Shear Strength Of Unsaturated Soils Research Articles

Influence of degree of saturation on the dynamic soil properties of a sandy soil

It is commonly assumed that the fully-saturated scenario represents the critical shear strength condition for geotechnical structure design; nevertheless, this is not the case when the dynamic shear strength of unsaturated soil is considered, because the water content affects the transfer of seismic waves through the porous medium and into a structure. It is important to better understand this behavior so the soil response during an earthquake could be established to prevent damage or failure of the structure. A series of cyclic triaxial tests with controlled matric suction was conducted to determine the principal dynamic parameters for a sandy soil for different degrees of saturation. Finally, to assess the influence of soil saturation on the dynamic soil properties of a sandy soil, a dynamic lateral load pile design was performed using Reese’s p-y curve method, and the parameter obtained from the laboratory testing were used to backcalculate the seismic coefficients used for structural design. Structural designs of laterally loaded piles considering different soil degrees of saturation demonstrated that the saturated soil condition while subjected to a dynamic condition does not represent the critical load case for seismic analysis.

UnSaLuDo: The development of an educational board game on unsaturated soil mechanics

Non-conventional tools of teaching are effective in delivering deep learning of scientific facts as well as skills associated with scientific disciplines. A well-known method to promote student engagement and learning in general is the use of games in classrooms. Games can provide a flexible option that could engage students, create active learning experiences, and enable them to experiment in safe playful environments. This paper presents UnSaLuDo, a board game developed by the GeoFUN group. The goal was to develop a tool that enables students to experience fun and integrational activities in a classroom setting while learning complex scientific targeted threshold concepts related to unsaturated soil mechanics. The topics covered by UnSaLuDo included fundamental concepts of cohesion and adhesion in liquids, surface tension, contact angle and capillary rise; water retention and water flow in unsaturated soils; volume change, deformation, and shear strength of unsaturated soils. This paper provides background information on the motivation for the development of the board game and an overview of the topics covered. The paper also presents all the components of the board game developed and suggests approaches for how it can be incorporated into geotechnics courses.

Effect of the optimum and residual moisture content on the strength characteristics of unsaturated sands

Soil exists mainly in unsaturated conditions. Therefore, accurate prediction of the soil shear strength for unsaturated conditions also becomes equally important for the geotechnical design of earth structures. This study primarily investigates the effect of the moisture content of unsaturated soil on its shear strength. The strength characteristics of silica sands with different grain sizes were studied using the modified triaxial apparatus and analytical methods. For this purpose, four series of triaxial compression tests on silica sands were performed by varying the moisture content of the test sample at compaction and shearing as optimum or residual moisture content. The test results showed that the test sample sheared at optimum and residual moisture content exhibited higher shear strength when the sample was initially compacted at residual and optimum moisture content, respectively. The moisture content at compaction and the soil grain size considerably influence the shear strength of unsaturated sandy soils. Furthermore, the analytical method used in this study for unsaturated soil shear strength prediction does not account for the effect of initial moisture content in predicting unsaturated soil shear strength.

Experimental observation and constitutive modelling of the shear strength of a natural unsaturated soil

Increasing frequency and intensity of extreme weather events in the Netherlands is raising attention on the unsaturated response of geo-infrastructures, promoting research projects to provide an overview of the impact of unsaturated conditions on the response of shallow soil layers and embankments, and to better address maintenance and mitigation measures. As part of this effort, we discuss the results of standard laboratory tests performed on initially unsaturated samples retrieved from the field and tested in natural conditions, as well as after controlled drying and wetting. The variation of the “undrained” (i.e. at constant water content) shear strength with the degree of saturation obtained from the laboratory tests aligns well with CPT measurements performed in the field. An elastic-plastic constitutive model with mixed isotropic-rotational hardening developed for saturated soft soils was extended to unsaturated conditions by following a robust approach previously developed for compacted clayey soils. Coupling between the mechanical and the hydraulic behaviour is provided by the water retention curve. The model nicely captures the response observed in the laboratory, until extreme dry conditions, which possibly alter the structure of the soil, the peak stress, and the brittleness after failure. The model is capable of reproducing the effects of the previous hydraulic history on the stress-strain behaviour observed from the laboratory tests over a wide range of degree of saturation.

