Suction Range Research Articles

Filter paper method for suction measurement using electrical resistivity

The filter paper method is an economical, easily accessible, and simple-to-use method to measure total or matric suction over a wide suction range. Difficulty in handling the filter paper can result in inaccurate water content determination of the filter paper leading to poor estimation of the suction. This paper examines the feasibility of using electrical resistivity of the filter paper in the non-contact mode to determine equilibrium time and to infer the total suction. It was shown that for a specific set-up, electrical resistance (ER) can be used in place of electrical resistivity. Relationships between ER and water content of the filter paper and between ER and total suction were first established. Finally, a prototype set-up is used to demonstrate how it can be used to measure the total suction of soil specimens. It was demonstrated that ER of the filter paper can be used to provide a reasonable estimate of total suction greater than 100 kPa with an error between −18 and +10%. Better accuracy can be achieved if each filter paper is calibrated before use.

Micro-scale analysis on shear characteristics of unsaturated soil by considering capillary suction and adsorption suction based on DDA

In this research, Discontinuous Deformation Analysis (DDA) is extended to analyze shear characteristics of unsaturated soil by separately considering capillary suction and adsorption suction at micro-scale. First, the developed matric suction model is validated by comparing simulation results with the analytical solution. Then, the shear characteristics of unsaturated cohesionless soil are simulated and compared with experimental results. The “peak effect” of the shear strength is reproduced, and it is quantitively explained by the proposed micro parameter CSN (number of the capillary bridge). Further, in the high suction range of cohesive soil, adsorption suction is the dominant factor for apparent cohesion. As the contact area of the clay layer decreases slightly in the high suction range, adsorption suction controls shear strength, therefore, there is seldom a “peak effect” of the shear strength for cohesive soil. The extended DDA provides an innovative approach to analyze the micro mechanism of matric suction in a wide suction range.

An elastoplastic solution for cylindrical cavity expansion under constant water content conditions in anisotropic unsaturated soils

In this paper, an elastoplastic solution is developed for cylindrical cavity expansion in anisotropic unsaturated soils under constant water content conditions. The most prominent features of the solution can be attributed to 1) the stress–strain behaviour of unsaturated soil is subtly coupled with the saturation-suction behaviour; 2) both the mechanical and hydraulic behaviours are modelled as elastoplastic processes; and 3) an anisotropic critical state soil model for saturated soil is extended to an unsaturated situation to consider the effects of the initial stress anisotropy and stress-induced anisotropy of the natural unsaturated soils. The problem is formulated as a system of first-order differential equations and solved as an initial value problem. A numerical code that can judge the hydraulic yield condition of soil particles during the cavity expansion process is developed based on the Runge-Kutta algorithm to solve the governing equations. Parametric analyses, which cover a wide range of suctions, overconsolidation ratios, degrees of initial stress anisotropy, are conducted to investigate the effects of these important factors on the expansion responses in unsaturated soils. The proposed solution could potentially be applied to interpret pressuremeter tests and model pile installation effects in anisotropic unsaturated soils.

A bound water model for numerical simulation of SWCC in the wide suction range based on DDA

In this study, Discontinuous Deformation Analysis (DDA) method is extended to simulate Soil-Water Characteristic Curve (SWCC) by considering both capillary water and bound water at the micro-scale. First, a simple and practical bound water model is proposed and incorporated into DDA program to simulate SWCC in the high suction range. Then, to prove the availability of the developed program, the extended DDA is used to simulate the SWCCs of two types of fine sand with kaolin clay in a wide suction range. The simulation results agree with the experimental results, which demonstrate that the extended DDA is an alternative method for simulating and analyzing SWCC of both cohesionless soil and cohesive soil. Finally, one of the influential factors specific surface area is theoretically analyzed. This study provides an approach to simulate the matric suction of unsaturated soil, and the extended DDA could be a potential tool for unsaturated soil research.

