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Unsaturated soils are the major component of both natural and artificial earth structures, and they exhibit less predictable behavior than saturated soils. Presently, the world is experiencing pronounced impacts from climate change. Sudden heavy rainfalls and droughts both result in fluctuating soil moisture levels, meaning that soil responds differently to infrastructure loads. Accordingly, a better understanding is needed of failure causes in unsaturated soils. This research was conducted to investigate the coupled behavior of unsaturated soil with varying degrees of compaction and moisture change. Samples were prepared at 81% and 88% degrees of compaction with moisture content ranging from 10 to 25% on both sides of optimum moisture content for silty soil. Different parameters such as suction, porewater pressure, and deformation were carefully measured at every stage of the triaxial test, from sample placing to the shearing phase. The results show that there is more compressive soil behavior when there is a lower degree of compaction, i.e., 81%, along with moisture content of 15–25%. Where there is a higher degree of compaction at 88%, dilative behavior is more evident, which changes to compressive behavior as the moisture content increases. Any change in moisture content (increase or decrease) during the lifecycle of any earthen structure necessitates a long time to stabilize its suction value (decrease or increase) if it compacted to a degree that still exhibits a compressive response. This slow recovery of suction, specifically from the wetting to the drying cycle, is not favorable for soil stability upon loading. [ABSTRACT FROM AUTHOR]

The presence of unsaturated soil is critical in geotechnical engineering since the matric suction may aid in accommodating the pile shaft capacity. The design of piles can be optimized by incorporating unsaturated soil mechanics principles. Hence, the amount of waste materials can be reduced, the duration of pile installation can be expedited, and the amount of energy used for casting the pile can be optimized, resulting in more sustainable design and construction of piles. Conventional α, β, and λ methods and modified α, β, and λ methods are the common models that are used for calculating the shaft capacity by incorporating soil–water characteristic curves (SWCCs). However, in our opinion, we feel that the investigation of the influence of seepage infiltration due to rainfall on the shaft capacity of piles, calculated using both analytical means and numerical analysis, has been dealt with inadequately in past studies. The objective of this study is to investigate changes in the shaft pile capacity according to suction changes due to rainwater infiltration for the greater reliability of the pile design, using both analytical and numerical studies with the finite element method (FEM). Sand and kaolin, which are typical components of coarse-grained and fine-grained soil, are used in this study. The laboratory results were incorporated into PLAXIS 3D (Version 22), and a coupled analysis was carried out, utilizing the meteorological conditions in Astana. The results showed that the decreases in matric suction in sand and kaolin are similar after their subjection to rainfall, yet sand produces a higher shaft capacity compared to kaolin. The modified β method offers a higher shaft capacity compared to the other methods due to the effective stress factors being taken into account. The modified α and λ methods are recommended for short piles because they are more sustainable, whilst the modified β method is preferable for long piles. Overall, unsaturated soil conditions should be applied to optimize the foundation design since they generate a higher shaft capacity. [ABSTRACT FROM AUTHOR]

The soil–water characteristic curve (SWCC) is a key constitutive relationship for unsaturated soil which can be unimodal or bimodal. For the fine-grained compacted subgrade soil with a bimodal pattern, the determination of SWCC is complicated and needs a wide-range suction measurement. In this paper, the bimodal SWCC of a subgrade soil derived from the compaction condition was measured and determined by incorporating pore size distribution. For this purpose, a series of laboratory tests were conducted, including the pressure plate method, filter paper method, and vapor equilibrium method, which were used to measure SWCC at the low, medium, and high suction range, respectively. The pore size distribution (PSD) data were obtained by mercury intrusion porosimetry (MIP) tests and used to predict SWCC. Based on the analysis of hydraulic paths and SWCC-PSD correlations, the SWCC of the subgrade soil should be determined to follow the actual hydraulic path. SWCC within a low suction range can be filled by PSD-based data to improve the fitting accuracy. Then, a graphical method is applied to predict the bimodal SWCC by combining the filter paper method, vapor equilibrium method, and PSD-based data. The prediction curves fit well with the test data for all selected compaction conditions. Furthermore, the prediction method can still provide good prediction performance in the absence of high suction section data, which is beneficial for the application of bimodal SWCC. [ABSTRACT FROM AUTHOR]