The combination of splitting tensile strength and unconfined compression tests on unsaturated soils to measure the evolution of unsaturated shear strength

The paper presents experimental results from unconfined compression and splitting tensile tests used in combination in order to estimate the cohesion intercept of unsaturated soils. Samples of different soils in the form of slurry were subjected to various suction values using the axis translation technique in a pressure plate extractor and then these samples were removed and subjected to unconfined compression and splitting tensile tests. Mohr’ circles for the two loading conditions were drawn and the tangents plotted in order to obtain the cohesion intercept for each soil and each suction. Cohesion intercept was then plotted against suction for each soil and the angle of shear strength increase due to suction increase was estimated and compared to the value of the angle of shearing resistance of the fully saturated soil. The method allowed insight into the evolution of unsaturated soil shear strength of slurried fine-grained soils, and seems to be an interesting low-cost alternative compared to established controlled suction methods to estimate unsaturated shear strength, yet it is based on two assumptions: first that the failure envelope is linear in the range between zero vertical stress and the stress corresponding to unconfined compression loading, and second that the suction applied to the samples does not change significantly from the moment a sample is removed from the pressure plate extractor until it is subjected to loading. The former assumption seems a fair one given the range of stress involved; the latter was investigated by conducting an unconfined compression test with suction measurement during loading. This test indicated that the latter assumption is a fair one too. Furthermore, results of the angle of strength increase relative to suction φbwere normalised by the angle of shearing resistance of each soil indicating that a reasonable relation for the evolution of φbseems valid.

Contribution of capillary pressure to effective stress for unsaturated soils: Role of wet area fraction and water retention curve

Estimating the contribution of capillary pressure to the effective stress is critical for studying the mechanical and hydraulic behavior of unsaturated soils. Based on experimental evidence, we clarify that the degree of saturation (Sr) rather than matric suction (s) is more suitable as a variable for modeling Bishop's effective stress parameter (χ). The wet area fraction, defined as the ratio of the wet external area of soil particles to the total surface area, is then introduced as an alternative to the parameter χ. Because the contact area between soil particles and pore water is affected by both hydraulic (matric suction) and mechanical (volumetric deformation) states, the soil water retention curve (SWRC) is embedded in the proposed model to simulate the influences of pore size distribution and hydraulic hysteresis on shear strength. The proposed model requires only the material parameters of soils in the reference state and then can be used to predict the strength characteristics in other mechanical or hydraulic states. The shear strength data under different test conditions and the corresponding SWRC data are adopted to validate the new model. The results reveal that the wet area fraction could well predict the contribution of capillary pressure to the shear strength of unsaturated soils.

Slope Stability Analysis Method of Unsaturated Soil Slopes Considering Pore Gas Pressure Caused by Rainfall Infiltration

The variation of pore gas pressure caused by rainfall infiltration is an important factor that leads to slope failures. The purpose of this study is to propose a new slope stability analysis method that considers pore gas pressure and examines the effect of airflow on slope stability by using a numerical method. A water-air two-phase flow analysis was conducted to investigate the distribution of pore air pressure, pore water pressure, and water saturation triggered by rainfall infiltration. Then the variation of the load resulting from pore gas pressure was incorporated into the slope stability analysis method based on the unsaturated soil shear strength theory and the residual thrust method to simulate the influence of airflow on the Tanjiahe landslide in China. In order to study the infiltration behavior with respect to initial saturation, water and gas flow analyses were performed considering various initial states of saturation under similar settings. Results showed that the pore gas pressure between the slope surface and the slip band clearly varied and that it decreased during the process from the slide bed to the deep direction. Then, the pore water pressure formed in the saturated zone was transferred by the airflow to the slope toe. As a result, because the pore gas pressure gradient increased the thrust of the slide mass, the safety factor decreased over time. Moreover, in the first step, the magnitude of infiltration decreased with an increase in initial saturation, while when the magnitude dropped to the minimum value, it then went up with an increase in initial saturation. The maximum value was usually reached at a saturation degree of 0% or 100%. When evaluating slope stability, the safety factor obtained by the slope stability analysis method that considered the water-gas coupling effect was much lower than when it was not considered during the process of a similar seepage. The impact on the slope failure was significant and may provide a practical reference for hazard assessments to control rainfall-induced landslides.