A fractal hydraulic model for water retention and hydraulic conductivity considering adsorption and capillarity

Commonly used hydraulic models that describe the water retention and transport properties of variably saturated soils mainly account for the capillary component of matric potential but ignore the adsorption component. Experimental evidence suggests that soil hydraulic conductivity may be underestimated by several orders of magnitude in the low saturation range for models ignoring the adsorbed film flow. Thus, this paper introduces a fractal method to characterise the effects of capillary and adsorption components of matric potential on soil hydraulic properties considering a simplified pore water transport process. Furthermore, the concept of specific thickness for the adsorbed film was defined to describe the adsorption and adsorption capacity of the soil. Based on these, a complete hydraulic model, including the soil water retention curve (SWRC) and relative hydraulic conductivity curve (RHCC), was derived to describe the water retention and transport properties of the soil in the entire matric head range. The performance of the proposed model was then evaluated using 13 sets of test data, from clean sand to bentonite, showing that it can capture the characteristics of water retention and transport properties well. The model parameters calibrated from the SWRC dataset could predict RHCC over the entire suction range without introducing any additional parameters. Additionally, the influences of the model parameters on soil hydraulic behaviours are also discussed in the paper. It was found that the pore size distribution of the soils mainly controlled the transport characteristics of capillary water, but had a slight influence on the hydraulic properties of adsorbed water, which was dominated by the type and content of minerals and specific surface area of the soil.

Prediction of Wetting Hydraulic Characteristics of Compacted Bentonites in Isochoric Conditions

Unsaturated hydraulic characteristics of compacted smectite clays are essential in many engineering applications. The knowledge of the accurate soil-water characteristic curve (SWCC) and hydraulic conductivity function (HCF) is vital for simulating the transient water flow through bentonites. HCFs are routinely estimated from SWCCs using the statistical models that consider probabilistic soil pore-size models. Recent studies, however, emphasize a strong influence of adsorption saturation on the wetting of smectite clays. In this work, the hydraulic diffusivities of four different bentonites at different compaction densities were estimated by back-analysis from water imbibition tests. A constant hydraulic diffusivity with water content was found to be a good approximation, and the diffusivity was equivalent to the self-diffusion coefficient of water due to the influence of adsorption forces in bentonites. A novel procedure is proposed based on the identified dominant mechanism to predict the HCFs and diffusivities directly from the wetting SWCCs of compacted bentonites. The proposed approach was used to predict the HCFs, and transient water flows of four Indian bentonites at dry densities of 1, 1.4, and 1.6 Mg/m3 and other bentonites from the literature. The proposed approach was superior to the existing statistical methods and provided excellent agreement with the back-estimated conductivity data from the water imbibition tests. On the other hand, the existing statistical models severely underpredicted the conductivities, especially in the higher suction range, due to the dominant influence of adsorption saturation.

Behavior of unsaturated cohesive-frictional soils over a whole range of suction/thermo-controlled stress paths and modes of deformation

The present work documents some of the most recent experimental evidence of the thermohydro-mechanical behavior of compacted soils over a whole range of suction- and/or thermo-controlled stress paths and modes of deformation, including data from a series of triaxial, true triaxial, plane strain, ring shear, and resonant column tests conducted on different types of cohesive-frictional soils in the low-to-medium matric suction range under either room temperature or thermally controlled conditions. The work has been accomplished at the Advanced Geomechanics Laboratory of the University of Texas at Arlington, focusing primarily on the following essential features of unsaturated soil behavior: (1) Loading-collapse and apparent tensile strength loci assessed from suction-controlled triaxial and true triaxial testing on clayey sand, (2) Critical state lines from suction-controlled plane strain testing on silty soil, (3) Peak and residual failure envelopes from suction-controlled ring shear testing on clayey soil, (4) Frequency response curves and cyclic stress-strain hysteretic loops from thermo-controlled, constant-water content resonant column testing on clayey soil, and (5) Residual failure envelopes from suction/thermo-controlled ring shear testing on clayey soil. The work is intended to serve as a succinct yet reasonably thorough state-of-the-art paper contribution to PanAm-UNSAT 2021: Third Pan-American Conference on Unsaturated Soils, Rio de Janeiro, Brazil, July 21-25, 2021.

