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Suffusion refers to the loss of small particles in a soil matrix without volume changes, caused by hydrodynamic forces and alternation of pore fluid chemistry. This study investigated the ionic concentration (IC)-induced suffusion of sand–clay mixtures under saturated and unsaturated conditions through laboratory soil-column experiments. The impacts of clay mineralogy and sand-grain size on suffusion were investigated based on the breakthrough curves (BTCs) obtained from the soil-column experiments. In addition, the degree of saturation during the experiments was measured to assess the impact of the saturation state on suffusion in the sand–clay mixtures. The observed BTCs in this study demonstrated that suffusion was more significant under saturated conditions than unsaturated conditions for the sand–kaolinite and sand–illite mixtures. In contrast, the sand–bentonite mixtures exhibited higher susceptibility to suffusion under unsaturated conditions. Moreover, for the sand–kaolinite mixtures, a higher susceptibility to suffusion was observed with smaller sand-grain sizes under saturated conditions, whereas the opposite trend was observed under unsaturated conditions. This study provides insights into the suffusion behavior of sand–clay mixtures, considering influential factors such as clay type, sand-grain size, and IC gradient under both saturated and unsaturated conditions, which highlight the importance of considering the saturation state for a comprehensive understanding of suffusion in the sand–clay mixtures. [ABSTRACT FROM AUTHOR]

The soil is usually unsaturated in practical engineering, and the slope failure surface generally presents three-dimensional (3D) characteristics. Therefore, the traditional plane problem calculation method does not accurately describe the slope's stability and safety assessment results strengthened through frame beam anchor plates (FBAPs). This work introduces a novel approach based on the limit analysis (LA) technique for assessing the 3D stability of unsaturated soil slopes supported by FBAPs to address this issue. The slope safety factor (Fs) expression was derived based on the energy balance principle. The soil slope stability the FBAP reinforces was quantitatively expressed as a function of slope parameters. Nonlinear distribution features of soil matrix suction and effective unit weight, as well as the internal energy dissipation rate created by the anchor plate, must be taken into account when calculating the internal energy dissipation and external work rate. The impact of the 3D effect, seismic parameters, unsaturated soil characteristics, and supporting structure on slope stability was discussed. Some design suggestions for slope reinforcement are put forward. According to the findings, the suction effect's beneficial effects on stability should be considered. Slopes with a width-to-height ratio (B/H) of less than 10 should consider the 3D effect's beneficial effects. The seismic action significantly negatively impacts slope stability, while using FBAP reinforcement structures can dramatically improve slopes' static and seismic stability. [ABSTRACT FROM AUTHOR]

Pile response in unsaturated soils is an area of interest due to its prevalence in real-world scenarios. Significant differences exist in the lateral behavior of single piles in partially saturated soils due to increased soil stiffness. In this study, a single pile was monotonically loaded laterally to investigate the performance of piles in soils with variable water table levels. Pile head lateral response showed that a mix of unsaturated-saturated soil led to an increase in lateral loads on the pile at a similar displacement compared with the fully saturated or dry soil conditions. The lateral response of the pile was obtained based on the curvature data from the strain gauges. A derivative method was used to generate the shear force and lateral resistance along the pile length based on finite differences at the seven strain gauge locations, thereby eliminating errors associated with curve fitting. Various lateral response plots, such as the normalized p-y curves, M-y curves, and maximum bending moment response plots, were used to assess the response to lateral load. Further, the effect of two different loading rates were assessed on the fully saturated and unsaturated-saturated soil, and the range of loading rates evaluated had minimal influence on the pile lateral response. Results indicated that considering the influence of the unsaturated soil could increase lateral resistance under similar lateral displacements. However, there is a risk of inducing large pile bending moments, potentially leading to pile flexural failure if the pile plastic moment is reached. [ABSTRACT FROM AUTHOR]

To address the engineering problem of wall collapse and destabilization during the construction of the slurry trench wall, the stability of the slurry trench wall under the condition of three-dimensional (3D) unsaturated stable seepage is studied using the upper limit analysis method and in combination with the three-dimensional rotating failure mechanism. The expression of the factor of safety (FS) for the unsaturated soil slurry trench was deduced by the gravity increase method. Additionally, a genetic algorithm was used to search for the minimum FS and the critical slip surface, and the obtained results were compared and validated with existing literature results. The effects of relevant parameters on the stability of the slurry trench under the condition of unsaturated seepage were analyzed in detail. The results of this paper indicated that ignoring the 3D effect of the slurry trench will underestimate the stability of the slurry trench. The presence of suction in unsaturated soil is beneficial to the stability of the slurry trench, and the impact of suction is closely associated with seepage conditions and the characteristics of the soil. The increase in surcharge load can lead to a decrease in the FS, and its effect on slurry trench stability increases with the enhancement of the three-dimensional effect. The impact of different seepage conditions on the sliding surface of the channel wall of clay strata is more significant, while the effect on the trench wall of sand strata is negligible. [ABSTRACT FROM AUTHOR]

Rainfall, evaporation, tides and waves make the soil at coastlines show dynamic unsaturated characteristics. In order to cope with landslides, collapses, and coastline receding, microbially induced calcium carbonate precipitation (MICP) was recently implemented to reinforce the soil in slopes and coastal zones using a spraying method. However, most of the researches on MICP focus on soil with a static saturation degree, and there are few numerical researches about the MICP reactions under dynamic saturation degree. Therefore, a coupled numerical model capable of describing the evolutions of bacteria, substance, calcium carbonate (CaCO3), porosity, and permeability in unsaturated soil over time reasonably was developed based on the convective–diffusion–reaction theory and the Richards' equation. The effect of MICP reactions on the parameters α, n, θs, θr in van Genuchten-Mualem (VG) model was considered. A large cylinder test (30 × 75 cm) about reinforcing sand based on the spraying method was carried out to validate the model. The CaCO3 distribution of the MICP infiltration and mineralization was fully reproduced and the concept of critical depth (Zcr) was proposed. The initial porosity, initial bacterial concentration, and spraying rate were the main parameters affecting the CaCO3 distribution and critical depth Zcr. When the bacterial concentration increased, the maximum CaCO3 content, critical depth Zcr, and normalized permeability coefficient (k/k0) indicated exponential increase, exponential increase, and exponential decrease. The saturated permeability coefficient of the sand was determined by the maximum calcium carbonate. The normalized permeability coefficient (k/k0) and critical depth Zcr showed an exponential decrease and a power function increase with the increase of the maximum CaCO3 content. [ABSTRACT FROM AUTHOR]

Realistic estimates of evaporation from unsaturated soils, which are important for many geotechnical and geoenvironmental applications, need to be considered in the context of soil settlement, and require thermohydromechanical (THM) analysis. Evaporation also depends on environmental parameters, including air temperature, air relative humidity, net radiation, and wind speed. Therefore, a consideration of atmospheric coupling in predicting evaporation is also necessary. In this study, the two-dimensional model EVAP1––which numerically estimates evaporation from unsaturated soil using THM and employing a soil–atmosphere model––was used to conduct a parametric study in order to investigate the effects of variation in environmental parameters and to study the effects of climate change on potential and actual evaporation and soil settlement. We found that the evaporation rate increased nonlinearly with increases in air temperature, net radiation, and wind speed, but decreases with an increase in relative humidity. However, the effect of a change in wind speed was less than the effect of a change in the three other environmental parameters. In addition, the change in evaporation rate differed in different regions with different air temperatures. For example, the temperature change had more of an effect on the evaporation rate at higher temperatures. In addition, neglecting soil settlement led to an overestimation of evaporation, albeit the amount of evaporation was almost the same whether soil settlement was considered or not, both at the beginning of the evaporation process, when potential evaporation was dominant, and at the end, when the water content in both cases had decreased almost to the residual water content, and the evaporation was minimized. The difference in the amount of evaporation was greater in the middle of the evaporation process, when actual evaporation dominated. [ABSTRACT FROM AUTHOR]