Granite residual soil is a special regional soil with special mineral composition and pore structure characteristics, which is easy to induce serious geological disasters or engineering problems, so it is particularly important to study its mechanical properties of unsaturated soil and its control mechanism. However, the effects of dry density and initial water content on soil–water characteristic curve (SWCC) and their mechanisms are still unclear. Therefore, samples with different dry densities (1.30 g/cm3, 1.50 g/cm3, 1.70 g/cm3) and initial water content (14 %, 20 %, 22 %) were set up in this paper. SWCC test was conducted on the two groups of samples under the humidification path and dehumidification path using filter paper method. Combined with scanning electron microscopy (SEM) test and pore-size distribution (PSD) test, the influence mechanism of different micro-pore structure on SWCC and hysteresis characteristics of granite residual soil was analyzed qualitatively and quantitatively. The results show that the samples with different dry densities basically coincide with each other in the high suction segment. The larger the dry density is, the smaller the range of transition zone in the low suction segment is. As the initial water content of the sample increases from 14 % to 22 %, SWCC changes from a single increase curve to a double increase curve, and the corresponding pore-size distribution curve (PSDC) changes from a trimodal curve to a bimodal curve. The SWCC of granite residual soil has obvious hysteresis effect, and the hysteresis area becomes smaller with the increase of dry density. The inflection point exists in the hysteresis area of SWCCS with higher initial water content. The distribution range of macropore is determined by dry density, and the distribution range of small pore is determined by initial water content. The transformation of SWCC from a single increase curve to a double increase curve is mainly controlled by the distribution range of small pores. The bottleneck effect in the migration process of pore water in the soil and the pore redistribution during water intake and water loss are the main reason for the hysteresis of SWCC. The results of this work provide some guidance for the study of unsaturated soil mechanical properties of granite residual soils.

AbstractThe contact angle plays an important role in matric suction of unsaturated soils, but few studies consider the effect of contact angle hysteresis on matric suction. In this paper, taking sandy soil as an example, the influence of contact angle hysteresis on the suction of unsaturated soil matric is studied by soil particle model calculation and experimental method. The results show under the same water content condition, the smaller the contact angle is, the larger the matric suction is and the phenomenon of water repellency appears. Under the low water content of unsaturated soil, when the contact angle is less than 90°, it will also produce water repulsion phenomenon, which is different from the hydrophilic characteristics of the contact angle is less than 90° in the capillary law. When the water content is the same, with the increase of contact angle, matric suction will decrease. With the increase of water content, matric suction is not affected by soil contact angle. In the process of dehumidification, the apparent contact angle of the sandy soil with the same water content less than receding contact Angle; there is a difference in matric suction between continuous measurement method and discontinuous measurement method, and continuous measurement’ matric suction greater than discontinuous measurement’ matric suction. This study provides some theoretical basis and reference for the study of matric suction of unsaturated soil.

Climate change is creating more extreme rain events and causing more landslides in recent years. Most of the previous studies focused on large-scale slopes such as mountainous areas. For the problem of small-scale landslides in urban areas, a hydro-mechanical analysis of soil slopes under rainfall is also demanded. In this study, a numerical model of a slope under rainfall and load is established by COMSOL Multiphysics. To facilitate the analysis for practical purposes, two built-in modules are used to model a unidirectional coupling of hydro‐mechanical behaviors of unsaturated soil slopes. The Richards’ module is first adopted to model an unsaturated soil slope under rainfall. Then the Extended Barcelona Basic module that considers the matric suction is adopted as the constitutive model. The effect of rainfall duration and saturated hydraulic conductivity on stress, settlement, and pore-water pressure are studied. When the rainfall proceeds, the matric suction head keeps reducing constantly. For a fixed location, the settlement decreases as ks increases. The wetting front is more apparent as the ks is reduced as the curvature of the pore pressure profile is more pronounced. The matric suction at a steady state is decreased when the ks becomes small. The simulated soil suction is comparable to these field monitoring data.

Soil improvement is often done through mixing the granular soils with fine-grained materials to obtain densified soil matrix and eventually increase the bearing capacity of the soil. In this paper, sand was mixed with three different quantities of bentonite. Then, the hydromechanical behaviour of sand-bentonite mixtures was evaluated at different matric suctions. Suction-controlled triaxial test and the pressure plate extractor were used to obtain the experimental data needed to derive the stress-strain curves and the Soil Water Characteristics Curve (SWCC), respectively. The results showed that the apparent cohesion that developed due to the existence of matric suction was increasing with the bentonite content increase. Residual suction value was increasing as the bentonite content increasing. It was concluded that the increase of water retention capacity has made the sand-bentonite samples responds very significantly with changing of matric suction. Therefore, addition of fine materials to granular soils would make the soil prone to wetting collapse upon reaching the saturation condition during the rainy seasons.

In order to reflect the creep characteristics of unsaturated silty clay, a triaxial compression consolidation drainage creep test was conducted under the condition of controlling the matric suction. According to the results of the creep test, combined with the empirical models, Mesri model and Log-modified model, the relationship between the initial tangent modulus and the matric suction was established, and two empirical models of unsaturated soil creep considering the effect of matric suction were constructed. The study confirmed the stress–strain through the ε/D–ε relationship curve, and determined the parameters F and n through power function. The methods for determining the strain–time relationship parameters of the two improved models are different. The improved Mesri model was obtained by fitting the ln ε–ln t relationship, while the improved Log-modified model was solved by the BFGS algorithm and the general global optimization method. By comparing the two improved models of unsaturated soil creep tests, it was found that the improved Mesri model can more accurately describe the creep characteristics of unsaturated soils, which confirms the rationality and feasibility of this model and method.