Hydromechanical behavior of unsaturated soils: Interpretation of compression curves in terms of effective stress

This state-of-the-art paper on the hydromechanical behavior of unsaturated soils focuses on the interpretation of the compression curves of unsaturated soils in terms of effective stress, with the goal of understanding the relative impacts of suction on the effective stress, net yield stress, effective yield stress and slope of the virgin compression line (VCL) during a monotonic increase in net stress. A database of compression curves was compiled for both high and low plasticity fine-grained soils under a wide range of suctions, isotropic or oedometric stress states, drainage conditions (constant suction or constant water content) and preparation techniques (impact compaction, static compaction, consolidation from slurry). Most of the compression curves plotted in terms of effective stress revealed a consistent hardening response with increasing suction and a slight suction dependency on the slope VCL. Interpretation of the compression curves in terms of effective stress led to load-collapse curves with a similar shape for a wide range of soils. Most soils evaluated had a greater rate of increase in effective yield stress with suction than the rate of increase in suction stress with suction, implying that these compacted soils may be susceptible to collapse upon wetting. Inconsistent trends were observed in some studies, which were attributed partially to natural variability but also experimental issues and limitations on the range of conditions investigated. Accordingly, recommendations are provided for future studies on the compression curves of unsaturated soils to ensure that results can be clearly interpreted in terms of effective stress.

General Model for the Uniaxial Tensile Strength Characteristic Curve of Unsaturated Soils

Characterization of tensile strength is critical to investigate the influence of desiccation and tensile cracks on the structural integrity of natural and artificial earthen structures and slopes. Although several models for the prediction of tensile strength of unsaturated soils have been proposed in the literature, critical gaps remain regarding a comprehensive model describing the tensile behavior of all soil types over a wide range of suction. This paper presents a general model for the tensile strength characteristic curve (TSCC) which establishes a relationship between the tensile strength versus water content in unsaturated soils. The proposed model is applicable to various soil types ranging from clean sands to silty and clayey soils. Tensile strength is characterized using the suction stress concept, and the TSCC model is built upon the concept that changes in tensile strength with water content (or degree of saturation) are primarily dominated by two distinct water retention mechanisms of capillarity and adsorption. Differences in the characteristics of capillary and adsorptive mechanisms and interparticle forces cause dissimilarities in the resultant TSCC in different soil types. Thus, a two-part suction stress characteristic curve (SSCC) was incorporated into the development of the TSCC to separately account for and distinguish interparticle forces and the resultant tensile strength under capillary and adsorptive mechanisms. The model was validated against laboratory-measured tensile strength reported in the literature for 10 soils, and was found to capture the tensile behavior of sandy, silty, and clayey soils well. Compared with several alternative models, the predictive accuracy of the proposed model was greater, particularly for clayey soils at low water contents (high suction). This superior performance can be attributed to properly accounting for the effect of adsorption mechanism, which is significant in clays.

Soil-water retention behaviour of fine/coarse soil mixture with varying coarse grain contents and fine soil dry densities

The interlayer soil identified in conventional French rail track corresponds to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such a mixture, different coarse grain contents fv and dry densities of fine soil ρd–f were considered. The filter paper method was applied to measure matric suction. Mercury intrusion porosimetry tests were performed to observe the microstructure of the fine soil. In terms of the gravimetric water content of fine soil wf with matric suction ψ, the soil-water retention curve (SWRC) was significantly affected by ρd–f for ψ < 715 kPa, while independent of ρd–f for ψ > 715 kPa. Interestingly, this threshold ψ corresponded to a delimiting pore diameter of the bi-modal microstructure of fine soil, which separated micropores from macropores. In terms of degree of saturation Sr with ψ, the SWRC was significantly affected by ρd–f in the full suction range, while it was independent of fv. These findings help to better understand the results for samples where dry density of mixture ρd is kept constant: an increase in fv resulted in a decrease in ρd–f and the suction changed accordingly. In this case, both fv and ψ affected the mechanical behavior.