The limit support pressure significantly influences the stability of the shield tunnel face and deformation control during tunneling. Current research predominantly focuses on soils under dried or saturated conditions. However, unsaturated soils, which are more commonly encountered in engineering practice and widely distributed, have not been as extensively studied. Based on limit equilibrium theory, a modified analytical wedge-prism model is proposed in this paper to predict the limit support pressure acting on the shallow shield tunnel face in unsaturated sandy soil where the soil arching effect is considered. Taking advantage of finite-element software ABAQUS (version 2021), a series of numerical trapdoor tests are conducted first to study the failure behavior of unsaturated sandy soil. Then, by combining the rotation trajectory of major principal stress with the shear strength theory of unsaturated soil, the loosening earth pressure solution above the shallow trapdoor can be derived analytically. The limit support pressure can be finally obtained through a force equilibrium analysis of the wedge body. The accuracy of the proposed model is validated by comparing the calculated results with current model tests and numerical simulations. Parameter analysis results reveal that both the loosening earth pressure and limit support pressure initially decrease and then increase with the rising degree of soil saturation. A critical saturation point is identified where the loosening earth pressure and the limit support pressure reach their minimum values, attributed to the full development of the soil arching effect. [ABSTRACT FROM AUTHOR]

Suction stress is one of the most significant factors affecting the serviceability and stability of soil structures. This study presents a framework for analyzing the stability of geosynthetic reinforced soil (GRS) walls and slopes under unsaturated conditions. An analytical formulation of suction stress-based effective stress was implemented into a limit equilibrium solution, namely, the top-down procedure. The developed framework enables the prediction of the tensile load distribution and connection load between the reinforcement and wall face considering the pullout resistance of reinforcements in GRS walls backfilled with granular and marginal soils under unsaturated conditions. The applicability of the framework was demonstrated by providing an illustrative example followed by three series of parametric studies to understand the effects of pore size distribution, air entry pressure, and infiltration rate on the performance of an unsaturated GRS wall. The results quantify the impact of suction, showing that as it increases, the maximum tensile loads and connection loads decrease while pullout resistance increases. Mostly affected by the suction effect are upper reinforcement layers, where combined effects of reduced tensile load and increased pullout resistance decrease connection load and reinforcement length requirements. The current study strongly discourages counting on the contribution of suction for the design of new GRS walls. The suction value cannot be accurately and reliably determined for the entire lifespan of the GRS wall, and it may decrease or diminish in an uncontrolled and random manner under infiltration. However, by quantifying the effect of suction, the proposed framework in this study provides a valuable tool for analysis purposes, enabling a rigorous interpretation of field-measured reinforcement loads during wall service as well as evaluation of the forensics of failed GRS walls. [ABSTRACT FROM AUTHOR]

Energy piles are often partially or fully embedded in unsaturated soils, particularly in arid and semiarid regions; however, suction effects on their thermomechanical behavior have not been fully understood. In this study, a series of physical modeling tests were carried out to investigate the behavior of energy piles in unsaturated soils. The soil used for testing is completely decomposed granite, which is a type of clayey sand sampled from Hong Kong. Five constant-temperature pile load tests and three constant-load heating and cooling tests were conducted under different conditions of pile surface roughness and initial soil suction. It is observed that the ultimate capacity of the energy pile (Qult) increases as the initial suction increases from 0 to 90 kPa, the pile roughness increases from 0.05 to 1, and the temperature increases from 10°C to 21°C. Moreover, the effects of roughness are more significant in unsaturated conditions than in saturated states. A suction increment can increase both shaft and toe resistance, while temperature and roughness mainly affect the shaft resistance. During cyclic heating and cooling, suction and roughness increment reduces the irreversible pile head settlement due to the increment of shaft resistance. Furthermore, the irreversible pile head settlement is greatly affected by vertical load. When the working load is relatively small (smaller than 0.5Qult in this study), the settlement increases with the number of thermal cycles but at a decreasing rate for both saturated and unsaturated conditions, and finally reaches a stable state. When the working load is relatively high (0.7Qult in this study), the response of the pile does not reach a stable state in saturated conditions even after five thermal cycles. These observations should be treated with caution because they may be specific and only applicable to the case of no overburden pressure. Exercising caution is crucial when interpreting these observations, particularly at a quantitative level, because they may be limited to the test soil and the low stress level utilized in the physical model tests. [ABSTRACT FROM AUTHOR]

Precipitation is one of the most important factors inducing shallow slope failures, and the shallow slope covering bedrocks is prone to instability after heavy rainfall. In one-dimensional (1D) seepage–deformation coupling issues, permeability coefficient and moisture vary with matric suction in unsaturated soil. Combining mass conservation, Darcy's law, and elastic theory, an analytical solution for coupled seepage–deformation in unsaturated soil slopes during rainfall infiltration is derived using the Fourier integral transformation method. The analytical solution can be applied to a 1D seepage problem in a soil slope with flux at the top and impervious bedrock in the base under heavy precipitation, and is conducive to study infiltration into the slope under rainfall conditions. To validate the accuracy of the proposed analytical solution in this study, it is compared with monitored pore-water pressure data from the Gufenping Landslide in the red-bed region located in Nanjiang, Sichuan, China. The compared result shows a good consistency between the analytical solution and the measured results, with a minor relative error. Investigation of the parameters demonstrates that the water-level rise is closely related to the coupling, which is influenced by precipitation duration, precipitation intensity, soil properties, and slope angle. The bottom boundary of the slope is considered to be impermeable in this study, which leads to rainfall accumulation at the base over time, and the coupled effect becomes more pronounced at the bottom boundary. [ABSTRACT FROM AUTHOR]

