Influence of stress history on the strength parameters of an unsaturated statically compacted soil

1999 ◽  
Vol 36 (2) ◽  
pp. 251-261 ◽  
Author(s):  
Tomoyoshi Nishimura ◽  
Yasunari Hirabayashi ◽  
Delwyn G Fredlund ◽  
Julian K-M Gan
Keyword(s):  
Shear Strength ◽  
Unsaturated Soil ◽  
Stress Ratio ◽  
Matric Suction ◽  
Strength Parameter ◽  
Direct Shear ◽  
State Variables ◽  
Total Stress ◽  
Stress History ◽  
Compacted Soil

Unsaturated soils are generally near the ground surface and are commonly overconsolidated due to environmental effects. The stress state variables for an unsaturated, in situ profile consist of the net total stress, (σ - ua), and matric suction, (ua - uw), where σ is the total stress (in three directions), ua is the pore-air pressure, and uw is the pore-water pressure. These stress state variables control the behavior of the unsaturated soil. A total stress ratio, TSR, was used in this study as a measure of the stress history. The total stress ratio is defined as the ratio of the compaction pressure to the current confining pressure. Shear tests were conducted using a modified direct shear apparatus on a statically compacted unsaturated soil subjected to various total stress ratios with controlled matric suction. The shear strength parameters (i.e., ϕ', ϕb, and c') for an unsaturated soil were measured using the modified direct shear apparatus. The total stress ratio influences the shear strength parameter ϕb of a compacted soil. The shear strength parameter ϕb decreases with matric suction regardless of the loading history. For a compacted soil with a total stress ratio of 1.0, ϕb was higher than that for the soil tested at a total stress ratio greater than 1.0, regardless of increase in matric suction, and was shown to be influenced by loading history.Key words: unsaturated soil, shear strength, stress history, compacted soil, direct shear test, matric suction.

scholarly journals Study on Water Characteristic Curve and Shear Characteristics of Typical Unsaturated Silty Clay in Shaoxing

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-15
Author(s):  
Ruiqian Wu ◽  
Youzhi Tang ◽  
Shaohe Li ◽  
Wei Wang ◽  
Ping Jiang ◽  
...  
Keyword(s):  
Shear Strength ◽  
Internal Friction ◽  
Unsaturated Soil ◽  
Direct Shear Test ◽  
Shear Test ◽  
Characteristic Curve ◽  
Friction Angle ◽  
Matric Suction ◽  
Silty Clay ◽  
Direct Shear

In order to probe into one simplified method to predict the shear strength of Shaoxing unsaturated silty clay, the test method combining unsaturated soil consolidation instrument and conventional direct shear instrument is used to study the shear strength, and the method is compared and verified with the results of equal suction direct shear test. The research results show that the soil water characteristic curve fitted by the measured data points and VG model has obvious stage characteristics in the range of 0~38 kPa, 38~910 kPa, and 910~10000 kPa. The shear strength of unsaturated soil measured by consolidation meter combined with conventional direct shear test is in good agreement with that measured by equal suction direct shear test in the range of 0~500 kPa. The results show that the shear strength, total cohesion, and effective internal friction angle of soil increase slightly with the increase of matric suction in the range of 0~38 kPa. When the matric suction increases from 38 kPa to 500 kPa, the shear strength and total cohesion force of the soil have similar stage characteristics with the SWCC, which first increases and then tends to be stable, while the effective internal friction angle changes slightly. Finally, taking the air-entry value as the demarcation point, an improved model of unsaturated shear strength is proposed by analyzing the error value. Compared with the measured value, the absolute value of relative error is basically kept in the range of 5%~10%, which is close to the measured value.

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.

Shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil

2010 ◽  
Vol 47 (10) ◽  
pp. 1112-1126 ◽  
Author(s):  
Md. Akhtar Hossain ◽  
Jian-Hua Yin
Keyword(s):  
Shear Strength ◽  
Normal Stress ◽  
Matric Suction ◽  
Water Retention Curve ◽  
Direct Shear ◽  
Normal Stresses ◽  
Soil Water Retention Curve ◽  
Strength Data ◽  
Decomposed Granite ◽  
Completely Decomposed Granite

Shear strength and dilative characteristics of a re-compacted completely decomposed granite (CDG) soil are studied by performing a series of single-stage consolidated drained direct shear tests under different matric suctions and net normal stresses. The axis-translation technique is applied to control the pore-water and pore-air pressures. A soil-water retention curve (SWRC) is obtained for the CDG soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus. Shear strength increases with matric suction and net normal stress, and the failure envelope is observed to be linear. The apparent angle of internal friction and cohesion intercept increase with matric suction. A greater dilation angle is found at higher suctions with lower net normal stresses, while lower or zero dilation angles are observed under higher net normal stresses with lower suctions, also at a saturated condition. Experimental shear strength data are compared with the analytical shear strength results obtained from a previously modified model considering the SWRC, effective shear strength parameters, and analytical dilation angles. The experimental shear strength data are slightly higher than the analytical results under higher net normal stresses in a higher suction range.

scholarly journals Relationship between the matric suction and the shear strength in unsaturated soil

2020 ◽  
Vol 13 ◽  
pp. e00441
Author(s):  
Idris A. Abd ◽  
Mohammed Y. Fattah ◽  
Haidar Mekkiyah
Keyword(s):  
Shear Strength ◽  
Unsaturated Soil ◽  
Matric Suction

A Study of a Large Compacted Clay Embankment-Fill Failure

1965 ◽  
Vol 2 (3) ◽  
pp. 274-286
Author(s):  
Alan E Insley
Keyword(s):  
Shear Strength ◽  
Stress Analysis ◽  
Safety Factor ◽  
Effective Stress ◽  
Soil Samples ◽  
Average Moisture Content ◽  
Compacted Clay ◽  
Total Stress ◽  
Compacted Soil ◽  
Effective Stress Analysis

The paper describes the failure during construction of a proposed 70 ft. high railway embankment fill. The fill was built of a uniform clay of medium plasticity which was used at an average moisture content of 3 per cent greater than had been provided for in the design. The fill failed under its own weight when it reached a height of 55 feet.In order to assist in the design of stabilizing works three test holes were drilled in the fill and soil samples recovered. Properties of field compacted and laboratory compacted soil samples are compared. The age of both types of samples is shown to have a significant effect on the test results.Both total and effective stress analyses of the embankment at failure have been performed using the laboratory values of soil strength. The total stress analysis gives a safety factor of 1.0 at failure whereas the effective stress analysis gives a safety factor of 1.2. The hazards of choosing the correct value of laboratory shear strength for the total stress analysis are discussed.

scholarly journals Analysis of the suction evolution during direct shear test in a silty sand soil

2020 ◽  
Vol 195 ◽  
pp. 03031
Author(s):  
Omar AL-Emami ◽  
Gabriela M Medero ◽  
Fernando A M Marinho ◽  
Melis Sutman
Keyword(s):  
Shear Strength ◽  
Water Content ◽  
Large Scale ◽  
Matric Suction ◽  
Vertical Stress ◽  
Silty Sand ◽  
Experimental Program ◽  
Direct Shear ◽  
Initial Void Ratio ◽  
Study Results

Shear strength of soils is one of the essential parameters for analysing and solving divers geotechnical problems (e.g. the bearing capacity of shallow footings pile foundations, slope stability and earth embankments). In this study, a series of conventional large-scale (300 X 300 mm) direct shear tests were carried out on saturated and constant water content silty sand specimens at ei = 0.6 and 1.0 tested under applied vertical stresses of 100, 200, or 400 kPa to investigate the influence of matric suction on the shear strength characteristics of the tested material. A loading steel cap was modified to allow the direct measurements of the matric suction using two commercial available Equitensiometer suction probes (EQ3). The experimental program indicated that, for both studied void ratios, the obtained shear strength of specimens under constant water content is found to be distinctly greater than those obtained from saturated samples. The results showed that the samples compacted at ei = 1.0 exhibited collapse behaviour during saturation stage, whereas same samples did not show any volume change during stabilisation stage when tested under constant water content condition. The study results also showed that the matric suction reduction during consolidation stage depends on initial void ratio of the tested samples as well as the level of applied vertical stress. Moreover, the matric suction evolution during shearing process of both studied void ratios specimens decreased with increasing the level of applied vertical stress.

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.

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.

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.

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