In-situ permeability tests using soil water contents that easily and quickly measure the saturated-unsaturated hydraulic properties in the unsaturated soils are proposed. The continuous constant head infiltration test and gravity drainage test with intrusive soil moisture device to measure the field-saturated hydraulic conductivity and the field-saturated volumetric water content were performed. The unsaturated soil hydraulic properties were estimated by transient soil water contents during these tests. Sensitivity analysis was carried out under the estimation parameters are limited. The change of the water content in the soil during the constant head infiltration test process and the gravity drainage test process were individually simulated by the seepage flow analysis. From these simulations, functional model of the unsaturated soil hydraulic properties considering the hysteresis phenomenon were identified.

This paper explores the mechanisms that lead to the degradation of clayey rocks when exposed to environmental effects as those caused by unloading and cyclic variations in relative humidity. The following aspects are evaluated: (i) the number of applied RH cycles, N, (ii) the amplitude of relative humidity cycles, RH, (iii) the stress level (p-ua) and (iv) the effect of the fluid used to induce rock saturation (liquid water or vapour). The implementation of non-conventional experimental techniques for inducing and tracking rock degradation, at 'macro' and 'micro' scales, is described. An experimentally-based framework of behaviour is presented which may be used in practice for the evaluation of the degradation potential of clayey rocks.

The soil-water characteristic curve (SWCC) is a relationship between water content in a soil and suction. The SWCC was first plotted by Edgar Buckingham, a soil physicist, in 1907 for six soils ranging in texture from sand to clay. It was adopted for use in unsaturated soil mechanics by the geotechnical engineering community. The SWCC is now almost treated as the index property of unsaturated soils. It has been used as a proxy for permeability and shear strength of unsaturated soil. Most soils have a sigmoidal SWCC, otherwise known as a unimodal SWCC as opposed to a bimodal SWCC which has been found for some soils. Although determining the SWCC is easier than determining permeability or shear strength for unsaturated soil, the test is still time-consuming and it is not easy to determine the entire SWCC. Incomplete or insufficient SWCC data may lead to an incorrect SWCC and hence inaccurate determination of permeability and shear strength. Progress has been made to expedite the experimental determination of SWCC as well as estimating the SWCC from basic soil properties using pedotransfer functions. In addition, SWCC has been represented using volumetric water content, gravimetric water content or degree of saturation. Different representations may have dire consequences on its application. Determining the SWCC using volumetric water content or degree of saturation presents challenges in estimating the instantaneous volume during the experiment. In this paper, the current state-of-the-art in determining, representing and estimating SWCC and its applications are described and critically examined.

Triaxial tests have been extensively used to characterize soil behavior. During triaxial testing, the soil deformation needs to be measured. However, measurement of deformation in unsaturated soils is considered a great challenge for researchers due to the existence of air phase. Over the years, a great deal of research effort has been dedicated to developing methods to measure soil deformation during triaxial testing. Among these methods, image-based methods receive increasing attention. This state-of-the-art report presented herein aims at documenting advancement in the image-based methods for unsaturated soil deformation measurement. The documentation presented this study includes the image analysis, X-ray CT, image correlation, multi-camera photogrammetry, and the photogrammetry-based methods. The pros and cons of each method are discussed and a comprehensive comparison between these methods is provided. Concluding remarks are included in the last section of the report.

The role of pore air pressure on seismic behavior of unsaturated sandy soil is discussed through recent numerical simulations with three-phase and simplified two-phase coupled analyses. Equations governing the dynamic deformation of unsaturated soil were briefly shown based on porous media theory and constitutive models. The validity of three-phase coupled analysis and the applicability of simplified two-phase coupled analysis are discussed through simulations of cyclic triaxial test, seismic behaviors of horizontal ground and embankments.

In order to study the effects of suction stress on undrained shear strength of unsaturated soil under low confining pressure, unconfined compression tests and constant water content shear tests, in which the suction value and the volume change were measured, were carried out using the compacted specimen of a silty soil. The calculated failure stress state parameters were then analyzed with the equivalent confining pressure by taking into account of three kinds of suction stress; one is obtained from the soil water retention curve corresponding to the degree of saturation and the measured suction, and the others are obtained from the soil water retention curve for the water content and suction value, s* decided by the suction stress - SWRC method (SSM). A unique failure line was obtained for both saturated and unsaturated cases. This verifies the relevance of new suction stress estimation method. In addition, the results also suggests that the unsaturated shear strength can be predicted from uniaxial compression test with specimens at different initial moisture contents if the SWRC for water content is known.

The mechanical behavior of unsaturated soils is one of the challenging topics in the field of geotechnical engineering due to world-wide distributions and construction of most engineered earth structures within these soils. However, the study of unsaturated soil behavior is complex because its pore space is partially filled with water and air. The shear strength of unsaturated soils is defined by using two independent stress state variables i.e. net normal stress (σ-μa) and matric suction (μa-μw). Direct measurements of unsaturated shear strength in laboratory are complicated, expensive, time consuming and required specialized testing equipment. An improved equation for unsaturated shear strength behavior of soils is proposed in this paper. The validity of the proposed improve method is examined for a number of suction strength data of different kind of soils available in the literature. It is found that the suction strength values predicted from the proposed equation are in well agreement with the experimental results. Furthermore, the proposed improve equation is also compared with some previously published suction strength equations using those known experimental data. The results show that most of the previously published equations are suitable for particular types of soil whereas the proposed equation provides well agreement for different types of soils. Therefore, the proposed improve equation is more suitable for the prediction of the suction strength and quite adequate for practical engineering purposes.