Understanding the moisture migration of unsaturated soil in response to temperature changes is essential in soil science and geosystems applications. The coupling of thermohydro (TH) processes is a result of changes in fluid density and viscosity; however, gravity can disrupt the moisture migration of unsaturated sand initiated by a thermal gradient when the sand is considerably porous. Moreover, the moisture redistribution caused by the gravity effect is influenced by the imposed boundary temperature and initial moisture content of the unsaturated sand. Therefore, this paper presents heating tests performed on sand specimens of different gradations and subjected to various thermal gradients in a modified and well-controlled soil testing cell to evaluate the coupling behavior of TH processes in unsaturated sand. Two sands were tested in the soil testing cell, which was equipped with thermocouples (TCs) to measure the temperature and time-domain reflectometry (TDR) sensors to measure the apparent dielectric constant (Ka); three temperature boundary conditions were imposed on the soil testing cell to create two thermal gradients. This study shows that gravity, thermal boundary conditions, initial moisture content, and matric suction influence the migration of moisture: the silty sand, as compared with pure sand, more properly displayed the phenomenon of coupled TH processes in unsaturated soil when tested at a temperature gradient of 200°C/m and initial moisture content of 6%. The study provides an advanced understanding of the paired migration of heat and moisture in porous media under various conditions, including thermal gradient, initial moisture content, and gradation. When unsaturated soils are under a temperature gradient, the pore water tends to move from the heat source to a cooler end in search of moisture equilibrium. Temperature-driven moisture flow is commonly investigated in soil science, agriculture, and geosystems. For example, the soil surrounding a geosystem (electrical cables and ground heat exchangers) experiences heating and cooling cycles when heat energy is exchanged from buildings to the soil via the geosystem. This heat exchange can effectively cool the buildings in the summer and heat the buildings in the winter. To maintain effective heat transfer in the soil, it is necessary to investigate the temperature-driven moisture flow, as it has been established that redistributed moisture also significantly impacts heat transport. At the same time, the moisture flow is also influenced by gravity, particularly for coarse sands. Moisture flow in coarse sands is more prone to the effect of gravity than that in fine sands. This paper evaluates how the physical and hydraulic properties of the soil, such as gradation and suction, can affect the coupled thermal and hydraulic processes. The gravitational effect can dominate the moisture redistribution depending on the direction of the temperature gradient. [ABSTRACT FROM AUTHOR]

This paper aims at proposing a calculation model to deal with the face stability of shield tunnels in unsaturated soil. Based on the existing results of centrifugal test and numerical simulation, a new failure mechanism is proposed, which combines the logarithmic spiral with upper loosening arch mechanisms. The failure mechanism considers the influence of the soil arching effect and the logarithmic spiral height coefficient. Formulas for calculating the loosening earth pressure of unsaturated soil are deduced. Subsequently, this model of loosening earth pressure is introduced into an analytical model to compute the limit support pressure on the tunnel face. Compared with other height coefficients and the degree of soil arching effect, when the height coefficient is 0.95 and the degree of soil arching effect is 10%–30%, the limit support pressure proposed in this paper is in good agreement with the centrifugal test results. Finally, comparisons are performed with the proposed model and classical models as well as typical centrifuge model tests. The comparative analysis shows that the proposed theoretical model is in good agreement with the numerical simulation. The outline of the failure zone by the theoretical model in this paper is in good agreement with the centrifuge results. [ABSTRACT FROM AUTHOR]

Field compaction control is arguably the most common yet critical quality control procedure in geotechnical engineering. Since the early 1930s, the systematic process for performing quality control of compacted soils has often been implemented by measuring the in-place dry unit weight (or density) and as-compacted soil moisture content after placement in a fill. However, the current practice overlooks several facts resulting from comparing soil prepared and compacted in the laboratory to soils placed and compacted in the field. These issues include comparing the compaction energy in the lab versus what is applied in the field, and the behavior of saturated soils in the laboratory to the performance of unsaturated soils in the field. To address some of these gaps, this study presents a new saturation-based framework for compaction quality control. The aim of this new framework is to reduce the uncertainties and assumptions of the compaction control process and provide practicing engineers with further insight into the key engineering attributes of compacted soils. The proposed saturation-based approach compares a degree of saturation difference with a normalized dry unit weight ratio, making saturation upon compaction the controlling diagnostic variable and the focus of the monitoring effort. In essence, the optimal compaction conditions will be referenced to a characteristic saturation state near 80%. Compared with the conventional quality control system for field compaction, the saturation-based approach is developed with the same field and reference data collected for most earth fill projects. The results of this approach enhance the engineering judgment required to match the laboratory reference values to the field conditions. For the purposes of illustration, the proposed saturation-based framework is applied to compaction control data of a large earth dam and compared against the conventional method side by side. [ABSTRACT FROM AUTHOR]

Gas permeability (ka) and gas diffusion coefficient (Dp) are vital for controlling gas transport through earthen landfill cover overlying municipal solid wastes or acid mine tailings. Soil clods generated during the construction of landfill cover can result in various soil microstructures. However, the effects of soil microstructure on Dp and ka are unclear. This study aimed to investigate and compare effects of soil microstructure on measured ka and Dp of compacted manufactured sand tailings (MST)-bentonite mixtures, considering the effects of specimen preparation method, compaction water content (wcomp), and wetting path. The related pore size distributions (PSDs) were measured by mercury intrusion porosimetry. The measurements show that the average ka of as-compacted specimens increased and then decreased as wcomp increased (i.e., a lower soil air content ε —gas-filled volume per unit soil volume), while Dp reduced monotonically. It was because the compacted MST-bentonite mixtures exhibited a multimodal PSD, in which macropores significantly increased the average advective flow velocity of gas. However, the average diffusive flow velocity of gas was independent of pore size, according to the kinetic theory of gas molecules. Conversely, ka and Dp both decreased as ε decreased when the water content of the as-compacted specimen was increased along the wetting path by spraying water, due to the wetting-induced collapse of macropores. These illustrated that Dp was mainly controlled by the ε and tortuosity of gas-filled pores, while ka was dominated by well-connected gas-filled macropores. Moreover, the PSD of specimens was affected by the specimen preparation method, bentonite content, and sieving treatment, thus affecting ka and Dp. [ABSTRACT FROM AUTHOR]

The bearing resistance of energy piles in the presence of temperature effects has not been thoroughly investigated, preventing the perfecting of energy pile design methods. Quantifying the relationship between soil suction and the temperature of unsaturated soils therefore becomes an important step in predicting the bearing resistance of energy piles. A new constitutive model based on interfacial energy and thermodynamic theories is therefore presented to predict the effect of temperature on soil suction as well as the soil–water characteristic curve (SWCC) in this paper. The analytical model for the nonisothermal matric suction was developed by combining five different temperature-dependent functions for the surface tension, air–water contact angle, void ratio, and thermal expansion of solid and water density, thereby providing a more complete approach than the one that considers surface tension only. The proposed formulation was expressed under a simplified form which is believed to be a useful and convenient tool to apply to a range of possible field situations. The temperature-dependent relationship of soil suction was then used to extend existing isothermal SWCCs to nonisothermal conditions that allow obtaining the SWCC at any temperature. The validity of the proposed model was verified by comparison to several test data sets for five different soils: swelling clay, hard clay, clayey–silty soil, ceramic material, and sand. The satisfactory agreement between predicted and measured curves proved that the proposed model had good performance in predicting the effect of temperature on the SWCCs of unsaturated soils. The nonisothermal SWCC model was then coupled with bearing resistance theory to produce a simplified method for analysis of energy piles. The results showed that the proposed method successfully predicted pile resistance at various temperatures when compared to experimental data. The pile resistance reduced as the temperature rose for a specific degree of saturation or if the soil was in an undrained condition. However, water evaporation may cause a decrease in water content and an increase in matric suction as the temperature increases. Therefore, as soils dry out, pile resistance may increase with increasing temperature. [ABSTRACT FROM AUTHOR]

