calculation of slope stability.

Measurement of Soil Suction and Soil-Water Characteristics of Bentonite-Sand Mixtures

2010 ◽  
Author(s):  
Pan Hu ◽  
Qing Yang ◽  
Maotian Luan
Keyword(s):  
Soil Water ◽  
Vapor Phase ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Phase Method ◽  
Soil Suction ◽  
Compacted Bentonite ◽  
Water Characteristics ◽  
Wide Range ◽  
High Level

The soil-water characteristic curve (SWCC) is a widely used experimental means for assessing fundamental properties of unsaturated soils for a wide range of soil suction values. The study of SWCC is helpful because some properties of unsaturated soils can be predicted from it. Nowadays, much attention has been paid to the behaviours of highly compacted bentonite-sand mixtures used in engineering barriers for high level radioactive nuclear waste disposal. It is very important to study the various performances of bentonite-sand mixtures in order to insure the safety of high-level radioactive waste (HLW) repository. After an introduction to vapor phase method and osmotic technique, a laboratory study has been carried out on compacted bentonite-sand mixtures. The SWCC of bentonite-sand mixtures has been obtained and analyzed. The results show that the vapor phase method and osmotic technique is suitable to the unsaturated soils with high and low suction.

Method for Quick Prediction of Hydraulic Conductivity and Soil-Water Retention of Unsaturated Soils

2018 ◽  
Vol 2672 (52) ◽  
pp. 108-117 ◽  
Author(s):  
Shaoyang Dong ◽  
Yuan Guo ◽  
Xiong (Bill) Yu
Keyword(s):  
Finite Element ◽  
Hydraulic Conductivity ◽  
Soil Properties ◽  
Soil Water ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Hydraulic Properties ◽  
Water Retention ◽  
Characteristic Curve ◽  
Soil Water Retention

Hydraulic conductivity and soil-water retention are two critical soil properties describing the fluid flow in unsaturated soils. Existing experimental procedures tend to be time consuming and labor intensive. This paper describes a heuristic approach that combines a limited number of experimental measurements with a computational model with random finite element to significantly accelerate the process. A microstructure-based model is established to describe unsaturated soils with distribution of phases based on their respective volumetric contents. The model is converted into a finite element model, in which the intrinsic hydraulic properties of each phase (soil particle, water, and air) are applied based on the microscopic structures. The bulk hydraulic properties are then determined based on discharge rate using Darcy’s law. The intrinsic permeability of each phase of soil is first calibrated from soil measured under dry and saturated conditions, which is then used to predict the hydraulic conductivities at different extents of saturation. The results match the experimental data closely. Mualem’s equation is applied to fit the pore size parameter based on the hydraulic conductivity. From these, the soil-water characteristic curve is predicted from van Genuchten’s equation. The simulation results are compared with the experimental results from documented studies, and excellent agreements were observed. Overall, this study provides a new modeling-based approach to predict the hydraulic conductivity function and soil-water characteristic curve of unsaturated soils based on measurement at complete dry or completely saturated conditions. An efficient way to measure these critical unsaturated soil properties will be of benefit in introducing unsaturated soil mechanics into engineering practice.

A UNIFIED FRACTAL MODEL FOR PERMEABILITY COEFFICIENT OF UNSATURATED SOIL

Fractals ◽  
2019 ◽  
Vol 27 (01) ◽  
pp. 1940012 ◽  
Author(s):  
GAOLIANG TAO ◽  
XIAOKANG WU ◽  
HENGLIN XIAO ◽  
QINGSHENG CHEN ◽  
JIANCHAO CAI
Keyword(s):  
Relative Permeability ◽  
Unsaturated Soil ◽  
Permeability Coefficient ◽  
Unsaturated Soils ◽  
Fractal Theory ◽  
Characteristic Curve ◽  
Fractal Model ◽  
Computational Effort ◽  
Model Parameters ◽  
Mechanical Coupling

Due to the significant challenges in the measurements, evaluation of permeability coefficient for unsaturated soil is of immense importance for investigating the seepage and hydro-mechanical coupling problems of unsaturated soil. However, the predictions of existing typical models reveal significance divergence for permeability coefficient of unsaturated soils even under identical conditions. In particular, the existing models are greatly restricted in their practical application due to their complexity in the form of integral expressions that require significant computational effort. Here, a simplified unified model is presented to estimate the relative permeability coefficient. First, a fractal-form of soil–water characteristic curve (SWCC) is derived from fractal theory. Then, on the basis of the proposed SWCC models, the classical models (i.e. Childs and Collis-George (CCG) model, Burdine model, Mualem model and Tao and Kong model, respectively) for evaluating the permeability coefficient of unsaturated soil are converted to be presented in fractal forms. It is interestingly found that the fractal forms of these models are enormously similar. Based on these observations, a simplified unified fractal model for the relative permeability coefficient of unsaturated soil is proposed, where only two parameters (i.e. fractal dimension and air-entry value) are included, thereby significantly reducing the computational efforts. The detailed procedure for determining model parameters is elaborated. The accuracy of this model is verified by comparing its predictions with the experimental data for over 12 types of unsaturated soils. The results highlight that, compared with existing models, the proposed model would be much more efficiently used for estimating the relative permeability coefficient of unsaturated soils, thereby facilitating its application for investigating the associated seepage and hydro-mechanical coupling problems in practice.

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.

scholarly journals Examination of the estimation of relative permeability for unsaturated soils

2015 ◽  
Vol 52 (12) ◽  
pp. 2077-2087 ◽  
Author(s):  
Feixia Zhang ◽  
D.G. Fredlund
Keyword(s):  
Lower Limit ◽  
Unsaturated Soils ◽  
Integral Formula ◽  
Characteristic Curve ◽  
Soil Conditions ◽  
Mathematical Algorithm ◽  
Soil Systems ◽  
Permeability Function ◽  
Unsaturated Permeability ◽  
Flexible Integration

The unsaturated permeability function is an important soil property function used in the numerical modeling of saturated–unsaturated soil systems. The permeability function is generally predicted by integrating along the soil-water characteristic curve (SWCC) starting at saturated soil conditions. The integration is based on a particular integral formula. The Fredlund–Xing–Huang permeability function is a flexible integration technique used for calculating the unsaturated permeability function. The original permeability theory published by Fredlund, Xing, and Huang in 1994 specified that the air-entry value (AEV), ψaev, be used as the lower limit of the integration when calculating the permeability function. However, as there was no analytical procedure available for the calculation of the AEV on the SWCC, it became common practice to start the integration procedure from a value near zero. The assumption was made that the error associated with starting the integration from an arbitrary low value was minimal. While this might be the case in some situations, the error can be quite substantial in other situations. This paper undertakes a study of the effect of the lower limit of integration on the calculation of the permeability function. Comparisons are made between starting the integration from various values below the AEV and starting the integration from the calculated AEV, ψaev. A mathematical algorithm is also proposed for the calculation of the AEV for integration purposes. The results show that the relative coefficient of permeability can be significantly underestimated when the lower limit of integration is smaller than the AEV. The recommendation is that the AEV always be used as the lower limit of integration in the Fredlund–Xing–Huang permeability equation.

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