A simplified procedure to estimate the shear strength envelope of unsaturated soils

2006 ◽  
Vol 43 (10) ◽  
pp. 1088-1095 ◽  
Author(s):  
Orencio Monje Vilar
Keyword(s):  
Experimental Data ◽  
Shear Strength ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Hyperbolic Function ◽  
Soil Suction ◽  
Suction Control ◽  
Saturated Sample ◽  
Simplified Procedure ◽  
Good Agreement

Procedures that allow the prediction of some properties of unsaturated soils or the minimization of the number of tests needed to measure them are advantageous because the control of suction during testing is a formidable task that is time consuming and involves a great degree of expertise. A simplified procedure is proposed in this paper to estimate the shear strength of an unsaturated soil. The procedure is based on an empirical hyperbolic function that has been successfully used to fit experimental data. The function requires two input values, namely the shear strength of a saturated sample and the shear strength of an air-dried sample tested without the need for suction control. Samples tested under a controlled suction larger than the maximum suction expected in the problem can, alternatively, replace the air-dried samples. Both alternatives were tested against results for various soils reported in the literature. The good agreement between the estimates and the experimental data indicates that the proposed procedure is promising and reliable for estimating preliminary unsaturated shear strength parameters.Key words: unsaturated soil, suction, shear strength, prediction, laboratory tests.

Model for the prediction of shear strength with respect to soil suction

1996 ◽  
Vol 33 (3) ◽  
pp. 379-392 ◽  
Author(s):  
S K Vanapalli ◽  
D G Fredlund ◽  
D E Pufahl ◽  
A W Clifton
Keyword(s):  
Shear Strength ◽  
Soil Water ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Experimental Studies ◽  
Matric Suction ◽  
Glacial Till ◽  
Soil Suction ◽  
Soil Water Characteristic Curve

Experimental studies on unsaturated soils are generally costly, time-consuming, and difficult to conduct. Shear strength data from the research literature suggests that there is a nonlinear increase in strength as the soil desaturates as a result of an increase in matric suction. Since the shear strength of an unsaturated soil is strongly related to the amount of water in the voids of the soil, and therefore to matric suction, it is postulated that the shear strength of an unsaturated soil should also bear a relationship to the soil-water characteristic curve. This paper describes the relationship between the soil-water characteristic curve and the shear strength of an unsaturated soil with respect to matric suction. Am empirical, analytical model is developed to predict the shear strength in terms of soil suction. The formulation makes use of the soil-water characteristic curve and the saturated shear strength parameters. The results of the model developed for predicting the shear strength are compared with experimental results for a glacial till. The shear strength of statically compacted glacial till specimens was measured using a modified direct shear apparatus. Specimens were prepared at three different water contents and densities (i.e., corresponding to dry of optimum, and wet of optimum conditions). Various net normal stresses and matric suctions were applied to the specimens. There is a good correlation between the predicted and measured values of shear strength for the unsaturated soil. Key words: soil-water characteristic curve, shear strength, unsaturated soil, soil suction, matric suction.

Shear strength of unsaturated soil interfaces

2009 ◽  
Vol 46 (5) ◽  
pp. 595-606 ◽  
Author(s):  
Tariq B. Hamid ◽  
Gerald A. Miller
Keyword(s):  
Shear Strength ◽  
Unsaturated Soil ◽  
Characteristic Curve ◽  
Matric Suction ◽  
Direct Shear ◽  
Direct Shear Tests ◽  
Strength Failure ◽  
Suction Control ◽  
Failure Envelopes ◽  
Fine Grained Soil

Unsaturated soil interfaces exist where unsaturated soil is in contact with structures such as foundations, retaining walls, and buried pipes. The unsaturated soil interface can be defined as a layer of unsaturated soil through which stresses are transferred from soil to structure and vice versa. In this paper, the shearing behavior of unsaturated soil interfaces is examined using results of interface direct shear tests conducted on a low-plasticity fine-grained soil. A conventional direct shear test device was modified to conduct direct shear interface tests using matric suction control. Further, the results were used to define failure envelopes for unsaturated soil interfaces having smooth and rough counterfaces. Results of this study indicate that matric suction contributes to the peak shear strength of unsaturated interfaces; however, postpeak shear strength did not appear to vary with changes in matric suction. Variations in net normal stress affected both peak and postpeak shear strength. Failure envelopes developed using the soil-water characteristic curve (SWCC) appeared to capture the nonlinear influence of matric suction on shear strength of soil and interfaces.