A nonlinear large strain consolidation model for unsaturated soils is presented in this paper. The model uses the Lagrangian–convective coordinate to track the flow of air and water through a deformed layer and the state surface approach to approximate the hydromechanical behavior of unsaturated soils. The model comprises a set of governing equations to integrate the water flow, airflow, and layer consolidation where large strain deformation of the layer occurs. The governing equations were discretized and solved using the finite-difference method, with the solutions verified against the analytical answers. The model was implemented to surcharge preloading studies with a focus on the effects of the initial wet degree of layers on consolidation. The results suggest that the initial wet degree affects the consolidation curves. Higher consolidation settlement occurs in the wet soil layers than in the less-wet layers. In the same layer, the pore air pressure dissipation is completed earlier than the pore-water pressure. [ABSTRACT FROM AUTHOR]

A soil's hydraulic conductivity is a highly nonlinear function of water content, decreasing many orders of magnitude from its saturated state to dry state. This nonlinearity is a macroscopic manifestation of microscopic soil properties of pore structure, pore connectivity, and mineral–water interaction. These microscopic soil properties are underpinned by two distinct soil–water interaction mechanisms: adsorption and capillarity. Herein, a soil hydraulic conductivity equation was developed by incorporating capillary pore flow and adsorptive film flow. The capillary pore flow is captured via a model of bundle of cylindrical capillaries, whereas the adsorptive film flow is established using a film thickness function incorporating adsorption mechanisms of the electric double layer, van der Waals, surface hydration, and cation hydration potentials. The transition between adsorptive film flow and capillary flow is delineated by a water cavitation probability function. The proposed soil hydraulic conductivity equation automatically can fulfill the five necessary physical constraints imposed by underlying soil–water interaction characteristics. The proposed hydraulic conductivity equation can capture the hydraulic conductivity data very well for a wide array of soils, and outperforms several established soil hydraulic models in full matric potential ranges, especially in the low matric potential range in which adsorptive film flow is dominant. [ABSTRACT FROM AUTHOR]

Traditional soil mechanics idealizes geomaterials as being in a dry or fully water-saturated state. However, there is a zone between the ground surface and the groundwater table, where the soil is unsaturated, which means the pore space is filled with air and water. This zone is affected by environmental changes during wetting and drying seasons through precipitation, evaporation, or evapotranspiration. The influence of water content, for instance, the degree of saturation, on the characteristics of soil under dynamic loading i.e. at small to intermediate strains, for example, small-strain shear modulus (Gmax), modulus degradation [G(γ)], and damping ratio [D(γ)] remain of interest in the geotechnical community due to the practical relevance of the problem. This study attempted to cover the existing gaps in the dynamic characteristics of soil with different degrees of saturation. Therefore, Hostun sand (HS) was mixed with 30% Amberger kaolin (AK) and compacted to Proctor density at the optimum water content. They were saturated in a special saturation cell applying back pressure. Then, the saturated samples were dried to different degrees of saturation. Resonant column (RC) tests were carried out on the prepared samples to determine the effect of the degree of saturation on Gmax, G(γ), and D(γ). Of note, in this series of tests, suction was not strictly controlled; however, the necessary volume change corrections were performed via various noncontact transducers. To describe the experimental observations and to evaluate the impact of the degree of saturation on Gmax, G(γ), and D(γ), the soil water characteristic curve (SWCC) of the adopted mixture was determined. The results showed a significant increase of Gmax with the degree of saturation following an S-phaped curve. Based on the new data, previous models to predict Gmax were inspected under a significantly wider range of the degrees of saturation and suction. In addition, the effect of the degree of saturation on the intermediate strain characteristics of the adopted mixture [G(γ)/Gmax] and D(γ) was evaluated, where modulus degradation was found almost unaffected by Sr while a moderate increase of damping ratio with Sr was observed. [ABSTRACT FROM AUTHOR]

A new method and instrument were developed for in situ measurement of the gas diffusion coefficient (Dg) and gas coefficient of permeability (kg) of unsaturated soil. Air containing tracer gas of volumetric concentration of 5%–10% was continuously pumped into a point source at a constant flux rate (q0) until approaching steady state. Then, the pseudo-steady-state tracer gas concentration (Cr0) and gas gauge pressure (ΔPr0) were recorded at a distance (r0) away from the point source. The Dg and kg were determined through the Cr0−q0 and ΔPr0−q0 relationships, respectively. Soil column tests (silty sand) and field tests (sandy lean clay) were conducted to validate the new method. The in situ measured Dg and kg were comparable with those of undisturbed specimens measured by a two-chamber apparatus. The in situ measurement of Dg and kg ranged from 2×10−8 m2 s−1 to 4×10−6 m2 s−1 and 1.9×10−8 ms−1 to 3.9×10−7 ms−1 , respectively. The measurement accuracy increased at deeper depth, due to reduced boundary effect at soil surface. The measurement of in situ Dg was more susceptible to the boundary effects than that of kg at shallower depth (i.e., 20 cm deep) in general. The field test results showed that the ratio of in situ Dg (or kg) to that measured by the element tests was generally between 1.2 and 3.3 at or below depth of 0.30 m, due to the in situ heterogeneity of soil. [ABSTRACT FROM AUTHOR]

Assessing the effect of rainfall on the short-term and long-term stability of earthen structures warrants studying the earth pressures of unsaturated soils under transient flow. This paper presents a variational approach applied to the limit equilibrium method for calculating the resultant force of active and passive earth pressures in unsaturated soils subject to transient flow conditions. The method is developed using the unified effective stress representation and utilizes a log-spiral slip surface. The latter eliminates the need to make any static assumptions in the derivation and leads to a more comprehensive failure mechanism than the planar failure mechanism used in most alternative methods. The proposed analytical solution accounts for the infiltration rate, batter angle, backslope inclination, and surcharge. For verification purposes, the results from the proposed formulations are compared against those from several existing analytical solutions. A set of parametric studies is performed to examine the effects of rainfall on active and passive earth pressures for different soil types. The results show that the soil type, mechanical parameters, and shape of the failure surface will significantly affect the earth pressure coefficient. The proposed formulations provide a valuable tool for evaluating the effect of rainfall on the behavior of earthen structures and forensic studies of failed earthen structures. [ABSTRACT FROM AUTHOR]

The tensile strength of unsaturated soils is essential to assess the safety and reliability of earthen structures over a prolonged period. This specific property of unsaturated soils plays a significant role in desiccation cracking and tensile failure of earth structures. The tensile strength of unsaturated soils can be measured through various direct and indirect testing methods in the laboratory and through different models available in the literature. These models can rapidly predict the tensile strength of unsaturated soils over a wide suction range. However, a practical framework to capture the tensile strength behavior with variations in soil type, tensile strength testing methodologies, and sample preparation methodologies has not been probed. In the present study, a novel tensile strength framework for both coarse- and fine-grained soils is proposed to determine the tensile strength characteristic curve (TSCC) over the entire saturation range from the respective soil water retention curves. A critical assessment of the variation of the parameters for the TSCC model for various types of soils, tensile strength testing methods, and sample preparation methodologies was also performed in this study. Further, the proposed framework was validated with several experimental data sets available in the literature for both coarse- and fine-grained soils. A good agreement was found between the experimental data and predicted results in most cases, with variation in soil type, sample preparation, and testing method. Therefore, the proposed model can be effectively used to determine the tensile strength of unsaturated coarse- and fine-grained soils over the entire saturation range and can be widely utilized for various engineering assessments in real-field situations. [ABSTRACT FROM AUTHOR]