The shear strength equations, based on two stress state variables, proposed by Fredlund et al. (1978) and Oberg and Sallfors (1997), respectively were compared at peak and critical states. Data of a set of drained direct shear tests for a fine sand tested under saturated and unsaturated conditions were used for the comparison. The comparison between the two equations suggests that suction contributing to the effective stress for sand used is often much smaller than predicted by the (sSr) term used in effective stress expressions of Oberg and Sallfors (1997). It is suggested that effective stress in unsaturated soils should be related to soil fabric and microstructure which caused an increase of shear strength parameters. Clusters or assemblages of particles, held by menisci, increased dilation and frictional resistance during shearing related to the different phases of water retention behaviour.

In describing the stress state of unsaturated soil, the two independent stress state variables advocated by Fredlund et al. (1977) is approved by many scholars. However, the experimental verification is not sufficient at present, which is mainly embodied in two aspects. One is that only isotropic compression tests are conducted, and the complex stress condition is not considered. The other is that the verification only involves volumetric strain and moisture change of specimens and does not involve strength and shear strain. In order to further verify the rationality of the two independent stress state variables for unsaturated soil, a triaxial apparatus for unsaturated soil was improved to control pore water pressure, confining stress and deviatoric stress. Two types of tests were conducted with remolded Q3 loess. The first type is consolidated undrained shear tests in which confining pressure, pore water pressure and pore air pressure were equivalent increased or decreased, and net confining pressure, suction and deviatoric stress were kept constant. It is found that volumetric strain and moisture change of specimens are very small and can be ignored. The second type is three groups of consolidated drained shear tests. Each group contains two tests with controlled same confining stress and same suction. One specimen directly sheared after consolidation. The other equivalent increased confining pressure, pore water pressure and pore air pressure after consolidation and kept net confining pressure, suction and deviatoric stress constant, and then sheared. It is found that the shear strength, volumetric strain, moisture change and generalized shear strain of the two specimens are very close. The results of this paper further illustrate that the two independent stress state variables for unsaturated soil are reasonable from the aspects of deformation (including volumetric strain and shear strain), strength and moisture change.

Evaporation of water from soils induces soil cracking, salinization and degradation, especially in arid irrigated areas. The factors that influence water evaporation from soil includes external type, referring to atmospheric conditions and interior type, covering surficial soil characteristics and water content conditions. In spite of the numerous assessment that have been made on the influence of atmospheric conditions and soil characteristics on evaporation of moisture from soils, many aspects still require further investigation due to soil complexity. In this study, laboratory-based evaporation tests were performed on clayey soil and three kinds of quartzite sands of different particle sizes using the same environmental conditions. Three different tests were performed on slurry samples to evaluate the effects of soil characteristics on its free water evaporation process at room temperature (20-22°C) and relative humidity (50 ± 2%) conditions: soil thickness, particle size of mixing sand and mixing sands ratio. It is shown that larger soil sample thicknesses accelerate water evaporation rate and extend the constant evaporation rate stage. Higher sand mix proportions lead to larger evaporation area and soil porosity, allowing higher evaporation rate and longer duration of the first stage and the starting of the falling rate stage at lower water content. However, soil particle size was not found to have a significant impact on evaporation rate on per unit weight of added soil basis.

The permeability test for permeable asphalt pavement cannot be applied to soil-based pavement in the field because this test erodes the ground surface due to the high positive applied pressure. Furthermore, the permeability test is based on the time required for a certain amount of water seepage to occur, which is not a hydraulic conductivity parameter that has a physical meaning. Therefore, in the present paper, we propose a measurement technique for water permeability and soil-water retentivity by field measurements using a suction infiltrometer. The results revealed that, even if the pavement layer is 10 cm in thickness, the proposed method can determine the field-saturated hydraulic conductivity. Furthermore, we determined the soil-water characteristic curve for soil-based pavement using the simultaneously measured water entry value.

The formation of desiccation cracks on soil surface is a common natural phenomenon as it is subjected to drought climate. The presence of cracks can significantly affect the hydro-mechanical behaviour of soil, and results in various geotechnical problems. With increasing frequency of severe drought climate, better understanding of soil desiccation cracking behavior is becoming an increasingly significant issue. In this investigation, laboratorial desiccation tests were performed on soils by simulating long term drought climate. A camera was mounted above the specimen to monitor the initiation and propagation behaviour of desiccation cracks. Image processing technique was employed to quantify and characterize the obtained crack patterns. It is observed that the desiccation cracking generally takes place in three stages: main-cracks firstly start on soil surface and form main-clods; subsequently, main-clods are split into several sub-clods by sub-cracks; after the size of all clods is stable, cracking terminates and the final crack pattern is formed. It is shown that the image processing technique is efficient to accurately determine the geometrical parameters of crack patterns during drying, including crack width, length, clods area, etc. The introduced density function is effective in describing the distribution characteristics of the geometrical parameters of crack patterns.

Several cracking test of swelling mudstone of Sichuan Central Redbeds are conducted on the condition of hydration. Based on tests result, the process of crack development in mudstone, its influencing factors and cracking mechanism are analysied. The results show: (1) the cracking process can be divided into three stages: moisture absorption and crack generation, crack development and rapid expansion, and stablization. Initial water content is negative correlation with cracking degree, and soaking water absorption has a greater cracking degree than capillary water absorption. Influenced by the content of clay minerals, the overall cracking degree of mudstone is larger than that of argillaceous sandstone. (2) In the process of moisture absorption, the swelling deformation of mudstone is uneven, and this uneven expansion also changes with time. (3) The cracking of swelling mudstone is caused by uneven swelling and softening effect. The uneven swelling of mudstone is caused by: i) The spatial distribution of clay minerals is not uniform; ii) The whole process of moisture absorption is not uniform in space; iii) There are visible or invisible initial cracks inside the swelling mudstone.

