• Proceedings of the Korean Geotechical Society Conference
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• 2000.11a
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• pp.377-384
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• 2000

The characteristics and prediction model of the shear strength for unsaturated residual soils was studied. In order to investigate the influence of the initial water content on the shear strength, unsaturated triaxial tests were carried out varying the initial water content, and the applicability of existing prediction models for the unsaturated shear strength was testified. It was shown that the soil - water characteristic curve and the shear strength of the unsaturated soil varied with the change of the initial water content. A sample compacted in the lower initial water content needs a higher suction to get the same degree of saturation while the shear strength of a sample with the lower initial water content displays a lower value. In order to apply the existing prediction models of the unsaturated shear strength to granite residual soils, a correction coefficient, α, on the internal friction angle, ø'was added.

• Journal of the Korean GEO-environmental Society
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• v.13 no.4
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• pp.45-51
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• 2012

The shear strength of weathered granite soils under unsaturated condition was evaluated by $K_0$ consolidated triaxial tests. Various matric suctions in the unsaturated triaxial tests were applied using suction-controlled triaxial test apparatus for weathered granite soils obtained in Daegu. Soil water characteristic curve (SWCC) laboratory tests for drying and wetting procedure were performed and van Genuchten curves were fitted by experimental results. The contribution of matric suction in unsaturated soils is directly correlated to effective stress and evaluated from SWCCs. The effective stresses were estimated from these SWCCs and the relationship between effective stress and unsaturated shear strength was determined. In the effective stress description, the unsaturated shear strength with respect to various suctions indicates unique relationship and almost the same as that of the saturated envelope.

• Journal of Korean Tunnelling and Underground Space Association
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• v.12 no.2
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• pp.117-127
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• 2010

Gound excavation is frequently executed in unsaturated soil conditions. In this paper, the effect of apparent cohesion in unsaturated soils on the ground behavior during underground excavation is studied. The VPPE (Volumetric Pressure Plate Extractor) test, the unsaturated triaxial test and the trap-door test were carried out to figure out how the behavior of soils varies depending on the variation of apparent cohesion. The test results show that the ground behavior is almost identical if the soil is either fully dry or fully saturated. However, if the soil is partially-saturated with the increase of water content, the ground behaves quite differently. In summary, the apparent cohesion in unsaturated soils plays key roles when excavating underground structures.

• Geomechanics and Engineering
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• v.23 no.4
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• pp.301-311
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• 2020

In this study, shear strength behavior of fine-grained soils was investigated under unsaturated conditions. The samples in the unsaturated state were subjected to a net normal stress (σ-u_a) of 40 kPa and different matric suctions (u_a-u_w) of 50, 100 and 150 kPa. The matric suction values applied in the triaxial tests were selected according to the bubbling pressures determined from the SWC curves. The study was carried out on prepared re-constituted cylindrical samples by uniaxial consolidation of soil slurries. First, consolidated drained (CD) triaxial compression tests were performed on the saturated samples and the cohesion and angle of internal friction were determined. After that, drained triaxial compression tests under matric suctions were performed on the unsaturated samples. In order to obtain unsaturated test results, cohesion and internal friction angle values of saturated samples were used. The nonlinear surface representing the shear strength surface was approximated consisting of two planes (double planar surface). The reason for the nonlinear behavior of some soils is that the amount of sand content contained in it is relatively high and the bubbling pressure/permanent water content value is relatively low.

• Proceedings of the Korean Society of Agricultural Engineers Conference
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• 2005.10a
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• pp.393-398
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• 2005

In order to treat flow problems for an unsaturated soils, it is required the mediation of the coefficient of permeability with variation of suction. The coefficient of permeability for soils can be determined by the direct or indirect methods. The direct methods of permeability can be performed either in the laboratory or in the field. The indirect methods can be predicted from the soil-water characteristics curve. Laboratory test methods to measure the coefficient of permeability can be categorized into two types, steady-state and unsteady-state. In this study, the coefficient of permeability for unsaturated soils was determined by the modified apparatus for steady state-method(Klute, 1972) and was predicted from the equations of Brooks and Corey(1964) and Van Genuchten(1980). Experimental results were compared with predicted coefficient of permeability and verified applicability of the two equations.