The frame beam anchor plate (FBAP) is a new supporting structure to reinforce backfill. The current studies of slope stability concentrate on homogeneous isotropic dry or saturated slopes according to two-dimensional assumptions. In engineering practice, however, the soils are heterogeneous and anisotropic and exhibit evident unsaturated characteristics, and the failure surface shows obvious three-dimensional (3D) characteristics universally. A new method for evaluating the 3D stability of unsaturated soil slopes reinforced with FBAPs is established in this paper. The upper-bound analytical solution of slope safety factor (Fs) is derived based on the energy balance equation incorporated with the 3D spiral failure mechanism and the gravity increase method (GIM). Comparisons verified the effectiveness of the methodology and the optimization program. The influence of unsaturated characteristics, support structure design parameters, and seismic force on the 3D slope stability is evaluated through parameter analysis. The results show that the shear strength prediction model is crucial in slope stability assessments. The stability of slopes can be significantly improved by about 29%–52% by using FBAPs. Seismic action has a significant negative impact on the slope stability. When kh increases from 0 to 0.3, Fs generally decreases by about 40%–55%. [ABSTRACT FROM AUTHOR]

This paper attempts to investigate the bearing capacity of a strip footing that rested on unsaturated homogenous sands and used upper bound finite-element limit analysis in association with linear optimization. The analysis is based on suction stress-based (σs) effective stress (σ′) analysis. Unlike the saturated soil, the size of the yield surface varies with the vertical position, and therefore, proper modifications are carried out in the upper bound formulations to incorporate the unsaturated soil property functions. A detailed investigation is carried out to estimate the load-carrying capacity of a strip footing that vary the mechanical strength, unsaturated soil properties, water table depth (hw), and climatic conditions. The numerical solutions are presented in terms of bearing capacity factors [BCF (Nγs, Nqs)] for the unit weight (γw) and surcharge pressure (qs). The nodal velocity patterns are drawn for several cases, and the presented solutions are compared with the available results. The design charts provided in this paper might be useful to design engineers. [ABSTRACT FROM AUTHOR]

Soils above the groundwater table and near the ground surface are often partially saturated, and the changes in the degree of saturation and matric suction caused by infiltration may have substantial effects on the stability of geotechnical systems. This work evaluates the significant effect of transient flow on the ultimate bearing capacity of strip footings founded on unsaturated soils under the simultaneous actions of vertical and horizontal loads and moment using the lower-bound theorems of the finite-element limit analysis and the incorporation of second-order cone programming. The unified effective stress theory is incorporated into the soil yield function to model the partially saturated state, and the closed-form solution for one-dimensional transient flow is adopted so as to evaluate the tempo-spatial variations of suction stress. For both sand and clay underlying soils, it is found that the increase of infiltration time leads to a notable decrease of the size of the normalized failure envelopes for either obliquely or eccentrically loaded shallow foundations. The impact of transient flow on the ultimate bearing capacity of surface footings is also observed to be greater for clay soils as compared with sand deposits due to the significant contribution of higher induced suction stress. [ABSTRACT FROM AUTHOR]

Rainfall infiltration can alter the pore-water pressure and decrease matric suction, which can lead to slope instability and landslides. Most of the previous numerical studies conducted on rain infiltration have assumed a two-dimensional (2D) failure mechanism, which neglects the three-dimensional (3D) characteristics of slopes. An analytical assessment framework was developed using the upper-bound theorem of limit analysis to determine the factor of safety and failure pattern in the case of a 3D vertical cut slope under diverse rainfall situations combined with the unsaturated soil mechanics. The Green–Ampt model was adopted to depict the infiltration process and the progression of the wetting front into the soil. The slope factor of safety was determined using the energy balance equation by adopting the strength reduction approach. The validity of the framework was verified using the numerical simulation method. The effects of duration, intensity, and patterns of rainfall and the 3D traits of slope on the slope failure pattern and stability were determined. A set of design charts was included for the preliminary design. The present study quantitatively determines the stability of slopes susceptible to rainfall infiltration. [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]

A simplified analytical method is proposed for one-dimensional unsaturated soil consolidation theory. First, in the one-dimensional compression test of unsaturated soil, the effective stress principle of saturated soil under compression is introduced to replace the constitutive relation expressed by the state of double stress in Fredlund's theory. Based on this, the governing equations composed of two bi-variable partial differential equations are obtained. In the process of derivation, the dissipation law of pore pressure during the consolidation process is analyzed and the theoretical rationality is demonstrated. When solving the system of equations, considering that the initial excess pore water and the gas pressure are caused by instantaneous loading, Hilf's theory is improved to calculate the change of pore pressure. This improvement is more in line with the actual situation and simplifies the approximate calculation method to obtain the analytical solution when the coupling effect of water and gas is considered. This method is verified by comparing with Terzaghi's theory and Fredlund's theory. It is shown that the solution of Terzaghi's theory is a special case of this method for saturated soil, and the results of the dissipative process obtained with the method is close to Fredlund's theory. [ABSTRACT FROM AUTHOR]

The accelerating pace of development due to groundwater overwithdrawal throughout the world places increasing demands on the unsaturated zone. Estimating evaporation from unsaturated soil has been important for many geotechnical and geoenvironmental applications. As an example, the long-term performance of soil covers, which are widely used in mining and landfill applications to protect the environment, depends on evaporation from their surfaces. Evaporation, in turn, depends on moisture flow within an unsaturated soil mass, which is generally coupled with heat flow. Evaporation also depends on the void ratio and hydraulic and air conductivities of soil, which are, in turn, affected by stress and strain and the resulting soil settlement. As a result, a thermohydromechanical (THM) analysis for predicting evaporation is necessary. A two-dimensional (2D) finite-element program, θ-Stock, can perform THM analysis of unsaturated soil but works only within the soil, not in the atmosphere above the soil. In this study, a 2D model of soil–atmosphere interaction employing atmospheric coupling and using a modified Penman equation was implanted in θ-Stock to develop program EVAP1, which numerically estimates potential and actual evaporation from unsaturated soil using THM analysis. Because of the coupled character of the phenomena, the boundary conditions were determined by the soil state and the environmental data. The program EVAP1 was verified with published experimental and numerical studies on evaporation. The results of analyses using the verified program showed that neglecting THM behavior, and hence soil settlement, leads to an overestimation of evaporation from unsaturated soil surfaces. [ABSTRACT FROM AUTHOR]

The bearing capacity of foundations constructed on unsaturated soils depends on the soil saturation degree and thus the soil suction. This paper presents a simple limit equilibrium approach to compute the foundation bearing capacity on unsaturated soil using lateral earth pressure theory. An imaginary retaining wall is assumed to pass the edge of the foundation and active and passive lateral pressures on both sides of the wall are computed using the distribution introduced in an earlier study. Equating total active and passive thrusts on two sides of the wall gives the footing bearing capacity. The soil–water characteristic curve is used to obtain soil suction corresponding to different moisture contents and drained shear–strength parameters are used to determine the foundation bearing capacity. The developed method is applicable to sands and c-φ unsaturated soils with varying saturation degrees. The predicted results have been compared with data from seven case studies reported in the literature and found a satisfactory agreement. Parametric studies have been conducted to investigate effects of foundation embedded depths, two-layer soils, various suction stresses, and ground slope changes on the footing ultimate bearing capacity. The results show that with increasing the soil suction, the footing bearing capacity increases significantly. [ABSTRACT FROM AUTHOR]