Micromechanical-Based Shear Strength Equation Considering the Stress-State Effect for Unsaturated Soils

Unsaturated soil shear strength is a crucial and useful parameter for predicting geostructure stability, soil erosion, seasonal variation, and land management. Unsaturated shear strength measurement, however, is frequently costly, complex, and time-consuming. The main objective of this paper is to present a new generalized equation for the shear strength estimation of unsaturated soils. The proposed equation is derived from a micromechanical equilibrium condition considering the interaction of air, water, and solid phases. The particle contact area effect is taken into account in the proposed model, even though it is thought to be negligible in existing equations. In comparison with existing equations, the proposed one has the benefit of being able to capture the nonlinear influence of saturation degree and matric suction on unsaturated shear strength. The proposed equation is compared with several other existing equations as well as experimental data for four different soil types to verify its validity. The findings indicate that the proposed equation has a potential application in estimating unsaturated soil shear strength and that it outperforms existing equations. The stress state also has a substantial impact on the shear strength properties of unsaturated soils, which was extended to include in the proposed equation. The results reveal that the proposed equation is capable of accurately predicting the variation of the soil–water characteristic curve and unsaturated soil shear strength as a function of the stress state.

NMR-Based Measurement of AWRC and Prediction of Shear Strength of Unsaturated Soils

NMR-Based Measurement of AWRC and Prediction of Shear Strength of Unsaturated Soils

Study on the stability analysis of rainfall slope based on G-A model considering moisture content

In this paper, the moisture content on the wetting front is obtained by using the Van Genuchten (VG) model of unsaturated soil, and then the distribution of moisture content in the upper part of the wetting front is simplified as a trapezoid. The Green-Ampt (G-A) infiltration model of infinite slope with unsaturated characteristics is derived. The analytical expression of safety coefficient (FOS) of infinite slope with rainfall is solved by combining the limit equilibrium method with the unsaturated soil shear strength theory phase. The results show that: 1) compared with the traditional G-A model and the combined rectangular and 1/4 oval model, the upper part of the wetting front is simplified to a trapezoidal model, which has great advantages in infiltration rate and cumulative infiltration, especially when the slope is large or the rain intensity is heavy; 2) since the distribution of soil moisture content above the wetting front is considered, the matrix suction at the wetting front is not neglected, and the safety coefficient calculated by the method proposed in this paper is closer to the actual situation than the traditional G-A model.

Unsaturated soil shear strength of agricultural soils influenced by reclamation sequences in coastal China

AbstractAgricultural activities have raised increasing concern regarding their effects on soil properties and shear strength, which are related to root growth, crop yield and engineering stability. We collected undisturbed soil samples (0‐15 cm) from a No‐Reclaimed (NR) system, a Short‐term Reclaimed (SR) system for less than 7 years, and a Long‐term Reclaimed (LR) system for more than 30 years, in coastal reclamation areas. Direct shear tests were performed and soil water characteristic curves (SWCC) were developed for undisturbed soil samples at 96 soil sampling sites across five transects. Principal component analysis (PCA) was used to quantify tillage effects on soil properties and unsaturated shear strength including effective cohesion (c'), effective friction angle (φ'), and friction angle related with matric suction (φb). The LR system had significantly increased percentage clay and silt, organic matter (OM), microaggregates, gravity mean diameter (GMD) and effective cohesion (c'), but significantly decreased percentage sand, cation content and electricity conductivity (EC1:5). The SR system had increased OM and GMD compared to the NR system. c' and φb were significantly positively correlated with bulk density (ρb), clay, silt, OM and GMD. The correlations between the internal friction angle and these properties were opposite to those of cohesion. Two principal components (PC1 and PC2) explained the majority of the soil variables. PC1 had a strong relationship with clay, silt, OM, c' and sand, while PC2 had a strong relationship with EC1:5, SAR, GMD and ρb. Thus, PC1 can be regarded as an indicator of soil texture; similarly, PC2 was able to represent soil salinity properties and structure. Confidence ellipses significantly separated the differences in soil between different reclaimed systems. The use of principal components (PCs) in the linear regression reduced the variables and estimated c' and φ' accurately. This study increases the current understanding of the relationship between soil properties and shear strength under different tillage systems, and assesses the efficacy of soil property evaluation after reclamation in coastal areas.Highlights Effect of reclamation on soil properties and unsaturated shear strength was investigated. Long‐term reclamation increased unsaturated shear strength significantly. Soil properties are well explained by principal components (PC1 and PC2). Principal component regression is recommended for predicting unsaturated soil shear strength.