Combining two methods for the measurement of hydraulic conductivity over a wide suction range

The hydraulic conductivity of unsaturated soils can span several orders of magnitude. The measurement of hydraulic conductivity over a wide suction range is a challenging task for decades. In this study, numerical tests are carried out to check the assumption, error sources, and applicable conditions of the wetting front advancing method and instantaneous profile method. Based on the numerical test results, a digital wetting front advancing method is proposed to measure hydraulic conductivity of unsaturated soils in the relative high suction range and the traditional instantaneous profile method is found to be robust in measuring hydraulic conductivity in the low suction range, e.g. for soil with a degree of saturation above 0.9. Further, a combination of the digital wetting front advancing method and the traditional instantaneous profile method is implemented in the same soil column test and leads to an accurate soil hydraulic conductivity function over a wide suction range. The proposed method is applicable to various soil types and the test duration is only 1 ~ 5 days, thus makes the measurement of hydraulic conductivity a routine experiment for unsaturated soil mechanics.

Water retention curves of a geosynthetic clay liner under non-uniform temperature-stress paths

This paper presents a novel suction-controlled chamber that permits the determination of the full water retention curves of geosynthetic clay liners (GCLs) under non-uniform temperature-stress paths. It investigates field conditions encountered in brine ponds (low confining stress settings) and heap leach pads (high confining stress settings) during construction and operation stages. Consequently, the analysis of the moisture dynamics in a GCL was defined under the wetting path (construction) and drying path (operation). High vertical stresses were found to facilitate a more rapid water uptake as capillarity is established faster than at low, confined stresses. In general, the drying curves increase the water desorption over the suction range investigated due to the low water viscosity caused by high temperatures. The wetting of the GCL at 20 °C and drying at 70 °C under either low, confined stress (2 kPa) or high confining stress (130 kPa) shows a reduction in the volumetric water contents. Furthermore, on the drying path, the coupled effect of elevated temperature and high confining stress accelerates water desorption leading possibly to potential desiccation.

Plane strain shear strength of unsaturated fiber-reinforced fine-grained soils

Discrete randomly distributed fibers are commonly used to improve the engineering characteristics of the soil and thus soil properties such as shear strength, compressibility, density, and hydraulic conductivity. Most studies have so far focused on describing the behavior of soils containing randomly distributed fibers under dried or saturated conditions. However, the water table may seasonally fluctuate, thus generating unsaturated soil conditions. Therefore, a better understanding of the hydro-mechanical properties of unsaturated improved soils is of high necessity. In this research, the shear strength parameters of fine-grained soils were evaluated using the biaxial device available at Ruhr Universität Bochum. The applied device was modified to test unsaturated fine-grained soils with various degrees of saturation using axis translation and vapor equilibrium techniques. The experiments were conducted on fine soils containing 0, 0.5, and 1% fiber contents under a wide range of matric suctions. The ductile behavior was more noticeable in samples with lower suctions and higher straw contents. Furthermore, the shear strength of both unreinforced and reinforced fine-grained soils considerably increased by an increase in the suction. Finally, shear band inclination increased by the suction while decreasing by straw content.