A novel semi-analytical solution is proposed to analyze the consolidation behavior of composite ground with a floating impervious column in unsaturated soil based on the equal-strain hypothesis. The basic governing equations in the reinforced zone and the underlying stratum are first obtained. Using the Laplace transformation and matrix analysis method, a final solution in the Laplace domain is derived. Afterwards, on the basis of Stehfest's method, an inverse Laplace transform is conducted to derive the solution in the time domain. The effectiveness of the proposed solution is then evaluated against the results of finite difference analysis. Moreover, a sensitivity analysis is performed to assess the effects of area replacement ratio, depth improvement ratio, and column stiffness on the dissipation of excess pore-water/air pressure and the average degree of consolidation. Results show that only vertical seepage occurs during the consolidation process of unsaturated composite ground under a condition of equal-strain. The consolidation rate of unsaturated soft ground can be improved by introducing floating impervious columns. The use of larger area replacement ratio, depth improvement ratio, and column stiffness can increase the consolidation speed and reduce the settlement of composite ground with a floating impervious column in unsaturated soil. [ABSTRACT FROM AUTHOR]

Constitutive coupled modeling has developed rapidly in recent decades, with numerous new models published. However, few models consider dynamic porosity, and experimental validation of such a model remains a challenge due to multiple variables. In this study, a new constitutive model for unsaturated soil with dynamic porosity was developed based on mixture theory and nonequilibrium thermodynamics, then the model was validated using test data from two experimental studies that yielded good results (relative average error AVRE = 0.8631–1.3046, R2 = 0.9028–0.9981). The sensitivity of the model to the four primary parameters was analyzed to investigate the influence of model properties on the hydraulic and mechanical behavior. Results show that the calculation of volumetric strain is most sensitive to Young's modulus (E), while the calculation of specific water volume is most sensitive to permeability (k). In addition, the sensitivity of the parameters changes with their value. Modeled results show that the porosity change significantly affects both hydraulic and mechanical behavior, even when soil undergoes relatively low deformation. Relative calculation error decreases notably after porosity change is considered (44.9% and 35.2% improvement in two different calculations). This study also finds that dynamic porosity affects the deformation energy of solids. [ABSTRACT FROM AUTHOR]

Liquefaction in soils has predominantly been associated with saturated soils. Under seismic activity, unsaturated soils are may also be susceptible to liquefaction. The implications of neglecting unsaturated soils that are close to saturation as the foremost rule for assessment of liquefaction can be risky and catastrophic. Hence, in this study, a cyclic double-walled triaxial device capable of applying suction via axis-translation technique was modified to conduct cyclic triaxial tests on suction-equilibrated specimens at a low suction state. In unsaturated soils, the presence of highly compressible air within the soil specimens may prevent the effective confining pressure from reaching zero, which is typically considered as initiation of liquefaction. Therefore, in this study, liquefaction was considered to occur when 5% double-amplitude strain was attained in the soil specimen. A series of suction-equilibrated cyclic triaxial test results on unsaturated soils were analyzed to study and verify the possibility of liquefaction in unsaturated soils. Additional suction-controlled monotonic triaxial tests were performed to compare the stress paths of the cyclic triaxial tests with their respective critical state lines to indirectly assess the hypothesis of liquefaction mitigation using an induced partial-saturation technique. It was observed that liquefaction was possible and detected in unsaturated cohesionless soils having relative density of 50% and a degree of saturation of above 70%. However, with desaturation the resistance to liquefaction in soils increased exponentially. This demonstrated the potential for induced partial saturation to mitigate liquefaction in moderately compacted soils whose degree of saturation is reduced to less than 70%. [ABSTRACT FROM AUTHOR]

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

The effects of different types of earthen landfill covers and vapor on landfill gas emissions are not well understood. In this study, an existing theoretical model of water and multicomponent gas (methane, carbon dioxide, nitrogen, and oxygen) flow was improved by incorporating vapor flow. The model was validated using a published soil column test simulating landfill gas emissions through landfill covers. Numerical simulations were conducted to investigate landfill gas emissions through a single clay-layer cover, a cover with capillary barrier effects (CCBE), and a three-layer cover during continuous drying. The CCBE retained the lowest water content after drying, whereas the bottom clay layer in the three-layer cover stayed almost saturated due to the protection afforded by the top two layers. Accordingly, methane emissions from the CCBE were the highest, whereas those from the three-layer cover were the lowest. Ignoring vapor will predict a drier soil state, and hence will cause methane emissions to be overestimated by 1–2.5 times. Effects of vapor became more significant as the overall gas coefficient of permeability of the cover increased. [ABSTRACT FROM AUTHOR]

Rainfall infiltration causes the collapse of unsaturated embankments due to loss of suction. In this paper, the Barcelona Basic Model (BBM), a constitutive model developed for unsaturated soils, was implemented in Fast Lagrangian Analysis of Continua (FLAC) for simulating the effect of varying rainfall intensities on compacted embankments. It was observed that large vertical deformations occur and considerably high positive pore-water pressures get developed, accounting for the collapse of an embankment. The results also show that rainfall intensity and duration affect the slope stability. A comparison was made between the results obtained using the Mohr–Coulomb model and BBM. It was found that the soil undergoes larger deformations when modeled using BBM as BBM accurately models the loss of suction upon saturation. Reliability analysis of embankment shows that probabilistic analysis provides a better assessment of slope stability rather than the factor of safety and shows that suction and hydraulic conductivity are critical parameters influencing slope reliability for unsaturated soil slope embankments. [ABSTRACT FROM AUTHOR]

Traditionally, the seepage through embankments and other geotechnical structures is estimated using the hydraulic properties of saturated soil. In most cases, the soil is not fully saturated, and the use of saturated soil properties can lead to erroneous results. A reliable assessment of slope stability needs comprehensive characterization of both unsaturated and hydraulic properties of saturated soil. The permeability coefficient of unsaturated soil is usually estimated using existing permeability models derived from the soil water characteristic curve (SWCC) of soil. The performance of these prediction models has not been consistent in the past. A proper determination of unsaturated soil permeability properties will be needed for better stability designs and checks of earth structures. In this study, an experimental program is designed to evaluate the influence of matric suction, net confining stress, and wetting and drying cycles on the hydraulic conductivity of a clayey soil over a wide range of suction values. Modified suction-controlled triaxial equipment was used to apply and maintain suction and hydraulic head in a soil specimen to measure the coefficient of permeability for drying and wetting cycles. Additionally, a series of fully coupled hydromechanic numerical analyses was performed to examine the effect of hydraulic properties of unsaturated soil on pore water pressure response and safety factors of slope for transient seepage conditions during drawdown. The effects of stress-dependent hydraulic properties and hydraulic hysteresis on transient seepage were also investigated. The findings from this study highlight the role of hydraulic properties of partially saturated soil in seepage and slope stability analysis of earthen dams. [ABSTRACT FROM AUTHOR]