This study proposes a new soil-water characteristic model to predict the variation of volumetric water content with the matric suction pressure for unsaturated soil conditions. The proposed model has been built utilizing the nondestructive electrical properties of the unsaturated soil. In addition, nine previous models have been collected and used in parallel with the new model to predict the soil-water characteristic relationships for four sets of different experimental data. The study has clearly revealed that the new proposed model agreed very well with the experimental data with R2 values of 0.90, 0.93, 0.95, and 0.92 for the four studied cases respectively. However, none of the collected nine models had the power of the new proposed model in terms of predicting the experimental data for all the studied cases. Furthermore, it was concluded that the new model parameters have been correlated strongly with the final volumetric water content. [ABSTRACT FROM AUTHOR]

This research presents a modified numerical simulation approach for predicting approximate values of unsaturated soil parameters with low water content. In order to understand how unsaturated soils behave and to create the soil–water characteristic curve, the suggested model attempts to predict the following parameters. First, the relationship between the water content and the matric suction in terms of particle separation distance. Secondly, the relationship between the effective stress parameter χ and the filling angle θ when the separation distance between soil particles is taken into account. Thirdly, the relationship between suction stress and filling angle θ , in terms of particle separation distance. Fourthly, the effect of particle separation distance on the curve of tensile strength in terms of the degree of saturation. The updated model assumes that all the particles are equally dispersed spheres of the same size. In this study, the updated model is exclusively applied to simple cubic packing order with zero contact angle. The model may also be used to predict matric suction, effective stress parameter, suction stress, and tensile strength while combining experimentally determined soil properties. This removes the requirement for sophisticated measuring devices or specific test procedures to illustrate how the separation distance affects the behavior of unsaturated soils. Compared to the experimental data, our model has shown positive results. In this paper, all curves have been programmed using MATLAB. [ABSTRACT FROM AUTHOR]

This paper discusses the second-order reliability-based design optimization (SORBDO) of unsaturated finite slopes. An efficient search algorithm is proposed to locate the most probable slip surfaces using the Morgenstern–Price method of slices. The variability of fitting parameters of the soil water characteristic curve (SWCC) and shear strength parameters have been taken into account for evaluating the reliability of unsaturated finite slopes. Six unsaturated shear strength (USS) models are considered for the analysis of unsaturated soils. The most conservative model for the optimum design of finite slopes is recommended. Moreover, the results indicate that the mean and standard deviation of the fitting parameters of SWCC have a significant effect on the critical centers, critical slip surfaces, and reliability index of the unsaturated finite slopes. The results demonstrate a significant change in the critical centers associated with critical slip surfaces due to the change in the mean and coefficient of variation of the fitting parameters of SWCC. Statistical analysis of the experimental data shows that a positive correlation exists between fitting parameters of SWCC related to air entry value (af) and residual water content (mf). Negative correlations are found between the fitting parameters of SWCC related to air entry value (af) and slope (nf). Similarly, the SWCC fitting parameters related to residual water content (mf) and slope (nf) have negative correlations for clayey soils. It is observed that the correlated value of the reliability index is overestimated with reference to the uncorrelated value for clayey soil slopes. The proposed design charts are beneficial for engineers and practitioners for the sustainable design of unsaturated finite slopes. Overall, the developed rigorous SORBDO framework offers a more rational approach for assessing critical failure surfaces and critical centers, considering the interdependency of random variables. [ABSTRACT FROM AUTHOR]

In general, the ultimate bearing capacity (UBC) of shallow foundations on unsaturated soils is characterized by the conventional shear strength (SS) parameters in which saturated theories are applied. However, in this case, it is clear that the foundations designed using the obtained values from the saturated cases not be economical. In recent years, procedures have been developed to estimate the UBC of foundations on unsaturated soils, that take into account drained and undrained loading conditions. However, these studies generally concentrate on sandy soils. The validity of the results proposed in the literature should be tested for other soils. Therefore, this paper includes a conventional direct shear box (DSB) test to determine the unsaturated SS of statically compacted silty soil, and a series of model tests were performed to determine the foundation's UBC. In the experimental model setup, the UBC values of different types and sizes of model foundations on silty soil layers with a different soil saturation degrees (SSDs)/matric suctions (MSs) and different void ratio values were measured. In addition, the soil-water characteristic curves (SWCCs) and SS parameters of unsaturated silt were obtained. Using the experimental results, a new equation is proposed for the characterization of the UBC of shallow foundations on unsaturated silty soils. Using this equation, the UBC of unsaturated soils can be determined based on the results of unconfined compressive strength tests (UC) measured on unsaturated soil samples and based on the degree of saturation and the fitting parameter. The results indicate that the measured bearing capacity values obtained via the model footing test, shows a good consistency with those obtained by the proposed equation. [ABSTRACT FROM AUTHOR]