The critical mechanics of the initiation of loess flow failure and implications for landslides

Abstract An essential hypothesis is that the flow failure landslides occurring in the Chinese Loess Plateau may initiate in the high-moisture-content loess in the capillary zone rather than in the saturated zone within the slope. Two effective stress paths, namely, the monotonically increasing loading and wetting-based constant loading under undrained conditions, are performed on intact and mechanically compacted loess, aiming at specifying their hydromechanical trajectories when subjected to an increasing water content. The results indicate that static liquefaction is initiated when the soil moisture reaches a threshold during the wetting process after exhibiting a sharp increase in pore water pressure accompanied by strain-softening behavior. Accordingly, we define this state as the initiation of flow failure under the unsaturated framework to distinguish it from static liquefaction, and first propose its corresponding critical mechanics and criterion. Its validity and physical justification are mainly evidenced by the following observations: i) Significant difference in the pore characteristics of the test samples before and after flow failure initiation; ii) Approximate equivalence between the measured and predicted hydromechanical thresholds at the flow failure initiation state; iii) Existence of the strength envelope corresponding to the flow failure initiation state; and iv) The uniqueness of the modified critical state line in the full suction range. This paper provides a general approach to accurately identify the flow failure potential for unsaturated loess, especially in the capillarity zone within a slope, which is of vital importance to the early identification and risk mitigation of loess landslides with a fluidization pattern.

Effects of Contact Angle on the Hysteresis Effect of Soil-Water Characteristic Curves during Dry-Wet Cycles

The hysteresis characteristics of soil-water characteristic curves (SWCCs) under dry-wet cycling conditions are very important for understanding unsaturated soil properties, so it is crucial to propose an accurate and efficient method for predicting the hysteretic behaviors of SWCCs. To this end, this paper investigates the hysteresis characteristics of SWCCs in the full suction range of seven kinds of Hunan red clay with different initial dry densities by combination of the pressure plate method, the paper filter method, and the saturated salt solution method. It is found that there are, respectively, strong and weak hysteresis zones in the drying and wetting SWCCs under dry-wet cycling conditions. By combining this feature and based on the drying curve, the soil volume and contact angle changes during the drying and wetting processes are employed to predict the hysteretic behaviors of SWCCs. To verify the validity of the prediction method, the predicted curves of the samples with different initial dry densities are compared with the measured curves. The results show that in the strong hysteresis zone, the hysteresis characteristics of the drying and wetting SWCCs are mainly resulted from the changes in the soil pore structure; in the weak hysteresis zone, the hysteresis characteristics are mainly influenced by the changes in the receding and advancing contact angles corresponding to the drying and wetting processes. The Young–Laplace theory is used to transform the changes of contact angle during the drying and wetting processes into the proportional relationship k of matric suction, and the corresponding wetting curve is obtained by smoothing the drying curve. It is found that the prediction effect in the high suction part (the strong hysteresis zone) is better than that in the weak hysteresis zone, which confirms that the hysteresis effect of SWCCs in the high suction part is influenced by the contact angle. Our proposed method can greatly reduce the test period and has a significant practical application value, which provides a new idea for the prediction of SWCCs under dry-wet cycling conditions.

Models for predicting the suction of heaving compacted soils using geotechnical properties

Soil suction is the major property that controls the behaviour of unsaturated soils. Suction estimation is challenging both in the lab and the field. Various instruments to measure the suction have been developed with the recent technological advancements. Nonetheless, there are still limitations in regards to the reliability, cost, suction range, accessibility, scope of activity, and appropriateness for use either within the field or lab settings. The filter paper method is probably the simplest procedure to measure the suction for the entire range both in the field and the lab. Nevertheless, the procedure takes time. To alleviate the requirement for conducting this test, it becomes imperative to develop mathematical predictive models for soil suction. In this study, a detailed survey was carried out across the Free State province, South Africa, and sampling points identified. Samples were tested for their geotechnical properties. The influence of the geotechnical properties on soil suction was studied. Multivariate regression analysis was carried out utilizing MINITAB 18 program to develop the mathematical predictive models.