This study developed a new method to simultaneously measure gas dispersion coefficient and gas coefficient of permeability (GCP) of an unsaturated soils with due theoretical consideration of steady-state gas transport mechanisms. Correspondingly, a soil column apparatus was developed. Based on the proposed method, gas mixture containing air (with a volumetric concentration >90%) and tracer gas were applied to the column base. The gas dispersion coefficient was estimated based on a measured steady-state tracer gas concentration gradient and a dispersive flux of the tracer gas, the latter of which was determined based on mass conservation of the tracer gas. On the other hand, the GCP was determined based on the total flux of gas mixture and gas pressure gradient following Darcy's law. The new method has the following advantages over existing methods: (1) has an ability to determine gas dispersion coefficient and GCP at any soil depth rather than an average value of a test region; (2) measures the two transport properties at varying soil moisture conditions rather than being limited by uniform moisture distribution; and (3) requires only a simple hand-calculation method to estimate the gas dispersion coefficient without the need to apply nonlinear regression on a gas breakthrough curve. The proposed method to determine GCP and gas dispersion coefficient of a silty sand was verified by independent element tests. [ABSTRACT FROM AUTHOR]

This paper presents a model for compression and wetting-induced collapse behavior of unsaturated soils using the disturbed state concept. The volume change and collapse behavior of unsaturated soils were described using their compression behaviors in two reference states, including unsaturated condition before collapse as an initially intact state and saturated condition after wetting-induced collapse as a fully adjusted state. Using a disturbance function derived from experimental results, the collapse behavior of the soil was related to its behaviors in two references states. A power form function was proposed to describe the volume change of the soil during the collapse stage. Using a piecewise continuous disturbance function, the compression and collapse behavior of unsaturated soil could be expressed in the full range of loading, including before-collapse, during-collapse, and after-collapse stages in the effective stress and net stress space. A series of laboratory tests were conducted on three different types of collapsible soils to verify the proposed model. Verification of the model with laboratory results and the data available in the literature showed the ability of the proposed model to describe the compression and the wetting-induced collapse behavior of unsaturated soils before, during, and after the wetting-induced collapse. [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]

The resilient modulus, MR, of subgrade soils is an important parameter in design and analysis of pavements. Subgrade soils are often unsaturated and can experience a wide range of suctions due to changes in water content and temperature induced by seasonal variation and climatic events. Laboratory- and field-measured data show that MR is affected by stress level, water content, temperature, and hydraulic hysteresis. However, none of the existing models explicitly accounts for the effect of temperature nor can they accurately predict MR in high suctions. In this study, a generalized model is developed that can predict MR while incorporating water content, temperature, changes in deviatoric stress, and hydraulic hysteresis. A base model is first presented to predict the variation of MR in regard to water content and is dictated by two distinct water retention mechanisms—capillarity and adsorption. Accordingly, a two-part model is employed to separately account for changes in MR with water content under capillary and adsorption mechanisms. This feature allows the proposed model to accurately capture the characteristics of MR over the entire range of suction. The base model is then extended to incorporate the effects of temperature, changes in deviatoric stress, and hydraulic hysteresis. The proposed model exhibited an excellent performance upon validation against a total of 218 experimentally measured MR values reported in the literature spanning 14 different test sets on nine different soils tested under different conditions. The predictive errors are significantly lower than that from four alternative models, including the model in the Mechanistic-Empirical Pavement Design Guide (MEPDG). The presented model is straightforward and can be used in practice to predict the MR of subgrade soils considering concurrent changes in water content and temperature. [ABSTRACT FROM AUTHOR]

Silty clay is a common embankment material whose moisture increases over time due to soil–atmosphere interactions and climate changes. The objective of this study is to investigate the unsaturated shear strength and microstructural evolution of compacted silty clay on wetting paths. Suction-controlled triaxial tests, scanning electron microscopy (SEM) observations, and mercury intrusion porosimetry (MIP) tests were performed on compacted silty clay specimens at various matric suctions and net confining pressures. The results show that both the matric suction and the net confining pressure have positive effects on the unsaturated shear strength of compacted silty clay. Nevertheless, they tend to cause different failure forms of the specimens, indicating distinct mechanisms behind their contributions to the soil shear strength on wetting paths. A change in matric suction does not lead to obvious evolution of clay particle orientation, but an increase in net confining pressure causes a weaker preferential orientation of clay particles. The shape of pore size distribution curves is slightly affected by the matric suction on wetting paths, while an increase in net confining pressure obviously reduces the proportion of large pores. The brittle failure of compacted silty clay appears due to the presence of open microcracks at high matric suctions. [ABSTRACT FROM AUTHOR]

The thermal conductivity function (TCF) is an important constitutive function establishing the thermal conductivity–water content relationship in unsaturated soils. The existing TCFs do not account for the effects of confining pressure, nor do they consider thermal induced changes in pore structure and degree of saturation. This study presents a general TCF by considering the temperature effects on pore structure, degree of saturation, different heat transfer mechanisms (i.e., conduction, convection, and latent heat of vaporization), and the confining pressure. The TCF is linked to a temperature-dependent soil water retention curve (SWRC) model to include the impact of temperature on pore structure and degree of saturation. The proposed model applies a decay function to the degree of saturation to account for thermally induced changes in heat transfer mechanisms through conduction and convection of pore water in both liquid and vapor phases and latent heat transfer due to vaporization. A new function is introduced into the TCF to incorporate the effects of confining pressure on thermal conductivity that corresponds to the void ratio changes. The proposed TCF was validated against experimentally measured data for several different soils at zero confining pressure and one soil at various confining pressures reported in the literature. The comparison showed that the proposed TCF can capture laboratory-measured data properly, with prediction errors significantly lower than those from several alternative models. [ABSTRACT FROM AUTHOR]

This paper examines the effects of monotonic and cyclic temperature changes of a model energy pile (diameter=25 mm , length=264 mm) on the variations in temperature and volumetric water content of surrounding unsaturated sand. Water flowed away from the pile during heating to 36°C and toward the pile during cooling to 5°C, causing soil drying and wetting near the pile, respectively. The change in volumetric water content was time-dependent, nonlinear, and slower than the change in soil temperature and continued to evolve after the soil temperature changes stabilized. Cyclic heating/cooling induced lower thermohydraulic changes in the soil than monotonic heating and cooling. The most significant changes in soil temperatures and volumetric water content were closest to the pile at a radial distance of 20 mm from the edge of the pile and reduced with increasing radial distance for all cases. The largest change in the degree of saturation was near the pile and was up to 6% for monotonic heating. Cyclic heating/cooling induced irreversible cyclic hydraulic responses near the pile with consecutive thermal cycles and caused a permanent reduction in the soil volumetric water content. However, these irreversible cyclic effects were dominant at a radius of 20 mm and reduced with increasing radial distance from the energy pile. The change in volumetric water content was time-dependent, indicating that the ratio of heating to cooling times during cyclic heating/cooling will have a significant effect on the reversibility of hydraulic responses under temperature cycles. [ABSTRACT FROM AUTHOR]