In current study, a test program is designed to observe the influence of wetting stress paths on behavior and instability of unsaturated soil in stress state space. The mechanical behavior was studied by performing three test series in constant matric suction plane. In first series, water infiltration was performed together with shearing starting from zero (0 kPa) deviatoric stress. In second series, first water was infiltrated at zero (0 kPa) deviatoric stress then samples were sheared in drained pore water pressure conditions. In third series, water was first infiltrated at zero deviatoric stress then specimens were sheared in undrained pore water pressure conditions. Instability was studied in constant shear stress plane by first shearing soil up to predefined level of deviatoric stress then infiltrating water keeping deviatoric stress constant on the specimen. This research also describes how unsaturated soil behavior and instability is varied with variation in net confining stresses when suction is decreased from initial measured value to start water infiltration with different wetting stress paths and their effect on shear state and failure surface in stress state space. The test results revealed that the movement of stress paths was independent of net confining stress, matric suction and shearing conditions employed in the study. [ABSTRACT FROM AUTHOR]

The soil water retention curve (SWRC) or soil–water characteristic curve (SWCC) is a fundamental feature of unsaturated soil that simply shows the relationship between soil suction and water content (in terms of the degree of saturation and volumetric or gravimetric water content). In this study, the applications of the SWRC or SWCC have been extensively reviewed, taking about 403 previously published research studies into consideration. This was achieved on the basis of classification-based problems and application-based problems, which solve the widest array of geotechnical engineering problems relevant to and correlating with SWRC geo-structural behavior. At the end of the exercises, the SWRC geo-structural problem-solving scope, as covered in the theoretical framework, showed that soil type, soil parameter, measuring test, predictive technique, slope stability, bearing capacity, settlement, and seepage-based problems have been efficiently solved by proffering constitutive and artificial intelligence solutions to earthwork infrastructure; and identified matric suction as the most influential parameter. Finally, a summary of these research findings and key challenges and opportunities for future tentative research topics is proposed. [ABSTRACT FROM AUTHOR]

In order to reflect the whole deformation process of unsaturated foundation soil, the consolidated drained triaxial compression test is carried out on the unsaturated soil of a foundation project. It is found that the partial stress-strain curve of unsaturated soil is similar to hyperbola, and the matrix suction has an obvious influence on the mechanical behavior of soil. The greater the matrix suction, the higher the partial stress of soil. According to the deformation characteristics and engineering characteristics of unsaturated soil, the Duncan-Chang hyperbolic model is selected as the basic model, and the statistical damage theory is introduced. Assuming that the micro element strength of unsaturated soil obeys the Weibull probability density distribution, the Duncan-Chang statistical damage model is established. By establishing the relationship between the initial tangent modulus and matrix suction, a new Duncan-Chang statistical damage model of the unsaturated soil considering matrix suction is established. The parameter analysis method is given, and the empirical expression of the Weibull distribution parameters is obtained, which is used to modify the model. The damage accumulation law of unsaturated soil under different matrix suction conditions is analyzed, and the partial stress-strain test curve of unsaturated soil is compared with the traditional Duncan-Chang model, which proves the feasibility and rationality of the model. The research results provide a certain reference for the study of mechanical properties and identification simulation of unsaturated soil.

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. [ABSTRACT FROM AUTHOR]

This paper presents micro–macro behaviors of unsaturated residual soils within high suctions. A series of unsaturated soil specimens was prepared using the saturated-saline-solution-based vapor equilibrium technique, considering different initial dry densities. The strength of these specimens was tested under high suction using the direct shear test. Micropore structures for residual soil tests were analyzed using mercury intrusion porosimetry and scanning electron microscopy. The macromechanical strength of residual soil under direct shearing and unconfined compressing conditions was improved by increasing the suctions and densities. And, the level of pore volume distribution measured in microscale on unsaturated residual soil was greatly dependent on matric suction and specimen density. Moreover, soil–water characteristic curves greatly decreased and trended to flat in the high-suction condition (more than 50 MPa). [ABSTRACT FROM AUTHOR]