Modeling the unsaturated soil zone in slope stability analysis

2014 ◽  
Vol 51 (12) ◽  
pp. 1384-1398 ◽  
Author(s):  
L. L. Zhang ◽  
Delwyn G. Fredlund ◽  
Murray D. Fredlund ◽  
G. Ward Wilson
Keyword(s):  
Shear Strength ◽  
Stability Analysis ◽  
Slope Stability ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Matric Suction ◽  
Slope Stability Analysis ◽  
Engineering Practice ◽  
Rate Of Increase ◽  
Nonlinear Shear

The linear form of the extended Mohr–Coulomb shear strength equation uses a [Formula: see text] parameter to quantify the rate of increase in shear strength relative to matric suction. When the [Formula: see text] value is unknown, a [Formula: see text] equal to 15° is sometimes used in the slope stability study to assess the influence of matric suction on the stability of a slope. In many cases, however, a [Formula: see text] value of zero is used, signifying that the effect of matric suction is ignored. Experiment results have shown that the relationship between the shear strength of an unsaturated soil and matric suction is nonlinear. Several semi-empirical estimation equations have been proposed relating the unsaturated shear strength to the soil-water characteristic curve. In this paper, the results of a study using two-dimensional slope stability analysis along with an estimated nonlinear shear strength equations is presented. The effects of using an estimated nonlinear shear strength equation for the unsaturated soils are illustrated using three example problems. Several recommendations are made for engineering practice based on the results of the example problems. If the air-entry value (AEV) of a soil is smaller than 1 kPa, the effect of matric suction on the calculated factor of safety is trivial and the [Formula: see text] value can be assumed to be zero. If the AEV of a soil is between 1 and 20 kPa, the nonlinear equations of unsaturated shear strength should be adopted. For soils with an AEV value between 20 and 200 kPa, an assumed [Formula: see text] value of 15° provides a reasonable estimation of the effects of unsaturated shear strength in most cases. For soils with an AEV greater than 200 kPa, [Formula: see text] can generally be assumed to be equal to the effective angle of internal friction, [Formula: see text], in applications where geotechnical structures have matric suctions around 100 kPa.

scholarly journals Numerical study on bearing capacity of a pile group next to a slope in unsaturated soils

2020 ◽  
Vol 195 ◽  
pp. 01006
Author(s):  
Amirreza Pourfatollah ◽  
Ali Pirjalili ◽  
Aliakbar Golshani
Keyword(s):  
Shear Strength ◽  
Bearing Capacity ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Numerical Study ◽  
Pile Group ◽  
Soil Strength ◽  
Strength Parameters ◽  
Water Characteristics ◽  
Soil Shear Strength

The bearing capacity of a pile group mostly depends on parameters of the soil shear strength affected by the soil-water characteristics, especially in unsaturated soils. The soil shear strength is entirely affected by hydraulic stresses in unsaturated soil, such as precipitation and evaporation. Further, the bearing capacity of the pile installed on unsaturated soil depends on hydraulic stresses applied to the soil. Furthermore, slope vicinity may cause a severe decline in the pile bearing capacity. The present study aimed to investigate a pile group in unsaturated soil adjacent to a slope and analyzed the effect of the rainfall on the soil strength parameters. Thus, a numerical study has been performed using a finite difference software,i.e., FLAC2D. Besides, to investigate the model in a real situation, the intensity and duration of rainfall are considered to evaluate changes in hydraulic stresses. Finally, the results show that the rainfall causes a considerable decrease in soil strength parameters in unsaturated soil, leading to the reduction of the pile group bearing capacity and slope stability.