Inherent anisotropy, resulted from sediment process, is an important aspect when considering the behavior of sandy soils. Also, accurate initial shear modulus G0 at very small strain is an essential parameter in many seismic designs and analyses of underground structures. In this research, the influences of inherent anisotropy on G0 were investigated by conducting a series of triaxial tests employed both local small strain (LSS) measurement technique and bender element (BE) method. Specimens were prepared using air pluviation method in an inclined container at different angles to create the inherent anisotropy induced by orientation of sand particles. The experimental results obtained on both saturated and unsaturated sands indicate that G0 was affected by depositional angle of the specimen. Moreover, those results showed greater G0 on unsaturated sand than that on saturated sand because of the application of matric suction.

Limited experimental data is available to verify the variation of suction stress that is typically experienced by partially saturated soils under large deformations. The purpose of this study is to present soil suction characteristic curve (SSCC) for compacted high plasticity clay in light of the recent experimental data obtained from temperature controlled/constant water content ring shear testing over a matric suction range varying between 0-200 kPa. Failure envelopes are used to indirectly calculate the experimental values of suction stress corresponding to each level of matric suction. The experimental suction stress was found to increase non-linearly with increasing matric suction at constant temperature. Also, suction stress increased with increasing temperature when compared at constant matric suction. Furthermore, the soil water characteristic curve (SWCC) parameters were obtained on test soil and used to estimate the SSCC using the predictive model proposed by Lu et al. (2010). However, the suction stress values were mostly overpredicted using this model. On the other hand, a simple polynomial (two-parameter function) model was found to best predict the suction stress over the matric suction range of 0-200 kPa for clay of high plasticity.

Unsaturated soil mechanics has rapidly become an integral part of geotechnical engineering practice. The negative pore water pressure is the major interest in unsaturated soil situations. The present study was an attempt to examine the effect of saturation on strength and consolidation properties of two soils namely CH and CL type of soil at various percentage of maximum dry density (MDD). Result showed an apparent raise in the cohesion value of yellow soil increased with amplification in the saturation value as well as the density value. Black cotton soil followed a reverse trend of decrease in the cohesion as well as the angle of internal friction value with increase in degree of saturation, but comparatively increased for a given degree of saturation with increase in value of density. No definite trend was observed in the value of coefficient of consolidation. The work included few test on centrifuge to quantify the settlement of the selected soils at 80% saturation with 85% maximum dry density and 95% maximum dry density. The centrifuge showed an appreciable value of settlement for yellow soil with 85% maximum dry density at 80% saturation while the other tests showed some settlement but not to a measurable value.

Recent case histories show the importance of incorporating unsaturated soil mechanics in geotechnical engineering practice for the design and construction of resilient and cost-effective systems. Unsaturated soils are a three-phase material- solid, liquid and gas, resulting in three interfaces. In addition to study the behaviour of flexible PVC plastics in unsaturated soils a method is proposed for characterizing an unsaturated soil deformation for a site based on reliability concepts using a sample of road embankment design. The present study includes understanding the behaviour of plastic reinforcement in unsaturated soils and its modelling for embankment. In this research paper three different types of soil have been tested. Cohesive (CH), cohesive friction (MI) and cohesion less (sand) are sampled and tested in the laboratory. Main factors considered to evaluate the effectiveness of PVC plastic material in all types of soils are shear strength parameters [cohesion(C) and angle of internal friction (Φ)] and settlement (deformation) by variations of moisture content and reinforcement materials. To find out settlement on embankment model centrifuge test is used: whereas triaxial compression test is used for shear strength parameters. The comparison of the results shows that PVC plastic granules are more effective in CH soil by increasing internal angle of friction and decreasing cohesion both by increasing amount of plastic granules and water content up to optimum moisture content, whereas the effect of plastic granules not significant on cohesion less soils compared with cohesive and cohesive friction soils. From centrifuge modelling it is concluded that the settlement for cohesion less soil declines for increase in water content as well as plastic granules, whereas for cohesive and cohesive friction soils sudden increase in settlement occurs.

Industrial by-product lignin is a kind of polymer organic matter that can be applied to the improvement of subgrade soils. Based on the lignin-lime composite modifier blending ratio recommended by the unconfined compressive strength test, the soil-water characteristic curve and unsaturated shear strength of the lignin-lime improved Yellow River alluvial silt were studied through a series of experiments. The microstructure of the improved silt was observed by scanning electron microscopy (SEM). The test results show that: after the improvement of lignin-lime, the cohesion of the improved silt increases exponentially with the decrease of saturation, while the internal friction angle does not change much. With the decrease of saturation, the strain softening phenomenon of the improved silt is intensified and the dilatancy is increased. Compared with pure silt, the lignin-lime modifier fills the pores between the soil particles, resulting in a bond between the loose particles and an increase in strength.

Plants increase slope stability through mechanical reinforcement by roots and transpiration-induced matric suction (hence increase in soil shear strength; known as hydrological reinforcement). The effects of root water status on root biomechanical properties (e.g. volume change and tensile properties), and eventually the shear strength of rooted soil, have not yet been fully investigated. This paper studies the hydromechanical properties of plant roots and estimates how these properties may affect unsaturated soil strength. The root water retention curve (RWRC), is introduced as a new concept to relate root water content and root suction. Unsaturated shear strength equations are hypothesised to utilise RWRC to couple the mechanical and hydrological reinforcement by plant roots. The potential importance of considering the effects of root hydromechanical properties on soil shear strength is highlighted, together with the need to fully test and develop the underlying models.