In general, the ultimate bearing capacity (UBC) of shallow foundations on unsaturated soils is characterized by the conventional shear strength (SS) parameters in which saturated theories are applied. However, in this case, it is clear that the foundations designed using the obtained values from the saturated cases not be economical. In recent years, procedures have been developed to estimate the UBC of foundations on unsaturated soils, that take into account drained and undrained loading conditions. However, these studies generally concentrate on sandy soils. The validity of the results proposed in the literature should be tested for other soils. Therefore, this paper includes a conventional direct shear box (DSB) test to determine the unsaturated SS of statically compacted silty soil, and a series of model tests were performed to determine the foundation’s UBC. In the experimental model setup, the UBC values of different types and sizes of model foundations on silty soil layers with a different soil saturation degrees (SSDs)/matric suctions (MSs) and different void ratio values were measured. In addition, the soil-water characteristic curves (SWCCs) and SS parameters of unsaturated silt were obtained. Using the experimental results, a new equation is proposed for the characterization of the UBC of shallow foundations on unsaturated silty soils. Using this equation, the UBC of unsaturated soils can be determined based on the results of unconfined compressive strength tests (UC) measured on unsaturated soil samples and based on the degree of saturation and the fitting parameter. The results indicate that the measured bearing capacity values obtained via the model footing test, shows a good consistency with those obtained by the proposed equation.

The Global Digital Systems Ltd. (GDS) unsaturated soil triaxial test system is used to determine the shear strength of unsaturated undisturbed silty clay soil and obtain the soil-water characteristic curve. The van Genuchten (VG) and Gardner (GD) models are used to fit the soil-water characteristic curve to predict and verify the shear strength of the soil. The results show a better fit for the GD than the VG model. The shear strength of the soil increases with an increase in the matric suction, but the rate of increase is relatively low. The most significant influence of the matric suction on the soil strength is observed at less than 150 kPa. The influence of the matric suction on the shear strength of unsaturated silty clay mainly depends on the total cohesion. The soil shear strength value predicted by the GD model is conservative, but the predicted value tends to approach the measured value with an increase in the matric suction. [ABSTRACT FROM AUTHOR]

Weather changes induce wetting and drying cycles in soil and consequently affect the matric suction and may create residual effects in soil, which is referred to as hydraulic hysteresis. The results of some researches in the literature indicate that effective stress parameter is changed under the influence of hydraulic hysteresis. In this regard, it is important to study the simultaneous effect of void ratio and hydraulic hysteresis on effective stress parameter in a wide range of suction. Therefore, in this research an experimental approach involving suction-controlled triaxial experiments was used to investigate the variations of effective stress parameter and degree of saturation versus suction at different initial void ratios along drying and wetting paths. The results of the experiments show that hydraulic hysteresis due to suction variation affects the effective stress parameter and changes the value of this parameter value during wetting and drying paths. Furthermore, the value of this parameter cannot be taken equal to the degree of saturation. In addition, effective stress parameter at a given value of suction depends on void ratio. The variation of effective stress parameter obtained from the experiments was compared with available models and their capabilities in predicting the soil behaviour were evaluated. [ABSTRACT FROM AUTHOR]

In order to explore the shear strength characteristics of unsaturated soil in Zhangjiakou area under different confining pressures and matrix suction, a triaxial shear test of unsaturated soil was carried out using an unsaturated soil triaxial apparatus. The water holding capacity, shear strength and shear strength indexes of unsaturated soil were analyzed, and a convenient prediction model for predicting the shear strength of unsaturated soil was found. The results show that after the shear failure of the soil, with the continuous increase of the matrix suction, the cohesive force also increases and the change of internal friction angle is small. There is an obvious linear relationship between the suction strength of the unsaturated soil and the matrix suction. Based on this, a shear strength formula with only one parameter a is proposed, and it is found that the parameter is related to the plasticity index of the soil sample. The strength of unsaturated soil can be predicted by measuring the plasticity index value of the soil. The research conclusions provide a theoretical basis for the strength measurement of unsaturated soil in northwestern Hebei. [ABSTRACT FROM AUTHOR]

The osmotic technique in geotechnical testing of unsaturated soils is a promising technique to control soil-water potential. This technique is an alternative to the axis-translation technique. In this technique an aqueous polyethylene glycol (PEG) solution is interfaced with a selectively permeable membrane. The geotechnical literature has regularly referred to membrane effects which tend to reduce the achieved matric suction in relation to that theoretically attainable by consideration of the osmotic pressure of the bulk PEG solution. In this review, the literature on membrane science is studied in order to identify potential causes for these membrane effects. Concentration polarisation, membrane fouling and reverse solute draw are the underlying causes of these membrane effects and should be given consideration in the geotechnical osmotic testing literature. A better understanding of membrane science and its application to geotechnical osmotic testing can aid in the optimisation of osmotic testing and the systematic improvement of the technique. [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology & Engineering Geology / Shuiwendizhi Gongchengdizhi is the property of Hydrogeology & Engineering Geology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Architecture & Civil Engineering is the property of Chang'an Daxue Zazhishe and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Gypseous soil is one of the most problematic types of collapsible soils which is affected by many geotechnical factors. The most important factors are the effect of loading and wetting and their relation to soil density, especially when the soil at unsaturation condition. Suction pressure is the main criteria in determining the deformation behaviour of unsaturated collapsible soil when these soils distributed in arid or semi-arid region. In this study, disturbed sample of sandy soil of more than 70% gypsum content is taken from Al-Ramadi city western of Iraq. This study interested to investigate the variation of matric suction with the dry density and their effects on deformation of gypseous soil. For this purpose, a soil-model device provided with high accurate Tensiometers and Time Domain Reflectometry sensors in addition to data logger is designed and manufactured. Tensiometer sensor is used to monitor and measure the matric suction, while the Time Domain Reflectometry is used to monitor and measure the volumetric water content in the soil mass. The results of the tests showed that there is a significant effect of soil dry density on the relationship between the matric suction and water content.