scholarly journals Safety Factor of Unsaturated Soil Slopes considering the Intermediate Principal Stress Effect and Different Profiles of Matric Suction

2021 ◽  
Vol 2021 ◽  
pp. 1-10
Author(s):  
Zhaoxin Li ◽  
Changguang Zhang ◽  
Jingyuan Zhao ◽  
Qing Yan
Keyword(s):  
Shear Strength ◽  
Principal Stress ◽  
Safety Factor ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Matric Suction ◽  
Intermediate Principal Stress ◽  
Stress Effect ◽  
Soil Slopes ◽  
The Stability

Profiles of matric suction are critical for assessing the stability of unsaturated soil slopes, and the strength of unsaturated soils is affected by the intermediate principal stress. This study presents a theoretical formulation of safety factor for infinite unsaturated soil slopes under four different profiles of matric suction using the limit equilibrium method. The unified shear strength equation under plane strain conditions is adopted to capture the effect of intermediate principal stress on the strength of unsaturated soils. The proposed formulation of safety factor is found to have good comparability and broad applicability. The validity of the proposed formulation is demonstrated by comparing its predictions with the results of the extended shear strength method and the finite element method available in the literature. Parametric studies show that the effect of intermediate principal stress on the stability of unsaturated soil slopes is significant; the difference of safety factor among four suction profiles is pronounced, and the safety factor is highest for a linear suction profile. In addition, the safety factor changes with the infiltration depth in two stages, decreases with the slope angle, and increases with effective strength parameters. The results of this study are capable of providing beneficial guidance for optimization designs and disaster preventions of unsaturated soil slopes.

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.

The shear strength of unsaturated soils

1978 ◽  
Vol 15 (3) ◽  
pp. 313-321 ◽  
Author(s):  
D. G. Fredlund ◽  
N. R. Morgenstern ◽  
R. A. Widger
Keyword(s):  
Shear Strength ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Research Literature ◽  
Test Results ◽  
State Variables ◽  
Limited Data ◽  
Laboratory Test Results

The shear strength of an unsaturated soil is written in terms of two independent stress state variables. One form of the shear strength equation is[Formula: see text]The transition from a saturated soil to an unsaturated soil is readily visible. A second form of the shear strength equation is[Formula: see text]Here the independent roles of changes in total stress σ and changes in pore-water pressure uw are easily visualized.Published research literature provides limited data. However, the data substantiate that the shear strength can be described by a planar surface of the forms proposed. A procedure is also outlined to evaluate the pertinent shear strength parameters from laboratory test results.

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.

The relationship of the unsaturated soil shear strength to the soil-water characteristic curve

1996 ◽  
Vol 33 (3) ◽  
pp. 440-448 ◽  
Author(s):  
D G Fredlund ◽  
Anqing Xing ◽  
M D Fredlund ◽  
S L Barbour
Keyword(s):  
Shear Strength ◽  
Prediction Model ◽  
Soil Water ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Strength Function ◽  
Soil Parameters ◽  
Angle Of Internal Friction ◽  
Soil Water Characteristic Curve

The measurement of soil parameters, such as the permeability and shear strength functions, used to describe unsaturate soil behaviour can be expensive, difficult, and often impractical to obtain. This paper proposes a model for predicting the shear strength (versus matric suction) function of unsaturated soils. The prediction model uses the soil-water characteristic curve and the shear strength parameters of the saturated soil (i.e., effective cohesion and effective angle of internal friction). Once a reasonable estimate of the soil-water characteristic curve is obtained, satisfactory predictions of the shear strength function can be made for the unsaturated soil. Closed-form solutions for the shear strength function of unsaturated soils are obtained for cases where a simple soil-water characteristic equation is used in the prediction model. Key words: soil suction, soil-water characteristic curve, shear strength function, unsaturated soil.

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