Disturbed state concept and non-isothermal shear strength model for unsaturated soils

Shear strength of unsaturated soils is an important engineering property that is required for addressing geotechnical problems, the prediction of which remains to be a challenging task for design engineers due to the complex interaction problem. This study presents a new shear strength equation based on the micromechanical model and the disturbed state concept for unsaturated soils. The original point of this study is considering the solid contact area ratio which was neglected in most of the existing equations. Using the proposed model, the non-linear relationship between the matric suction, saturation degree, and the shear strength of unsaturated soils are described. Validation of the shear strength model was verified against the experimental data and several current models on six different types of soils. The results indicate that the proposed model has a good performance in predicting the shear strength of unsaturated soils, and generally is better than other existing models. In response to varying climatic conditions, the analytical model was then extended to consider the effect of temperature on the shear strength of unsaturated soils. The comparison between predicted and measured results was carried out on compacted silt for three different temperatures. The results show that the proposed model is capable of accurately predicting changes in unsaturated shear strength as a function of temperature.

An analytical model for predicting the shear strength of unsaturated soils

The prediction of shear strength of unsaturated soils remains a significant challenge due to their complex multi-phase nature. In this paper, a review of prior experimental studies is first presented in order to outline important pieces of evidence, limitations and some design considerations. Then, an overview of existing shear strength equations is summarised, with a brief discussion. A micromechanical model with stress equilibrium conditions and multi-phase interaction considerations is presented to provide a new equation for predicting the shear strength of unsaturated soils. The validity of the proposed model is examined using published shear strength data for different soil types. The shear strength predicted by the analytical model was found to be in good agreement with the experimental data and to provide high performance in comparison with existing models. Evaluation of the results using two criteria – the average relative error and the normalised sum of squared error – proved the effectiveness and validity of the proposed equation. Using the proposed model, a non-linear relationship between shear strength, saturation degree, volumetric water content and matric suction was observed.

Stability analysis of unsaturated soil slopes under reservoir drawdown and rainfall conditions: steady and transient state analysis

A finite-element upper-bound procedure is presented to assess the stability of a soil slope under reservoir drawdown and rainfall conditions. In order to overcome the scientific challenge of generating a kinematically admissible velocity field, discretizing the problem of interest into finite elements is carried out for this specific purpose, and based on which the upper bound formulation of slope safety factor is derived. The slope stability analysis is transformed to the optimization of a linear programming model under four constraint conditions, by virtue of an interior-point algorithm. Steady-state and transient water flows are discussed. Based on the soil–water characteristic curve (SWCC), matric suction above the phreatic surface is considered so as to provide a reliable assessment for unsaturated slope stability. True cohesion of soils and apparent cohesion stemmed from matric suction are combined to describe the shear strength of unsaturated soils. After validating the robustness of the proposed procedure by limit equilibrium and FEM, it is further applied to evaluate the slope stability under (rapid, general, slow) reservoir drawdown and rainfall conditions.

Shear Strength of Unsaturated Soils with Different Plasticity

A series of shear box tests were conducted on four different types of soils in this study. The first set , (clay1) is the host clay, Middle Delta Nile clay “MDNC” which was obtained from Mid Delta Nile zone, Qalyubiyya Governorate, Egypt. The second set, (clay2) consists of the host clay MDNC but mixed with 40% sand. The third set of tests (clay3) was carried out on natural low plastic clay samples brought from El-Tal El-Kbeer area in Ismailia Governorate. The fourth set (clayey sand -SC) consists of 25% of the host clay MDNC mixed with 75% sand. The purpose is to predict the unsaturated shear strength for various soil types. The results illustrate that for clay1, clay2, and clay3, the relationships between the unsaturated shear strength and matric suction are approximated to two linear failure envelopes intersected at matric suction equals to plastic limit. Each segment has an angle of internal friction due to matric suction  b ranging from 1.80 o to 2.90o for the first zone and  b ranging from 0.28 o to 0.20o for the second zone. Meanwhile, for SC soil it appears to have one linear failure envelope for all saturation levels with angle of internal friction due to matric suction;  b=1.19 o.. Design- Expert® software applied the least square method (LSM) to fit a mathematical model for the experimental data. The Response Surface Methodology (RSM) of the program is used to predict the required model. The paper proposed two general equations to calculate the shear strength of unsaturated clays and one equation for granular soils, which are presented at the end of the paper with their coefficients.