Internal dumps in mines are the main source of risks that threaten the safe production of mines, and improving their impermeability and stability is one of the core issues in this study. In this study, fly ash was used as the surface soil improver for the internal dump, and different fly ash additions (0%, 5%, 10%, 15%, 20%) were designed to improve the properties of unsaturated soil materials. The unsaturated properties, permeability, and strength properties of soil materials were studied through centrifuge tests, permeability tests, and direct shear tests. The results show that the matrix suction of the five kinds of improved soil materials with different addition amounts of fly ash is improved, but when the addition amount is greater than 15%, the improvement effect is weak. A secondary response surface model of fly ash addition, volumetric water content, and matrix suction was established to predict matrix suction. With the increase in the amount of fly ash added, the permeability coefficient of the soil shows a downward trend of the power function, which can have a better impermeability and release erosion effect. After fly ash is added, all soil materials show typical stress hardening behavior, and there is an optimal value (15%) for the improvement of the soil antideformation ability. Affected by the pore size, particle gradation, and fly ash hydration products, the shear strength and cohesion of the sample first increased and then decreased, while the internal friction angle first decreased, then increased and then decreased. The above research results are of great significance for improving the stability and impermeability of the internal dump in mines. [ABSTRACT FROM AUTHOR]

The soil water retention curve (SWRC) or soil–water characteristic curve (SWCC) is a fundamental feature of unsaturated soil that simply shows the relationship between soil suction and water content (in terms of the degree of saturation and volumetric or gravimetric water content). In this study, the applications of the SWRC or SWCC have been extensively reviewed, taking about 403 previously published research studies into consideration. This was achieved on the basis of classification-based problems and application-based problems, which solve the widest array of geotechnical engineering problems relevant to and correlating with SWRC geo-structural behavior. At the end of the exercises, the SWRC geo-structural problem-solving scope, as covered in the theoretical framework, showed that soil type, soil parameter, measuring test, predictive technique, slope stability, bearing capacity, settlement, and seepage-based problems have been efficiently solved by proffering constitutive and artificial intelligence solutions to earthwork infrastructure; and identified matric suction as the most influential parameter. Finally, a summary of these research findings and key challenges and opportunities for future tentative research topics is proposed.

This study focuses on investigating the shear behavior of the Shanghai clay-concrete interface under cyclic loading using a large-scale multifunctional shear apparatus. Considering that soil-structure interfaces are typically found in shallow soil layers or even above the ground surface, the interface zone is unsaturated in its natural state, the shear experiments were therefore conducted on the tested soil with different water contents. The corresponding matric suction of the tested soil under different water content and initial dry densities was determined by the filter paper method. The experimental findings indicate a substantial influence of matric suction on the interfacial strength, revealing a pronounced non-linear relationship. The interface exhibits the highest shear strength during the initial shear, which gradually decreases with an increasing number of shear cycles. This reduction can be attributed to the weakening of the bonding effect at the interface caused by cyclic shear. On the other hand, the matric suction decreases with water content increases, leading to a decrease in interface shear strength. Additionally, the vertical cumulative displacement of the soil increases with higher water content. • Cyclic shear experiments were conducted on the interface between unsaturated Shanghai clay and concrete. • The interface strength exhibits a non-linear relationship with the matric suction. • The contribution of matric suction to the interface strength is significantly influenced by the number of shear cycles. [ABSTRACT FROM AUTHOR]