Modeling the WRC and volume change behavior of GCLs considering net stress and temperature

Geosynthetic clay liners (GCLs) are used in cover and bottom lining systems to mitigate contamination from landfills. During their service period, they are often subjected to varying net mean stress levels, temperature fluctuation, and water content, which could affect their water retention and volume change behavior severely. These changes under thermo-hydro-mechanical conditions can be evaluated through experimental methods or mathematical models. In the present study, the combined effect of net mean stress and temperature on water retention and volume change behavior of GCL in the unsaturated regime were studied. A theoretical thermo-hydro-mechanical framework for GCLs over a wide suction range is proposed based on experimental observations. The proposed framework was implemented in a simple water retention model and volumetric constitutive model to capture the thermo-hydro-mechanical behavior of GCL through several sets of experimental data available in the literature. The predicted results from the proposed method were in good agreement with the experimental results. Also, the proposed method required minimal input parameters to predict water retention and volume change behavior over a wide suction range. The proposed framework is therefore quite useful for engineering applications of GCLs under the combined effect of suction, net mean stress, and temperature.

Water retention characteristics of unsaturated bentonite–sand mixtures under controlled-temperature

Unsaturated bentonite–sand mixtures (BSMs) are accepted generally as ideal buffer/backfill materials in geologic repositories because of its excellent performance. Water retention characteristics are the important factor that affect the engineering properties of BSMs, but little attention has been devoted to that of Gaomiaozi (GMZ) BSMs under controlled-temperature in high suction range. This study evaluated the soil–water characteristic curves (SWCCs) of compacted GMZ BSMs using the vapour equilibrium technique with different sand content and temperature under unconfined conditions. It was found that the sand grains size had a negligible effect on the SWCCs in the high suction range. As the temperature increased, the decreasing adsorption and the increasing evaporation capacity promoted the decrease of water content. The BSMs changed from the compact structure to a loose structure with increasing sand content, which strengthened the flow of vapour and evaporation of water from bentonite and leaded to an observable reduction of water retention characteristics of GMZ BSMs. We presented a new empirical equation for unsaturated BSMs in the high suction rang considering effects of temperature and sand content, and that processed the data more efficiently and described satisfactorily the effects of temperature and sand content on the SWCCs of GMZ BSMs.

Hysteresis of the water retention curves of geosynthetic clay liners in the high suction range

This paper examines two needle-punched geosynthetic clay liners' water retention behaviour at high suction ranges using the vapour equilibrium technique where super-saturated salt solutions controlled the relative humidity. This study shows that the bentonite form and its mineralogy affect the absorption/desorption of GCLs and their corresponding water retention curves. In particular, a granular bentonite-based GCL was found to absorb more and release less water than a powdered bentonite-based GCL due to its higher montmorillonite content and larger pores. The water retention curves of both GCLs exhibited very little hysteretic behaviour at high suction. Repeated wetting-drying cycles shifted the WRCs of both GCLs slightly downward with minimal impact on their degree of hysteresis.

Measurement and Uniform Formulation of Soil-Water Characteristic Curve for Compacted Loess Soil with Different Dry Densities

To investigate the effect of dry density on the soil-water characteristics of compacted soil, loess used as filling in the land-making project of the Yan’an new district was collected and compacted to five initial dry densities of 1.40, 1.50, 1.60, 1.70, and 1.80 g/cm3, respectively. The soil-water characteristic curves (SWCCs) of all specimens in the range of 0–105 kPa were measured using the filter paper method. The measured data were fitted using the Fredlund and Xing equation for each initial dry density. The SWCCs have obvious differences in a suction range below 100 kPa and overlap when the suction range is higher. This suggests that the SWCC of compacted soil is independent of the initial dry density in the high suction range, but the correlation with the initial dry density exists in the low suction range. Therefore, the correlation functions of the parameters in the Fredlund and Xing equation with respect to the initial dry density were regressed, respectively. By substituting these functions into the Fredlund and Xing equation, the state surface function of θ w − ψ − ρ d was obtained and can reflect the SWCCs of all densities of the filled soil to support the further investigation of the unsaturated behavior of compacted soil.