This paper presents charts derived from stability analyses of curvilinear slopes in non-homogeneous unsaturated soils. The charts enable quick and inexpensive stability analyses to be conducted in practice. The soils are assumed to obey the Mohr-Coulomb failure criterion. Suction effects are captured using the effective stress concept. Cohesion and the contribution of suction to the effective stress are linear functions of depth. It is shown that a stable curvilinear slope profile is uniquely governed by dimensionless parameters. The charts are presented using these dimensionless parameters and may be used in the preliminary design of stable curvilinear slopes. In general, suction has a similar influence to cohesion, and as the contribution of suction to effective stress increases, steeper curvilinear slopes become stable. An example of application is included showing the design of a curvilinear slope.

Unsaturated swelling soils expand in volume significantly during a wetting event (for example, rainfall infiltration, snow melting, or irrigation), which can lead to substantial change in the stress regime within and degradation of mechanical properties of the shallow soil mass. These changes can, though not completely, interpret many likelihoods of shallow landslides in swelling soils that has been frequently observed. In this work, an infinite slope formulation is proposed to address the mechanism of the shallow slope failure, where the unsaturated soil is described by the extended Mohr-Coulomb elasto-plastic constitutive law, in particular, the material parameters of the yield surface is altered according to the accumulation plastic deviatoric strain (extended to unsaturated soils from the classic softening), to simulate the degradation of mechanical properties. In this way, the influence of (i) the wetting induced swelling, (ii) the swelling induced stress change, and (iii) the associated softening behavior, on the stability of shallow layer is quantitatively examined. The proposed infinite slope formulation is applied to two typical swelling soil slopes: a synthetized one and a real field case. A good agreement between prediction and available experimental data from a published field study is obtained, illustrating the important implications for the engineering design of swelling soil slopes.

During the construction of the South-to-north water transfer middle line and Jilin-Hunchun high-speed railway line, a lot of excavations in swelling rock and soil were performed to achieve acceptable grades, eventually forming extensive swelling rock and soil slopes. Many of them slid after water storage and rainfall events. One interesting observation is that most of the landslides are south-facing. In this paper, the causes of this direction variation were thoroughly examined on the basis of the geological characteristics of project sites. It is concluded that the water storage and rainfall event are the direct external trigger of landslides while the intensive soil-atmosphere interaction is the main reason for the landslides in south-facing direction. Because of the influence of slope orientation, the solar radiation, temperature and humidity on south-facing and north-facing slopes are materially different. The unloading fissures caused by excavation unloading and the shrinkage fissures act as a main channel for the exchange of moisture and energy between soil and atmosphere. Owing to the intensive solar radiation and high temperature on the south-facing slopes, they are subjected to dramatic wetting-drying and freezing-thawing cycles, which can lead to a remarkable decrease in the strength of swelling rocks and soils. They are therefore more possible to slide.

It is well known that water content is the key factor affecting the loess strength. In arid and semi-arid regions, the salt content in pore solution of loess is relatively high. Rainfall or irrigation can change the water content of loess and induce geological disasters such as loess landslide and loess mud flow. In this paper, Heifangtai, Gansu Province, is chosen as the study site, where is the most typical place of loess landslides. The occurrence of loess landslides is closely related to water, salt. In order to investigate the interaction rules of these three factors, a series of pressure plate apparatus tests and undrained shear tests were conducted on saturated loess, to investigate the degradation mechanism of loess strength caused by the interaction between water and salt.The results showed that: NaCl concentration had a significant effect on the matrix suction, i.e. the water retention capacity of loess sample increased as the increase of NaCl concentration,especially in the boundary effect zone (low suction zone, soil almost saturated), concerning the remolded loess samples having a same dry density.The increase of NaCl concentration in pore water can also lead to the decrease of shear strength of saturated loess, especially for the cohesion. The results of Scanning Electron Microscopy (SEM) tests gave a microscopic explanation for the above results. In conclusion, the enrichment of salt leads to the increase of water holding capacity of approximately saturated loess, which is closely related to the decrease of undrained shear strength in saturated conditions. This will help us to understand the mechanism of loess landslide in this area.

This study focused on soil-water characteristic curve (SWCC) such as hydro-mechanical properties for crushed bentonite The SWCC recognized as significant physical property, play to evaluate hydro conductivity and swelling property High suction was applied used vapor pressure technique with various salt solutions. The controlled suction ranges had from 2.8 MPa to 296 MPa in this tests that could corresponded to ranges from 98 % to 11 % in RH (i.e. relative humidity). SWCCs were measured to crushed bentonite having two different dry densities thought drying and wetting process. Also, swelling pressure was observed in one-dimensional condition.

This present study investigated the chemical influence on the water retention curves of compacted GMZ bentonite. Sodium chloride solution and calcium chloride solution were mixed with the bentonite before compaction. Water retention curves were determined by measuring suctions with a the high-precision humidity sensor. The chemical influences on water retention curves were predominant in the suction range less than 70 MPa. In the suction range, an increasing of solution salinity resulted in an upward movement of water retention curve, i.e. an increasing of water retention capability of the bentonite. The solution type also had notable influence on the water retention behaviour. A higher water retention capability of the bentonite added with calcium chloride solution was observed compared to that with sodium chloride solution. In the suction range larger than 70 MPa, the influences of the solution type and salinity on the water retention behaviour were insignificant, and the water retention curves with different solutions were almost identical.