There exist many structures founded on unsaturated soil deposits. Shear strength augmentation due to the evolution of the matric suction within the unsaturated porous media enhances the bearing capacity of the overlying foundation. This paper presents the evaluation of the pseudo-static seismic bearing capacity of the shallow foundations resting on unsaturated soil deposits using limit equilibrium method. Adopting the Coulomb failure mechanism and Bishop effective stress concept, the bearing capacity equations are solved. The distribution of the matric suction beneath the footing is assumed to be linear. The results of the bearing capacity evaluation are validated against some experimental data found in literature for the static condition. For the seismic loading consideration, the pseudo-static method is utilized. The dual effect of the earthquake acceleration vertical component is thoroughly discussed and a suction transition point is introduced in which the minimum bearing capacity is observed to bear the same value for both upward and downward directions. The increase in the matric suction throughout the soil deposit leads to the increase in the soil shear strength, thus posing more resisting forces as well as higher ultimate bearing capacity. The offered solution is deemed a consistent and useful tool for the accurate prediction of the seismic bearing capacity of shallow footings resting on unsaturated soil deposits. [ABSTRACT FROM AUTHOR]

An evaluation method was presented on the basal heave stability in unsaturated soils. A basal heave failure mechanism was established wherein the admissible velocity field was constructed in the light of the Prandtl failure mode. The shear strength theory based on two independent stress state variables was employed for unsaturated soils. The unified strength theory was introduced to account for the influence of intermediate principal stress. Analytical solution of safety factor (Fs) was then deduced based on the upper bound theory of limit analysis, and relevant numerical solutions were solved by means of the strength reduction method. It is found from parametric analysis that the intermediate principal stress and matric suction have a significant impact on Fs. Specifically, Fs is seriously underestimated if they are ignored. The theoretical framework in this work provides a new idea for analyzing the basal heave stability of unsaturated soils. [ABSTRACT FROM AUTHOR]

A series of partially drained (pore air pressure drained and pore water pressure undrained), fully undrained (both pore air and water pressures undrained) and fully drained (both pore air water and pressures drained) triaxial tests were conducted in unconfined conditions to explore the mechanical behaviour of an unsaturated silty soil. The water contents of test specimens were kept at 10, 15, 20 and 25% with the respective degree of saturation of 24, 36, 49 and 59.5%. The void ratio was 1.10 with a dry density of 1.25 g/cm3 while maintaining the relative compaction of all specimens at 80%. During the constant water content (CW) and fully undrained (FU) tests, the peak shear strength was within an axial strain range of 0–2.75% and the volumetric strain of all specimens did not vary within this range. Under shearing infiltration (SI) and constant loading infiltration (CLI) conditions, it was revealed that a significant magnitude of matric suction still exists in the specimens at the peak/post-peak deviatoric stress which depicts that a slope can fail during a rainfall under an unsaturated condition due to negative pore water pressure (matric suction) in the soil. The results of the triaxial tests suggest that the peak shear strength increases with matric suction linearly up to a matric suction of 15 kPa and the rate of increase gradually decreases with further increase in matric suction. Moreover, the failure envelopes have revealed an existence of a critical suction value of 15 kPa corresponding to 27% water content (i.e. on the dry side of optimum water content) and the increase in shear strength due to suction becomes insignificant afterwards. It is also observed that the peak shear strength increases nonlinearly with the increase in matric suction with a good agreement between the failure envelopes for CW, FU and SI tests. [ABSTRACT FROM AUTHOR]

Until now, all of the calculations about determining of water content around the particles in unsaturated soil have done with supposing the spherical shape of particles and most of quantities such as matric suction and interparticle stress due to water meniscus were calculated by using that simple assumption. In this paper, three packing methods of ellipsoidal particles are studied and by considering changes in meniscus geometry, matric suction and water content volume are evaluated as functions of meniscus radii for soil particles under the pore-water regime. This analysis provides a theoretical basis for describing several well-known phenomena in unsaturated soil behavior. A series of rigorous analytical equations was developed for describing the total volume of menisci between contacting ellipsoidal soil grains by treating the zero contact angle. Matric suction characteristic curves modeled using the analytical solutions displayed behavior similar to real unsaturated soils by using the ellipsoidal particles. The results show that by increasing the filling angle, the volumetric water content of horizontal arrangement is about 2 to 5 times higher than other states, while by changing in the radius ratio of the lens, this amount is more for vertical alignment. So, the matric suction for elliptical particles is quantified with little inclination towards the water content change which achieved four times larger for double diameter ratio.

The complexity of unsaturated soil means that its permeability coefficient and seepage law are important topics in soil mechanics and geotechnical engineering. In this study, a new type of equipment for measuring the seepage of unsaturated soil was used to study the steady seepage of unsaturated loess soil collected from northern Shaanxi province in China. The entire process of the steady seepage of unsaturated soil under different seepage hydraulic gradients was obtained, and the permeability coefficient in the steady seepage state was measured. When the soil water content was low, the soil moisture remained only in small pores or at the junction of soil particles, the effective seepage area of the sample was reduced, and the seepage path was extended. Therefore, the lower the water content, the smaller the permeability coefficient of unsaturated soil and the longer the time required for the seepage to reach a steady state. The matric suction of the unsaturated loess soil was measured under different water contents. With decreasing water content, water remained only in the smaller pores, which provided shrink films with a smaller radius of curvature. This allowed the shrink films to withstand a larger pore air pressure and transmit a smaller pore water pressure, so that the matric suction of the loess soil increased with decreasing soil water content. The Gardner model provided an accurate fit of how the matric suction varied with the volumetric water content of the unsaturated soil. The seepage velocity and seepage hydraulic gradient were correlated well with a linear relationship, indicating that the seepage law of unsaturated loess soil accords with Darcy's law. The experimental results show that the new unsaturated soil permeameter has good performance and controllability, thereby providing support for studying the steady seepage of unsaturated soil. [ABSTRACT FROM AUTHOR]