This study focuses on the stability of unsupported vertical cuts in unsaturated soil. For this, a series of numerical analyses were carried out to estimate the stability of unsupported vertical cuts excavated in an unsaturated glacial till considering three possible practical scenarios: (1) excavation time, (2) depth and location of a tension crack, and (3) rainfall infiltration into a tension crack. The results showed that the influence of excavation time and rainfall infiltration into a tension crack may not be significant if an unsupported vertical cut is made with a certain safety margin (i.e., safe height). A tension crack did not affect the overall safety of an unsupported vertical cut in a case where the location and depth of the tension crack are within 30% and 20% of safe height, respectively. The proposed approaches were then applied to the deep unsupported vertical cut made into a clay (9.75 m; Kwan 1971) for the validation. The failure in the vertical cut was initiated due to the development of a tension crack, which matched the numerical analyses results obtained extending both effective and total stress approaches considering the influence of matric suction. [ABSTRACT FROM AUTHOR]

In this investigation, an approximate analytical solution to the axisymmetric consolidation problem of unsaturated soils improved with vertical drain is presented under the equal-strain condition. The smear effect of soil, the well resistance of vertical drain, and the application of time-dependent loading are considered in the solution. Matrix analysis is used to solve the solution under constant loading for comparison against the available solution. The influence of soil parameters, smear parameters, and well resistance parameters on the consolidation behavior is investigated under ramp loading. Moreover, two types of loading which are noncyclic forms (i.e., instantaneous, multistage ramp, and exponential loading) and cyclic forms (i.e., triangular, sinusoidal, and cosine cyclic loading) are incorporated in the analyses. It is found that a higher hydraulic conductivity ratio in the undisturbed zone and a larger radius of the influence zone can speed up the consolidation process. The consolidation process is slowed down by increasing the radius of the disturbed zone and reducing its hydraulic conductivity, and the influence of hydraulic conductivity is more significant. The effect of radius and hydraulic conductivity of vertical drain on the consolidation behavior is similar. The consolidation behavior generally follows the pattern of the applied loading apparently. [ABSTRACT FROM AUTHOR]

Unsaturated soil is widely distributed in geotechnical projects, of which the consolidation and settlement are of great importance. Based on the consolidation theory of unsaturated soil, proposed by Fredlund and Hansan, this paper deals with one-dimensional consolidation of single-layer unsaturated soil with six types of boundary conditions, which can be denoted by arbitrary specified time-dependent functions. The analytical solutions in the time domain are provided directly, by homogenization of nonhomogeneous boundary conditions and the eigenfunction expansion method. Until then, the solutions for excess pore-water pressure and excess pore-air pressure in the single-layer unsaturated soil with all possible Dirichlet or Neumann boundary conditions are provided. Finally, the analytical solutions in this paper are validated against published results, and responses of excess pore-water pressure and excess pore-air pressure in the soil layer subjected to different time-dependent boundary conditions are investigated. [ABSTRACT FROM AUTHOR]

In the past, the capillary effect is rarely considered for microcontact models when two adjacent particles are separated by a certain distance. In this paper, a liquid-bridge contact model of unsaturated particles is presented, which is applied to the discrete-element software PFC. Based on this, simulations of uniaxial tensile for unsaturated soils under (2D) situation are carried out, and influence of a water retention angle on uniaxial tensile strength are studied. The simulation results show that the uniaxial tensile strength of unsaturated soil decreases gradually with the increase of the water retention angle, obvious cracks generate at both ends of the soil samples, and the soil sample possesses a residual strength. According to the particle displacement field, velocity field, and contact force chain, the failure forms of samples with different water retention angles are determined. Based on the rose diagram of normal contact force, the changes of its direction and value during failure process are observed directly, so as to analyze the evolution law of contact force for unsaturated soils. [ABSTRACT FROM AUTHOR]

Electroosmosis often dewaters soil into unsaturated conditions. Due to the shrinkage deformation and decrease of effective voltage that occur during the electroosmotic consolidation as the soil changes from the saturated to unsaturated state, an analytical solution with the consideration of the coupled deformation and time-decreasing effective voltage was developed for one-dimensional (1D) electroosmotic consolidation in unsaturated soils. The effect of nonuniform distribution of the initial water content was also considered. To validate the effectiveness of the proposed analytical solution, a laboratory test was conducted. Thereafter, the proposed solution was used in a parametric study to investigate the effects of the coupled deformation, distribution pattern of the initial water content, and decrease rate of the effective voltage. The results indicated that the coupling effect increases with a decrease in the desaturation coefficient, and the coupling effect is significant for soft soils with high retention capability and compressibility. The nonuniform distribution of initial water content also had a great influence on the consolidation rate. When the initial water content increased from the anode to the cathode, the consolidation rate was fast in that direction but slow in the other direction. Finally, with the consideration of time-decreasing effective voltage, the calculated maximum matric suction and drainage decreased and the completion time of electroosmotic consolidation was shortened. In summary, the analytical solution developed in this study could capture the actual problems for electroosmosis consolidation in field applications to some extent. [ABSTRACT FROM AUTHOR]

Stability analysis of tunnel faces is usually carried out under dry or saturated assumptions. However, in practice, soils are often unsaturated due to the presence of matric suction, and unsaturated components often have nonnegligible effect on soil strength. For a more realistic solution, the influence of unsaturated components on the stability of a three-dimensional tunnel face was considered for the first time. To meet this requirement, the three-dimensional rotational failure mechanism and the extended form of Mohr–Coulomb failure criterion were used to express the face failure in the limit analysis framework. The strength reduction technique was employed to estimate the stability of ta hree-dimensional tunnel face and the safety factors were calculated by the bracketing method. The proposed method is verified by comparing with the results obtained by FLAC3D. The effects of matric suction distribution, tunnel geometry parameters and shear strength nonlinearity on the stability of three-dimensional tunnel face were analyzed in the parameter analysis. The results show that considering the effect of suction stress is beneficial for improving the tunnel face stability, but it is influenced by the matric suction distribution, the tunnel geometry, soil types, and the shear strength nonlinearity. The main novelty of this study is to develop an effective method to evaluate the face stability of tunnels in unsaturated soils. [ABSTRACT FROM AUTHOR]

Poisson's ratio is an elastic property needed as input in a wide range of geotechnical engineering applications. Despite experimental evidence, there is currently no model in the literature to explicitly account for the effect of saturation on the Poisson's ratio. This paper presents the concept of Poisson's ratio characteristic curve (PRCC), which establishes a relationship between Poisson's ratio and degree of saturation (or matric suction). The PRCC concept is developed based upon the observation that variations of Poisson's ratio are mainly dominated by water retention mechanisms. A sigmoidal function is employed to describe the PRCC with two fitting parameters, funicular degree of saturation and pore fluid continuity, and both are related to the soil water retention curve (SWRC). The functional form is calibrated and validated against Poisson's ratios calculated from measured compressive and shear wave velocities from experimental data sets for 22 different soils from the literature. Further, a set of laboratory tests is performed to measure wave velocities using bender elements and determine the Poisson's ratio of Bonny silt at different suctions. The PRCC fitting parameters are shown to be linearly correlated with the SWRC fitting parameters. To illustrate the PRCC application, three sets of laboratory-measured data of at-rest earth pressure coefficient for different unsaturated soils are collected from the literature and compared against the predicted values using the PRCC model and those using constant Poisson's ratio values. The values using the PRCC model closely match the measured values, whereas using a constant Poisson's ratio can significantly underpredict or overpredict the at-rest earth pressure coefficient. The proposed model can readily be incorporated into analytical and numerical models, leading to more accurate assessments of unsaturated soil behavior. [ABSTRACT FROM AUTHOR]