Bentonite/sand mixture is used as one of the materials of the engineered barrier included in the geological disposal for the high-level radioactive waste. It is expected that the swelling of the barrier will be occurred due to decrease in the suction with the increase in the degree of saturation during the post-closure site management of them. The thermo-hydraulic-mechanical (T-H-M) analysis is usually conducted to simulate the behavior of the engineered barrier. Soil-water characteristic curve (SWCC) of the Bentonite/Sand Mixture is one of the important parameters in the T-H-M analysis. In this paper, the Kita-Sako model is employed to derive the SWCC of the bentonite/sand mixture. To check the applicability of the model for the Bentonite/Sand Mixture is the purpose of this paper. The concept of the model is firstly introduced and then the applicability of the model for Bentonite/sand mixture is checked. Finally, the improvement of the model is discussed, comparing the calculation results with the soil test results.

On-site quick identification of the degree of expansion of expansive soil is of great significance to practical engineering. A set of expansive soil identification device was developed based on Particle Image Velocimetry (PIV) technique, which can provide a constant temperature environment and stable image acquisition conditions under outdoor environment. This device was used to carry out laboratory tests on remolded expansive soils, and to establish a method of quick identification of the degree of expansion of expansive soil based on PIV technique. The results show that the average displacement of all the pixels in the sample first increases and then decreases in a certain period of time, and thereafter tends to be stable in the later stage of the experiment. The mean value of displacement increment (MVDI) was used as the criterion to judge the stability of the sample displacement when it was less than 0.01 mm for two consecutive times. The mean value of the accumulated displacement increment (MVADI) before the displacement stabilization of samples with different degrees of expansion shows a positive correlation with the free swell index under the same initial state (i.e., density and moisture content) of the sample. Undisturbed expansive soils were taken for field tests, and the relation between the MVADI and the free swell index obtained from standard testing procedure was established, which is consistent with the results of laboratory tests.

Clay has wide application in engineered barriers, such as buffer barriers in deep geological nuclear waste disposal, clay liners in landfill, and earthen slopes and embankments, to name a few. However, most clay soils have a high potential to crack under extreme weather induced moisture loss. From a macro-mechanical view, clay tensile strength which is a critical factor governing the cracking behavior is highly dependent on the moisture content and corresponding matric suction. In this study, a series of laboratory tests on two typical clays (kaolinite and sodium bentonite) were carried out to investigate the relationship between clay tensile strength and moisture content by using a specially designed device which can measure tensile strength directly (Stirling et al. 2014, 2015). In addition, the study also investigated the effect of silica sand additive on the tensile strength at a wide range of moisture contents. The results demonstrate that both moisture content and sand additive have a significant impact on the measured tensile strength of the clays.

Slope failure induced by rainfall is related to change of effective stress, and both saturated soils and unsaturated soils had been evaluated in effective stress theory. The effective stress comprised confining pressure and pore-water pressure, and unsaturated soil necessary to two stress variables such as net normal stress and matric suction. Before occurrence of slope failure, soils stated under stress condition, which was not isotopic compression stress. Therefore, it was estimated that lateral stress was lower than overburden pressure or dead loading pressure in practice. Mechanical deformations as consolidation or creep behavior was revealed, and associated to slope failure, differential settlement and lateral flow. At least, these problems have been recognized of geotechnical disasters.

The paper presents preliminary results of an experimental study aimed at evaluating the influence of soil microstructure on the collapse behaviour of compacted loess from Xi'an, Shannxi province, China. Collapse behaviour was evaluated from one-dimensional compression tests in which compacted specimens were loaded to different vertical stresses, under constant water content conditions, prior soaking. Mercury intrusion porosimetry (MIP) tests and Scanning Electron Microscopy (SEM) analysis reveals a strong influence of the stress level on the soil microstructure formed by soaking under zero lateral deformation conditions.

Soft soil is a highly saturated soil and its low shear strength undermines the reliability for construction. Hence, to reclaim soft soil for construction, more research is required to improve our understanding on the changes in soft soil properties with ongoing dewatering. Shrinkage behavior of soft soil is studied in this paper. Some of the previous studies focused on the simulation of soil volume change and matric suction change while neglecting the soft soil settlement during fully saturated stage of shrinkage. The aim of this research is to predict the settlement of soft soil at the stage where soil is fully saturated and moisture content is high. During this stage, suction of soil is assumed to be zero and the decreased pore water pressure induced by evaporation is taken as the main reason for settlement. With the soil shrinkage happening, the pore water pressure decreases due to water loss and effective stress increases, being the mechanical reason for soil settlement. To predict the settlement during fully saturated stage of soil shrinkage, the ramp loading consolidation theory is applied. The shrinkage test of saturated kaolin with initial moisture content of 150% has been conducted in laboratory and vertical settlement was recorded. Ramp loading consolidation simulation method is applied to soft soil settlement estimation and the prediction fits well with the experiment.

This paper aims to investigate the mechanism of an unstable slope with a long-time displacement. The unstable slope is located along the coast and covered with colluvium underlain by mudstone. Rainfall is considered the main source for triggering the slope displacement. Slope inclinometers are installed at the site, and sensors for measuring soil water content are installed in the colluvium and in the mudstone deposits. The water content in the colluvium with a thickness of 0.5~1 m above the mudstone increases considerably during rainfall, and the mudstone deposit is also highly saturated. These data imply that discontinuities or thin permeable deposits, e.g. sand layer, may exist in the mudstone and provide channels for rainfall-induced subsurface runoff to reach the inside of the mudstone. The mudstone deposit underneath the colluvium undergoes displacements during high-intensity rainfall based on data of slope inclinometers. Creep behavior in the mudstone deposit may take place due to its high saturation. The high saturation with a thickness of 1 m above the colluvium-mudstone interface during rainfall and high saturation in the mudstone deposit are major reasons for the long-time displacement in the colluvium slope.

