Unsaturated Silty Research Articles

The migration process and temperature effect of aqueous solutions contaminated by heavy metal ions in unsaturated silty soils

Adsorption-desorption experiments of three heavy metal ions (i.e., lead, copper, cadmium) in silty soil were carried out at different temperatures, and the microscopic characteristics of silty soil loaded with the three heavy metal ions were analyzed. A one-dimensional soil column was used to discuss the influences of heavy metal ion types and concentrations on the soil moisture distribution and the migration level of different heavy metal ions, especially during the dynamic change process from an unsaturated state to a saturated state. Studies show that the adsorption of heavy metal ions onto silty soil is closely related to the mineral composition and functional groups in silty soil. In addition to physical adsorption, the adsorption of heavy metal ions is closely related to the hydrolysis reaction of mineral components such as kaolinite, calcite, dolomite, plagioclase and quartz. Under constant temperature, the types and concentrations of heavy metal ions play an important role in the moisture migration of unsaturated soil. In the presence of heavy metal ions, the penetration of lead ions is the greatest, followed by copper ions and then cadmium ions. The greater the ion concentration is, the stronger the penetration of heavy metal ions in silty soils.

On The Compression Behaviour of Unsaturated Silty Tailings: An Experimental Investigation

On The Compression Behaviour of Unsaturated Silty Tailings: An Experimental Investigation

Analysis of liquid–vapor mixed migration mechanism in unsaturated soil based on the effect of temperature on soil microstructure

Moisture migration in unsaturated soils is a result of the interaction between temperature and soil microstructure. In order to reveal the mechanism of moisture increase of subgrade soils under diurnal cycle conditions, a series of macro and microscopic tests were carried out on the unsaturated silty clay and sand soil, including liquid–vapor mixed migration tests simulating a one-dimensional subgrade, environmental scanning electron microscope (ESEM), and matrix suction test. Then, the soil microstructure in microscopic images was investigated using the particle (pores) and cracks analysis system (PACS). Next, the relationship between the thermal effects of the soil–water characteristic curve (SWCC) and changes in soil microstructure was analyzed. Finally, the change mechanism of liquid–vapor mixed migration based on the change in soil microstructural under thermal effects was analyzed. The results showed under the diurnal cycle, both the silty clay and sand soil columns appeared in the phenomenon of a “diurnal cycle of water vapor migration”, which led to moisture accumulation at the top of the soil layer. In silty clay soil column, moisture was primarily driven by water vapor pressure and migrated upwards. Additionally, moisture redistribution led to changes in soil microstructure, which in turn influenced the process of moisture migration. The moisture content in the upper soil layer increased making both inter-aggregate and intra-aggregate pores decrease. The moisture content in the lower soil later decreased, leading to the water-holding capacity of the lower soil layer to increase. So, the moisture migration gradually decreased at night. In the sand soil column, moisture migration was mainly driven by gravity potential and migrated downwards. Moisture redistribution made inter-aggregate pore and matrix suction of the upper soil layer increase, leading to an increase in moisture migration at night.

Investigation of the critical state behavior of unsaturated silty sand with different fine contents in terms of net and effective stresses

This paper investigates the effect of fine content and matric suction on the behavior of silty sand soil through both net and effective stress variables in the critical state soil mechanics framework. For this purpose, a set of CD triaxial tests in saturated and unsaturated states were performed on the samples of sand with different fine contents. The results showed that in term of net stress variables the position of the critical state line (CSL) in stress space for a given non-plastic fine depend on suction. However, these lines approach each other by choosing a proper effective stress relationship. The CSLs in the coordinates of the void ratio in the term of effective stress changed as a function of the non-plastic fine content and matric suction. However, using the concept of the equivalent intergranular void ratio and considering a special mean effective stress variable depending on suction, the CSLs were converged so that a single line can be defined to describe the behavior of the silty sand at the critical state. Finally, the stress-dilatancy relationship of two constitutive models was examined and it was shown that the use of the equivalent state parameter could facilitate the use of these relationships.