• Geomechanics and Engineering
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• v.19 no.1
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• pp.1-9
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• 2019

Shallow failures occur frequently in both engineered and natural slopes in expansive soils. Rainfall infiltration is the most predominant triggering factor that contributes to slope failures in both expansive soils and clayey soils. However, slope failures in expansive soils have some distinct characteristics in comparison to slopes in conventional clayey soils. They typically undergo shallow failures with gentle sliding retrogression characteristics. The shallow sliding mass near the slope surface is typically in a state of unsaturated condition and will exhibit significant volume changes with increasing water content during rainfall periods. Many other properties or characteristics change such as the shear strength, matric suction including stress distribution change with respect to depth and time. All these parameters have a significant contribution to the expansive soil slopes instability and are difficult to take into consideration in slope stability analysis using traditional slope stability analysis methods based on principles of saturated soil mechanics. In this paper, commercial software VADOSE/W that can account for climatic factors is used to predict variation of matric suction with respect to time for an expansive soil cut slope in China, which is reported in the literature. The variation of factor of safety with respect to time for this slope is computed using SLOPE/W by taking account of shear strength reduction associated with loss of matric suction extending state-of-the art understanding of the mechanics of unsaturated soils.

• Geomechanics and Engineering
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• v.31 no.2
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• pp.223-235
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• 2022

Bearing capacity of shallow foundations is often determined for either dry or saturated soils. In some occasions, foundations may be subjected to external loading which is inclined and/or eccentric. In this study, the ultimate bearing capacity of shallow foundations resting on partially saturated coarse-grained cohesionless and fine-grained cohesive soils subjected to a wide range of combined vertical (V) - horizontal (H) - moment (M) loadings is rigorously evaluated using the well-established limit equilibrium method. The unified effective stress approach as well as the suction stress concept is effectively adopted so as to simulate the behaviour of the underlying unsaturated soil medium. In order to obtain the bearing capacity, four equilibrium equations are solved by adopting Coulomb failure mechanism and Bishop effective stress concept and also considering a linear variation of the induced matric suction beneath the foundation. The general failure loci of the shallow foundations resting on unsaturated soils at different hydraulic conditions are presented in V - H - M spaces. The results indicate that the matric suction has a marked influence on the bearing capacity of shallow foundations. In addition, the effect of induced suction on the ultimate bearing capacity of obliquely-loaded foundations is more pronounced than that of the eccentrically-loaded footings.

• Journal of the Korean Geotechnical Society
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• v.33 no.6
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• pp.37-48
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• 2017

Soil-water characteristic curve (SWCC) has been widely used to estimate the shear strength and coefficient of permeability for unsaturated soils. In general, it is divided into the drying path in which the water is discharged and the wetting path in which the water is permeated, and it has a hysteresis indicating different suctions at the same volumetric water content. In reality the field behavior of unsaturated soils is much closer to the wetting path during the infiltration. The drying path has been practically used for various analyses because obtaining the wetting path takes longer than the drying path. Although many approaches for estimating wetting path have been studied till now, these are complex and do not fit well. Therefore, a simple method for estimating wetting path based on empirical approach in this study is proposed in unsaturated soils, and a feasibility study is conducted as well.

• Journal of the Korean Geotechnical Society
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• v.26 no.6
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• pp.21-28
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• 2010

Decomposed granite soil is the most common type of soils. The measurement of the stress-strain-strength behavior of anisotropic decomposed granite soils is very important for the deformation and stability analysis of slopes, retaining walls, excavations. A series of unsaturated-drained triaxial compression tests were performed to know unsaturated strength properties. The sample had three different angles of the axial (major principal) direction to the sedimentation plane (compaction plane): 0, 45 and 90 degrees. The compression strain of specimens subjected to an isotropic compression was strongly influenced by the sedimentation angle. In addition, the time dependence was independent of the sedimentation angle in relation to the deformation behavior during the secondary compression process. The effect of the sedimentation angle on the triaxial compression strength and deformation was clearly shown with low confining stress. The effect of the sedimentation angle on the compressive strength and deformation was more evident in saturated specimens. A new method of predicting the shear strength of unsaturated decomposed granite soils, considering compaction angles, was proposed.

• Proceedings of the Korean Geotechical Society Conference
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• 2004.03b
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• pp.334-341
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• 2004

The coefficient of permeability is one of the most important properties in unsaturated soils. The permeability varies with change in the water content as the soil water characteristic curve(SWCC) does. Thus the permeability curve of unsaturated soils has the similar shape with the soil-water characteristic curve(SWCC). Therefore, the methodologies have been studied to simply predict the unsaturated permeability from the SWCC. In this study, the experimental tests of SWCC and permeability were carried out for domestic weathered granite soils. The SWCC test results were fitted to Fredlund and Xing's SWCC equation and then it was found that there are some relationships between the parameters of SWCC equation and the basic soil properties. Accordingly we used an ANN(artificial neural network) model to obtain the SWCC parameters from the basic soil properties. Finally, the coefficients of permeability were predicted from these results by a prediction model.