Sediment disasters become more serious and most of the sediment disasters of the natural slopes are due to shallow surface failures. The surface layers normally consist of sand or sandy soils. In order to clarify the mechanism of the surface failures, we have conducted a series of model tests. It was found from the model tests as the rainwaters were accumulated at the toes of the slopes, they triggered slope failures. It is important to drain promptly the accumulated rainwaters from the surface layers in order to prevent the large surface failures. Filter gabions with pipes placed at the toes of the slopes were introduced. The objective of this paper is to evaluate the performance of the filter gabions with pipes to prevent surface failure. The investigations were conducted by using the saturated and unsaturated consolidation analysis method with an elastoplastic model considering two suction effects. At first, the consolidation analysis method was verified by simulating slope model experimental tests with and without filter gabions. The analyses of the slope models with filter gabions having drainage pipes installed were then performed. Four cases with different pipe lengths were analyzed. More reductions of pore water pressures (PWPs) were observed as the pipe lengths increased. In the case with the longest pipe installed, the failure prevention performance was the largest.

Landslide hazards have caused loss of human lives, failure of structures, damage to agricultural lands and the natural environment. Rainfall-induced landslides constitute a major proportion of landslides in different parts of the world. Efforts are make to understand the mechanism that triggers landslide due to rainfall. One often cited trigger factor of rainfall-induced landslides is critical rainfall. The critical rainfall can vary in duration from 1 hour to antecedent rainfall of several days. However, such critical rainfall is obtained from observation at a particular location and may not be used or extrapolated to other locations. This paper collates case studies of rainfall-induced landslides that occurred in different parts of the world. Information collated includes location, soil type, permeability, slope angle, slope height, average annual rainfall of that area and the critical rainfall responsible for the landslide. The relationships between critical rainfall and other variables are examined critically. The study found that only soil type, slope angle and critical rainfall were useful. The study shows that 1-hour and 1-day critical rainfall increases with slope angle. It appears that the critical rainfall has an exponential relationship with slope angle. For coarse-grained soils, the 1-hour critical rainfall exponential relationship with slope angle can be used as the trigger rainfall for slope failures. For fine-grained soils, the 1-day critical rainfall exponential relationship with slope angle can be used as the trigger rainfall for slope failures. However, the findings are based on limited case studies. These relationships should be further validated with more case studies.

Bentonite pellet mixtures have been recognized as an effective backfilling material for the sealing of boreholes and tunnels during the construction of engineering barrier system in deep geological disposal of high-level radioactive waste. Once installed in the repository, the complicated hydro-mechanical coupling conditions may affect the swelling properties of pellet mixtures. In this study, a series of swelling pressure tests and swelling deformation tests were conducted on GMZ bentonite pellet mixtures. Results showed a good linear relationship between final swelling pressure and void ratio in swelling pressure test. Similar phenomenon was also observed between the upper loading and void ratio in swelling deformation test. Furthermore, suction effects on swelling pressure and swelling strain of bentonite pellet mixture indicated that, at high suction stage (> 4.2 MPa), the swelling of specimens was mainly controlled by the crystalline swelling and the exfoliation of clay particles. At lower suction stage (< 4.2 MPa), significant increases on swelling pressure and swelling strain were found, which might be related to osmotic swelling.

In order to investigate the volume change behavior of saturated GMZ bentonite during cyclic loading-unloading processes, oedometer tests with cyclic loading-unloading process were conducted on saturated compacted GMZ bentonite and slurry specimens. A significant hysteretic loop was observed in compression curve of saturated compacted GMZ bentonite during cyclic loading-unloading process. Compression curve of the slurry specimen presented a bilinear shape. Under smaller stresses (<0.5~2MPa), the unloading-reloading loop was insignificant. However, under higher stresses (>0.5~2MPa), the unloading-reloading loop was significant. The unloading-reloading loop could be explained by the competition between the mechanical and physico-chemical effects on the microstructure changes. When stress was higher than 10MPa, compression curve of the slurry gradually closed to and coincided with the compression curve of the compacted bentonite specimen. Therefore, normal consolidation line of GMZ bentonite could be determined based on the overlapped part of the compression curves.

Foundations on expansive soils pose a unique challenge to geotechnical engineers. Oftentimes they cost more to design and construct than foundations on ordinary soils. Free-field heave is the fundamental parameter on which foundation heave is calculated. A method for prediction of free-field heave using oedometer test data was outlined in Nelson and Miller (1992). A refinement of that method is presented in Nelson et al. (2006). To determine free-field heave, it is necessary to have a measured value of percent swell from a consolidation-swell (CS) test and the swelling pressure, σ'cv, measured in a constant volume (CV) test. Only the CS test is commonly conducted in geotechnical engineering practice. Therefore, it is convenient to have a relationship between the swelling pressure, σ'cs, measured in the CS test and σ'cv, so that heave prediction can be determined from only a single test. Several investigators have proposed relationships between σ'cv and σ'cs (Edil and Alanazy, 1992; Reichler, 1997; Bonner, 1998; Thompson et al., 2006; Nelson et al., 2006 and 2012). Nelson and Chao (2014) proposed a relationship between σ'cv and σ'cs based on the experimental results of expansive soil behavior and facilitated the heave prediction from a single oedometer test for each soil. Their method used the parameter "m" which relies on specific soil property. This paper further investigated the change of parameter "m" for various soil samples. The soil samples used in this study were obtained from two different locations in Myanmar and Thailand. Bentonite and sand mixture was also tested in this study for comparison purposes. Furthermore, the relationship between the "m" parameter and the degree of saturation of the soil samples were evaluated in this study. It was concluded that the linear trendline determined by Nelson and Chao (2014) provides the most simple and accurate equation for determination of the m value and the m value decreases as the degree of saturation of the soil increases.