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

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.

Bearing capacity evaluation for shallow foundations on unsaturated soils using discretization technique

Bearing capacity evaluation for shallow foundations is conventionally completed according to the equations derived for dry or saturated soils neglecting the effect of matric suction. However, many shallow foundations are mostly constructed upon the soil layer above the groundwater table, where a large portion of soils under shallow foundations is in fact in unsaturated conditions. The failure of shallow foundations is usually related to shear failure which is one of the main engineering properties required in foundation design. The shear strength of unsaturated soils depends on the matric suction which usually changes with depth. Since the discretization technique of limit analysis is particularly suitable for considering the variation of soil parameters, the bearing capacity for shallow foundations considering the shear strength of unsaturated soils is therefore evaluated by this technique. The results obtained by the present technique are compared with the ones obtained from different authors, and good agreement is observed. Then a parametric analysis is performed using the present technique, and the bearing capacity for shallow foundations are given considering different soil types, foundation widths, effective internal friction angle φ′, air-entry pressure 1/α with various n values, different water table depth with surface flux boundary conditions, different unit weight of soil γ, and surcharge load q0. Some useful conclusions are given and a useful method is therefore provided for the bearing capacity evaluation of shallow foundations on unsaturated soils.

Efficient approach in modeling the shear strength of unsaturated soil using soil water retention curve

Efficient approach in modeling the shear strength of unsaturated soil using soil water retention curve

Soil-water retention curve model for fine-grained soils accounting for void ratio–dependent capillarity

This paper presents a soil-water retention curve (SWRC) model for fine-grained soils. Compared with existing studies, the proposed model accounts for the distinct roles of the volume change of soils on capillarity and adsorption mechanisms. The capillary water is described by a relation that includes the characteristics of the pore-size distributions as parameters, while the absorbed water is modeled by a novel proposition that both considers the phenomenon of capillary condensation and allows for the decoupling between the degree of capillary and adsorptive saturation. Based on this feature, the void ratio effects are considered in a way in which they only affect capillary water, i.e., consistent with how volume change influences soil microstructures. The relative contributions of void ratio effects and hydraulic hysteresis on the path- and history-dependence of a SWRC in Sr–s–e space, where Sr is degree of saturation, s is matric suction, e is void ratio, for deformable unsaturated soils are examined. The significance of discriminating the effects of volume change on capillary and adsorptive water is illustrated by applying the SWRC model to computing the shear strength of unsaturated soils with different void ratios. The model performance is assessed by comparing against test data reported for four types of fine-grained soils in the literature and those tested for natural loess in this work.

Improved prediction of water retention characteristic based on soil gradation and clay fraction

As an important function for determining both the hydraulic and mechanical properties of soils, the soil–water characteristic curve (SWCC) based on the particle size distribution underestimates the water content at high suction head. In this study, the quantitative relationship between the clay content and residual pore fraction is established based on soil gradation parameters (uniformity coefficient Cu and curvature coefficient Cc); the van Genuchten equation is employed to describe the soil–water characteristic curve; and then a semi-physical and semi-statistical method based on clay content is proposed to predict water-holding of unsaturated soils, especially for the soil with higher clay contents or low water contents (high matric suctions). Eighty-seven soil samples containing nine texture classes from the unsaturated soil hydraulic database (UNSODA) are used to verify the proposed approach. Two traditional models proposed by Hwang et al. (2011) and Meskini-Vishkaee et al. (2014) are introduced to compare the predictive effect with the improved method. Although three methods all show good performance, the improved prediction method based on uniformity coefficient (Cu=d60/d10) has a better estimation accuracy than the two others according to statistical parameters, including root-mean-square error (RMSE), sum of square errors (SSE), and determination coefficient (R2). The ranking order of average uniformity coefficients in the three soil classes (sandy, silty, and clayey) agrees well with the ranking order of clay contents. Meanwhile, various dy/dx are used to characterize soil gradation and have similar goodness of fitting to residual pore fraction. In summary, the improved prediction model can effectively predict soil–water characteristic curves, especially for soil with low water contents (high matric suctions) or high clay contents. The model benefits the predictions of mechanical properties, permeability coefficient, shear strength and slope stability of unsaturated soil at low water contents.