Modelling unsaturated silty tailings and the conditions required for static liquefaction

The potential for static liquefaction of tailings is a major focus in the design and operation of tailings storage facilities. This research models the behaviour of unsaturated tailings, with a variety of degrees of saturation, addressing the propensity for static liquefaction during monotonic loading. Unsaturated triaxial tests, including constant suction conditions and constant water–air mass conditions, were performed. A bounding surface plasticity model was used to simulate the results. The constant mass condition is relevant to undrained closed-system loading, which may prevail during fast deformation after the tailings becomes unstable, when the air and water in the pore space remain locked inside the tailings. Boyle's law and hydraulic hysteresis were accounted for to model the changes of pore air and water pressures, and suction, with the change in tailings volume. Good agreement was achieved between test results and model simulations. Additional simulations to mimic rising water tables under constant total stress states in the field, situations that may trigger instabilities, are also shown. Results are added to charts which relate peak and post-liquefaction strengths, as well as collapse lines, to measures of initial state, for unsaturated conditions, which may be of use in practice.

Effect of initial void ratio on the tensile strength of unsaturated silty soils

Effect of initial void ratio on the tensile strength of unsaturated silty soils

Bearing Capacity of Shallow Foundations on Unsaturated Silty Soils

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.

Influence of aggregate structure on the compressibility of an unsaturated compacted silty sand

It is well recognised that the compressibility of unsaturated soils is affected by soil structure. However, the effects of soil aggregation and pore structure on the compressibility behaviour of unsaturated soil are still not well understood. In this study, the effects of soil aggregation and pore structure on the compressibility of unsaturated silty sand were investigated. Specimens were purposely prepared at two compaction water contents to similar void ratios. They were saturated after compaction and dried to suction values of 0, 50, 150, or 300 kPa following a similar hydraulic loading path, after which they were subjected to isotropic loading and unloading. Mercury intrusion porosimetry (MIP) and micro-X-ray computed tomography (μ-XCT) were used for microstructural studies. At a given suction, specimens compacted on the dry side are less aggregated (smaller aggregate sizes) showing higher compressibility, whiles specimens compacted on the wet side are highly aggregated (larger aggregate sizes) revealing lower compressibility. The low compressibility of the highly aggregated soil is attributed to the additional drying-induced intra-aggregate strength due to the suction stiffening effects from meniscus water within the aggregates.

Experimental Study on the Effect of Moisture Content on Shear Strength of Unsaturated Silty Clay

Experimental Study on the Effect of Moisture Content on Shear Strength of Unsaturated Silty Clay

Exploring the effect of basalt fibers on maximum deviator stress and failure deformation of silty soils using ANN, SVM and FL supported by experimental data

Because the experimental trials in civil engineering field are difficult and time-consuming, the application of artificial intelligence (AI) techniques is attracting considerable attention, with their use enabling successful results to be more easily obtained. In this study, we investigated the effect of fiber size, fiber amount, water content, and cell pressure on maximum deviator stress (MDS) and failure deformation (FD) of basalt fiber (BF)-reinforced, unsaturated silty soils using three AI techniques: the artificial neural network (ANN), support vector machine (SVM), and fuzzy logic (FL). The numerical analyses and experiments were conducted using varying amounts (1, 1.5, and 2%) and lengths (6, 12, and 24 mm) of BF, and a total of 180 samples were prepared for the detailed investigation. In order to compare model performances, R2 and MAPE goodness-of-fit metrics were used. The experimental results revealed that the addition of BF generally increased the MDS of the soils, which corresponds to the shearing resistance. According to AI models result, FL outperformed the SVM and ANN, with a R2 value of 0.938, especially in FD prediction. The sensitivity analysis was performed to ascertain the effect of the inputs on the MDS and FD response variables. Results revealed that fiber length and cell pressure have substantial influence in MDS estimations.