The mechanical behavior of expansive soils is influenced by the concentration of salts in their pore water. Research has shown that volume change of soil can occur due to a difference in salt concentration in the pore water between different zones of the soil as a result of either osmotically-induced consolidation or osmotic consolidation. The effect of the salt concentration of the pore water in unsaturated soil mechanics can be expressed as osmotic suction. Very little work has been done to quantify the mechanical equivalence of osmotically-induced and osmotic consolidation. This study attempts to quantify the mechanical stress equivalence of consolidation of an expansive soil submerged in a salt solution. Two remoulded soil samples of kaolin - bentonite mixture in proportions of 70% - 30% and 90% - 10%, (kaolin - bentonite) by dry mass were submerged in different concentrated salt solutions to investigate the effect of osmotic suction. Results showed that osmotic suction caused an additional settlement over the consolidation settlement under a mechanical stress but does not affect the soil compressibility. The osmotic coefficient of volume change (mπ) is only a fraction of the coefficient of volume change (mv).

The soil water retention curve is very critical in the assessment of unsaturated soil behavior. In the past, with the help of the axis translation technique, the pressure plate method was usually used to measure the soil water retention curve. However, this test was very time-consuming and required using specimens with the same loading history which was difficult to achieve. In this study, a simple device is proposed to measure the water retention curve of poorly graded sand with silt through a discrete evaporation test in which the soil suction and water content are measured using a high-capacity tensiometer and a digital balance, respectively. Measurement of representative soil suction was achieved through sealing the specimen to allow the water redistribution in the soil after a certain time of free evaporation. Results from this evaporation test indicate that the time required for the soil water retention curve can be significantly reduced from weeks to 1-2 days when compared with the conventional pressure plate method.

In order to improve the efficiency of the Soil Water Characteristics Curve (SWCC) determination and shorten the testing time, a new SWCC obtaining method utilizing the Continuous Pressure Method (CPM) was developed. The newly developed system allows continuous measuring of the suction by pressurizing the sample and continuously measuring the developing pore water pressure using a micro-tensiometer installed at the center of the sample. The suction (s) is defined as the difference between the applied air pressure and the measured pore water pressure at the center of the specimen (s = ua - uw , where ua = air pressure, uw = pore-water pressure). Both the drying and wetting (SWCC) can be obtained in 2 to 3 days. It is found that the SWCC under both the drying and the wetting phases obtained using newly developed apparatus are in well agreement with those obtained using the conventional multi-step flow method. In addition, the optimum pore-water pressure measuring point was clarified by measuring the pore water pressure at four different levels and comparing the results. Where it was found that the SWCC obtained considering the pore-water pressure being measured at different levels agree well with SWCC obtained considering the pore-water pressure being measured at the center of the specimen. Finally, it was concluded that the pore-water pressure measured at the center of the specimen can be considered as a representative point for obtaining reliable SWCC.

A series of experimental tests were conducted to analyze the change of dielectric constant with varied particle composition and moisture content, and the experimental data were compared with several commonly used dielectric constant models. Results show that both the empirical models and the volumetric mixing models are more applicable to sandy soils and will overestimate the dielectric constant of cohesive soils due to the neglect of the effect of bound water. Actually there are a large number of strong and weak bound water in cohesive soils and the dielectric constant of bound water is much smaller than that of free water. To describe the effect of mesoscopic particle composition and bound water, a new logarithmic dielectric constant model is put forward. The application of this model is verified by others' experimental data and the calculation of dielectric constant of cohesive soils can be more accurate with this model. The moisture condition of soils can be judged roughly according to the ratio of the increment of dielectric constant and volumetric water content.

The soil-water characteristic curve (SWCC) is an essential property in unsaturated soil mechanics. The test methods to determine SWCC are mainly adopted or adapted from either soil science or agriculture. Determining the SWCC is very labour intensive and time-consuming. Hence, many pedo-transfer functions (PTFs) for SWCC were developed in soil science and agriculture. The PTFs can be grouped into point and parametric PTFs. The point PTFs give the water contents at several suctions while the parametric PTFs give the values of the parameters in SWCC equations. The suctions commonly used in point PTFs are 4 (or 3), 10, 33, 100 and 1500 kPa. In this paper, a simple method is proposed to estimate unimodal SWCC using an ensemble of point PTFs. The proposed method is illustrated using six coarse-grained and six fine-grained soils from literature. The results show that the proposed method is able to estimate a unimodal SWCC with good accuracy.

For unsaturated soils, the soil water characteristic curve (SWCC), which represents the relationship between degree of saturation and suction, is a crucial function for determining both the soil hydraulic property and its coupled mechanical properties. However, because the tests for measuring the SWCC take a long time and the process is complicated. It is very necessary to propose a simple and practical method for predicting the SWCC of unsaturated soil. The grading curves and grading parameters of the five soils in the literature were obtained. And the fitting parameters a, n, m of the Fredlund and Xing model for five soils were obtained also. By means of the regression analysis between the grading parameters and fitting parameters of SWCC, a simple and practical method was proposed for predicting the SWCC curve based on the grading parameters. This may be helpful for engineering practice at an early stage when not much experimental data are available.