Tensile and shear behaviour of unsaturated soil have been investigated separately in the past. However, there is no comprehensive study on the transition from tensile to shear failure in compacted clayey soils. A series of uniaxial tensile, direct shear and triaxial compression tests were conducted on compacted Regina clay specimens to investigate the strength mobilisation and failure mode under different loading paths. It was observed that the matric suction increases the apparent cohesion and tensile strength of the material. At a given constant matric suction, the compacted clay displays a transition from tensile to shear failure with increasing confining stress which can be approximated by a bilinear failure envelope encompassing the transitional behaviour. The friction angles in tensile regime are greater than those in compressive regime, which can be explained by the transitional change in failure surface morphology from tensile ruptured to shear smoothened one. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical approach is established to determine the active wedge angle of unsaturated soils, based on the limit equilibrium method, while matric suction varies linearly with respect to depth. In the previous researches, constant suction distribution profile within the soil has been pointed out, however, generally linear and nonlinear suction profiles develop above the ground water table. The range of suction value covers transition zone and residual zone of unsaturation. Sensitivity of the proposed model, with respect to different parameters, shows that the value of active wedge angle is more dependent on the internal friction angle, in comparison to other properties. Therefore, instead of a complex relation, simple equation can be used. The results show that the active wedge angle for unsaturated soils is equal to π 6 + 7 8 ϕ . Comparisons between the presented model and experimental data show that, for some values of internal friction angle, the active wedge angle in unsaturated state is close to saturated state. [ABSTRACT FROM AUTHOR]

Collapse of soil skeleton due to variation in its loading and boundary conditions impacts the strength and stability of soil in its unsaturated state. The present study is focused on the influence of matric suction on collapsible behaviour and stiffness degradation of compacted unsaturated cohesive soil. Collapse characteristic of unsaturated soil is assessed under sudden inundation and varying vertical stress application. A series of dynamic triaxial, double oedometer and filter paper tests were conducted on unsaturated Ahmedabad cohesive soil under as-compacted conditions at varying dry density and water content to determine cyclic degradation index (δ), collapse potential and matric suction (ua – uw), respectively. Significant influence of matric suction was observed on collapse potential and stiffness degradation of unsaturated cohesive soil. Larger collapse potential at higher matric suction clarified the meta-stable state of compacted unsaturated cohesive soil under dry conditions. Cyclic degradation index (δ) with number of loading cycles was found to be decreasing with increase in matric suction representing brittle nature of soil possessing higher matric suction signifying soil skeleton to be highly volatile and unsafe under dynamic loading conditions. [ABSTRACT FROM AUTHOR]

The present study focuses on some tentative laboratory tests using a newly-constructed modified direct shear test apparatus. The single-stage and multistage direct shear tests were performed to determine the shear rate and test scheme of unsaturated shear test. Shear strength parameters of unsaturated soil in different conditions are obtained and the tests indicate good agreement with typical theories of unsaturated soil, the nonlinear matric suction failure envelope is determined. Some shear strength equations are also fitted through the experimental results.

To study the influence of matric suction on the strength characteristics of soil, consolidated and drained triaxial shear tests on undisturbed silty clay were conducted using GDS unsaturated soil triaxial apparatus under different matric suction conditions. The results show that the peak strength of soil increases with the increase of matric suction, and the rate of increase increases first and then decreases. With increased matric suction, the total cohesion increases and the extent of this increase gradually decreases, while the effective internal friction angle decreases slightly and then tends to be stable. The effect of matric suction on shear strength is mainly reflected in the total cohesion. Matric suction has the most significant influence on soil strength at 50 kPa to 100 kPa. In the presence of matric suction, the area of action of water film on soil particles determines the change of total cohesion, and the pore air pressure and pore water determine the change in effective internal friction angle. The angle used to describe the increase in the shear strength of soil with an increase in matric suction is φb=28.2°. According to Fredlund's strength theory, the formula of shear strength parameters of unsaturated silty clay in the study area under consolidation and drainage conditions was given, and the relationship between strength parameters and matric suction was fitted. [ABSTRACT FROM AUTHOR]