Experimental study on critical state behaviour of unsaturated silty sand under constant matric suctions

There are relatively few experimental studies on the hydromechanical and critical state behaviours of unsaturated silty sand. In this paper, suction-controlled, consolidated, drained triaxial shear tests were conducted on silty sand under saturated (net confining pressures of 100, 200, 300 and 400 kPa) and unsaturated (suctions of 30, 100 and 200 kPa and net confining pressures of 100, 200 and 300 kPa) conditions. For unsaturated silty sand, the effects of dry density, matric suction and net confining pressure on the hydromechanical and critical state behaviour were investigated, and the critical state parameters were obtained. The main findings for unsaturated silty sand included shear shrinkage at low dry density (1·6 g/cm3) and dilatancy at higher density (1·96 g/cm3). The peak shear strength was positively correlated with dry density, matric suction and net confining pressure. The critical state volumetric strain and critical state degree of saturation were negatively correlated with dry density and matric suction but positively correlated with net confining pressure. A new dilatancy equation that includes the state parameter for unsaturated silty sand was established. A unique critical state line passes through the origin in the effective stress [Formula: see text]–deviator [Formula: see text] plane, and its slope is the saturated critical state stress ratio [Formula: see text]. In the lnp*–v plane ([Formula: see text] is the specific volume), the critical state lines corresponding to different matric suctions are parallel.

Study on the frost heave behavior of the freezing unsaturated silty clay

The frost heave behavior of unsaturated soils is related to the water-vapor redistribution, whereas the water-vapor migration processes cannot be directly observed due to the limitations of measurement techniques. To reveal the frost heave behavior of the unsaturated silty clay, five specimens for unsaturated silty clay were prepared. One-side freezing tests for the five specimens were carried out under uniform conditions. The test durations are 12 h, 24 h, 48 h, 96 h, and 192 h, respectively, and the water content profiles in the freezing process were studied by testing final water content profile of each specimen as the profile at that time. The heat-water-vapor migration and frost heave behavior of the freezing unsaturated silty clay were interpreted. The results showed that the temperature variations can be divided into three stages (a sharply cooling stage, a slowly cooling stage, and a steady stage) according to the change rate of temperature, and the duration times of the first and the second stage are short. A mix migration of liquid water and water vapor in unsaturated soils can be experimentally determined during the freezing, and a thin ice forms at the top of the specimens due to the vapor migration and condensation. Meanwhile, the liquid water in unfrozen zone mainly migrates to frozen fringe, and segregated ice forms in the freezing unsaturated soil once enough water accumulates at the frozen fringe. In addition, the frost heave of the specimens is discrete under the uniform test conditions. We selected the relative error and absolute error of the frost heave as evaluation indexes, and further evaluated the repeatability of the test results. Evaluation results showed that the one-side freezing test results are repeatable, and it is an useful testing method to study the heat-water-vapor migration through one-side freezing tests with different test durations under uniform testing conditions. Moreover, the confusion that the frost heave mainly occurred in the top layer of high-speed railway subgrade was explicit solved by the behavior revealed in this paper.

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

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

Seismic Compression of Unsaturated Silty Sands: A Strain-Based Approach

The vast majority of surface structures are located on or surrounded by unsaturated soil deposits and may suffer excessive settlements during earthquakes. However, the fundamental understanding of the mechanisms by which the degree of saturation impacts the volumetric deformation of soils during seismic loading is still not mature. Consequently, it is critical to develop and calibrate seismic compression models while considering these mechanisms. The objective of this paper is to experimentally investigate the impacts of the degree of saturation, fines content, and desaturation technique on the seismic compression of sand and silty sands. The experimental program involved undrained cyclic direct simple shear tests on specimens prepared using suction control and wet-compaction techniques with similar relative densities but different levels of saturation. A strain-based predictive model was adapted and modified to capture the observed trends in the seismic compression of soils with different degrees of saturation. The suitability and applicability of the model were verified by comparing the measured and estimated compression values in this study with ones reported in the literature for other soils and desaturation approaches.