Matric suction significantly attributes on both strength and stiffness of the geomaterials. Earthen structures constructed by soil containing fines are profoundly found to be at an unsaturated condition. The unsaturated soil is often subjected remarkably significant and continuous changes in matric suction with variation in moisture content. This study focuses on an experimental investigation of Matric suction variation and its attribution on stiffness by means of elastic wave measurement on fine sand (Toyoura sand) with two types of fines; kaolin and non-plastic silt as well as natural soil containing fines(Edosaki sand). The small-scale triaxial apparatus equipped with disk transducer for measuring elastic wave and pressure membrane technique for evaluating matric suction is employed. A cylicndrical specimen is prepared by sandy soil with several amounts of fines and subjected to change in moisture content by injecting water inside the specimen. Both compressional and shear waves velocities are measured in conjunction with an associated matric suction at low stress level that replicates the soil behavior of the shallow depth. This investigation reveals that the presence of fines in soil appears to play an important role in mechanical behavior of sandy soil. The small strain stiffness as well as dynamic properties seems to be affected by the moisture condition in the soil.

This paper describes development of a triaxial permeameter apparatus for measuring coefficient of permeability of unsaturated soils by directly measuring the head difference between two points in a specimen using local pin-type sensors. The local pin-type sensors are made of stainless steel wrapped with membrane filter to measure suction. The apparatus includes the measurement of inflow rate by Mariotte's bottle and a weighing scale, while outflow rate is measured by burettes and a differential pressure transducer (DPT). Typical results show that the steady state flow condition can be achieved by using membrane filters to allow passage of water but prevent flow of free air. At steady state flow condition and reasonably stable measured head by the local pin-type sensors, permeability values were obtained by simple average.

This study discussed water migration behavior in a bentonite-based buffer from an unsaturated state to saturation by the laboratory experiments measuring water absorption and swelling pressure. It investigated and considered water migration behavior in an unsaturated bentonite from the viewpoint of material specifications such as the kinds of bentonite and dry density. From the experimentally obtained results, water migration behaviors in various unsaturated bentonites as buffer are analogous to water-diffusion from initial water content to around 90% saturation degree.

Bentonite swells after absorbing water and forms colloids that can release gel particles into solution causing the erosion of bentonite particle. The process of colloidal erosion can be described by the free expansion of bentonite. A new designed one-dimensional free swelling tests is performed to simulate one dimension erosion of compacted bentonite. Compacted specimens are made into two initial dry densities and water contents. De-ionized water and NaCl, Na2SO4, CaCl2 solutions with concentrations of 0.1, 0.5, 1.0 and 2.0 mol/L were used to soak the specimens. The effects of initial dry density, initial water content and saline solution on the free swelling behavior of compacted commercial bentonite are analyzed. Results show that, the final total swelling deformation is almost independent of initial water content, but increases with the initial dry density and decreases with the increase in osmotic suction of the infiltration solution.

In this study, gas breakthrough tests were conducted on GMZ01 bentonite specimens with dry densities 1.3, 1.5 and 1.7 Mg/m3 under rigid boundary conditions using the residual capillary pressure (RCP) technique. Prior to the gas breakthrough tests, water permeability tests were performed for determining intrinsic water permeability and establishing full water saturation for the follow-up gas breakthrough tests. The intrinsic water permeability measured ranges between 11 and 150 nDarcy (1.10×10-20 and 1.50×10-19 m2). As dry density increases, the residual capillary pressure differences of the initially water-saturated bentonite specimens recorded increase from 0.12 to 0.61 MPa. Meanwhile, the time to breakthrough was recorded to increase sharply as dry density increases. In addition, the maximum effective gas permeability decreases from 8.91×10-18 to 8.92×10-19 m2 as dry density increases from 1.3 to 1.7 Mg/m3. These results indicate that the capillary drainage in the bentonite specimen is highly related to the capillarity effects in the largest pores. The higher dry density, the smaller flow channels, resulting in more difficult for gas phase to dispel water from the interconnected flow pathways and a better sealing capacity with a higher residual capillary pressure.

This paper presents numerical simulation of an infiltration test performed on MX-80 block-pellet laboratory sample. The simulation is performed with Thebes Code, a fully coupled THMC finite element program. The focus of the simulation is to recover the observed swelling pressure and dry density of the initially inhomogeneous system during the hydration process and at the final fully saturated state. The numerical results show that the simulation is capable to capture the qualitative response reported in the experimental results. However, the quantitative predictions still need improvements. That could be achieved by better simulation of the three-dimensional geometry of the pellets and further incorporation of the microstructural effects in the employed hydro-mechanical constitutive models.

This paper explores the influence of the particle size on the water retention properties of a colluvial deposit derived from banded iron formations in the Pilbara region in Western Australia. A small-scale experimental program aimed at evaluating the water retentions curve of four particle fractions (1.18 - 2.36 mm, 0.60 - 1.18 mm, 75- 600 μm and < 75 μm) is described. Water retention curves, defined in terms of the gravimetric water content vs total suction, were obtained by exposing the specimens to a controlled wetting path using the vapour transfer technique. Total suction was measured using a dew-point psycrometer. The experimental results show an important increase in the maximum water retention capacity with decreasing the particle size. Finally, the influence of the water storage of each particle fraction on the water retention curve of mixed specimens, i.e. those composed by several fractions, is discussed.