Effects of Structure on the Compression Behavior of Unsaturated Loess

The influence of structure plays a strong part in hydromechanical properties of soil. A series of suction-controlled oedometer tests have been conducted to explore the compression behavior of intact and compacted loess. The main findings show the following: (1) the yield stress of intact loess is higher than that of compacted loess at a given initial void ratio and under constant suction; (2) the slope of the normal compression line [λ(s)] of compacted loess is larger than that of intact loess; (3) the λ(s) value of intact loess initially increases sharply as suction increases and then trends to a slow decrease as suction increases further, while λ(s) of compacted loess continues to increase; (4) the λ(s) value of compacted loess can be fitted precisely with the Barcelona basic model; and (5) intact loess exhibits anisotropic behavior, in which a vertically trimmed specimen has a higher yield stress and λ(s) value than a horizontally trimmed specimen. The results showed that intact specimens and compacted specimens have different hydromechanical properties. Moreover, anisotropic behavior has also been found in unsaturated silty loess.

Loading rate effect on mechanical properties of an unsaturated silty sand

This paper presents the results of an experimental study on the effects of testing rate on stress-strain behavior and volumetric changes of soil. A series of suction-controlled triaxial tests has been performed on reconstituted specimens of a silty sand (SM), at different stress-rates and strain-rates, respectively. The stress-strain paths were applied by using a modified version of a Bishop and Wesley device (USPv2), capable of applying independently pore-water and air pressure at both ends of the soil sample. During the isotropic compression stages loading rates of 2 and 32 kPa/h have been applied under constant suction values of 15 and 45 kPa. The drained deviator stages were conducted at the same suction levels under strain rates of 0.25 and 2.50 %/h. Results are presented in terms of applied loading rates as a function of the specimens specific volume, preconsolidation pressure, soil compressibility and deviatoric stress against strain rate. A comparison of results was made to a former study, under similar testing conditions of suction and loading rates at University of Napoli Federico II. The effect of loading rate on the soil behavior seems to have an insignificant effect on the specific volume variations, for the imposed values during the testing campaign.

Effect of grain size on the shear strength of unsaturated silty soils

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

Experimental Study on Mechanical Behavior of Unsaturated Silty Sand in Constant Equivalent Granular Void Ratio

In this paper effect of fine mineralogy on mechanical behavior of unsaturated silty sand in different fine contents and divers confining pressure has been studied. All samples were molded with constant equivalent granular void ratio well-known parameter already proposed for characterizing silty sand behavior in saturated state. This manner of study allow to investigate also the validity of equivalent granular void ratio concept in unsaturated state. For this purpose, a series of triaxial tests were performed on the sand specimens with different percentages of silt in the undrained saturated (CU) and unsaturated (CW) conditions. The results showed that the material types and aggregate distribution of the fines have enormous effects on the silty sand behavior. In addition, the shear strength in the unsaturated specimens changed as a function of the initial applied matric suction. A fewer performance of equivalent intergranular void ratio in the case of unsaturated state in comparison of saturated states was observed.

Experimental Measurement of Creep Deformations of an Unsaturated Silty Soil with Matric Suction

Experimental Measurement of Creep Deformations of an Unsaturated Silty Soil with Matric Suction

Effect of Temperature on the Strength Characteristics of Unsaturated Silty Clay in Seasonal Frozen Region

The strength characteristics of unsaturated silty clay after the cooling and single freeze-thaw cycle were studied by using Global Digital Systems Ltd. (GDS) triaxial test system after controlling initial matric suction. The results demonstrate that, the initial matrix suction and freezing process can strengthen the shear strength of the soil, which makes the stress-strain curve of the soil show certain strain hardening characteristics at different temperatures. when the temperature is negative (including 0°C), the curve has a flat transition section whose strength does not change with deformation, and the flat transition section becomes shorter as the temperature and confining pressure decrease. The strength of the soil at negative temperature and the speed at which it reaches its peak value are much greater than those of the soil after positive temperature and single freeze-thaw cycle, and the contribution of confining pressure to the strength decreases with the decrease of temperature. The effect of temperature change (15°C to −15°C) on soil strength is mainly reflected in the total cohesion, and the effect of freeze-thaw on the effective internal friction angle is more significant. The strength and deformation characteristics of soil under the freezing process and freeze-thaw cycle are determined by the initial matric suction, the form of connection between soil particles, the migration of unfrozen water, and the degree of water-ice phase transformation.