Suction Stress Characteristic Curve Research Articles

Mechanical and thermal properties of mud dauber nests under atmospheric drying

Mud dauber wasps construct soil nests to protect their offspring from predators, extreme temperatures, and rainwater. The mechanical and thermal properties of these nests are important for the reproductive success of mud daubers. The previous study showed that the high densities and strengths of mud dauber nests were due to the repetitive tapping and atmospheric drying used by mud daubers during nest construction. This study investigated the effect of atmospheric drying on the mechanical and thermal properties of mud dauber nests. The soil shrinkage curve, elastic modulus, suction stress characteristic curve, soil water retention curve, shear strength, and thermal conductivity function of mud dauber nest soils were measured by performing drying cake tests, direct shear tests, unconfined compression tests, and thermal conductivity measurements. This study showed atmospheric drying could increase Young’s moduli (from hundreds to thousands of kPa), the magnitudes of suction stress (up to 64 kPa), and shear strengths (e.g., unconfined compressive strength increased up to 2100 kPa) of mud dauber nests. The thermal conductivity was reduced by 47% due to atmospheric drying. Investigation of mud dauber nests under atmospheric drying could provide insights and inspiration to improve human manufacturing and manipulation of soils for earthen building construction.

Prediction method of soil–water characteristic curve and suction stress characteristic curve based on void ratio: a case study of Yan’an compacted loess

Prediction method of soil–water characteristic curve and suction stress characteristic curve based on void ratio: a case study of Yan’an compacted loess

Experimental and discrete element modeling study on suction stress characteristic curve and soil–water characteristic curve of unsaturated reticulated red clay

Experimental and discrete element modeling study on suction stress characteristic curve and soil–water characteristic curve of unsaturated reticulated red clay

Study on Soil Water and Suction Stress Characteristics for Unsaturated Clay Soil of Airport Engineering Based on Laboratory Tests

For the unsaturated soil in Feidong China, this study examined the suction stress characteristics based on the soil-water characteristic curve (SWCC), which was different from traditional research ideas. At the same time, the unsaturated consolidation device was adopted for SWCC tests, with consideration of the influence of yielding stress of soil, which was different from the traditional test approach of the soil-water characteristic curve. The results were estimated using the van Genuchten model, which was revealed that this is well-fit for the studied unsaturated soil, and the triaxial shear-strength tests were conducted with suction control. Then, the suction stress characteristic curve (SSCC) was analyzed, and SWCC-predicted data were compared with triaxial test-derived suction stress data. For the studied unsaturated soil, the deviatoric stress increased with the net inner stress p − u a at the same matric suction. At the same net inner pressure, the deviatoric stress increased with the matric suction, which verified the hardening activity of matric suction on the tested unsaturated soil strength. Besides, triaxial test-derived suction stress data greatly conformed to SWCC data-derived SSCC that was determined using identical parameters used in the SWCC model.

Biofilm addition improves sand strength over a wide range of saturations

Bio-mediated ground improvement is an attractive alternative to traditional admixtures for strength improvement of shallow surfaces because it is environmentally friendly. Since biofilms contain extracellular polymeric substances (EPS), they can be considered as an alternative to current technologies to improve soil strength. EPS containing biofilms are porous materials with charged surfaces, and therefore adsorption and capillary condensation can result in water retention. Currently, most of the literature work only tested soil improvement under the dry condition. Therefore, the influence of water retention by biofilms on the strength of improved soil remains unclear. Our goal is to evaluate the strength of a biofilm-enhanced sand over a wide range of saturations and explain the trends through the suction stress characteristic curve (SSCC), which quantifies interparticle adsorptive, capillary, and cementation forces as a function of saturation. We used homogenized anaerobic granule biofilms from an existing upflow anaerobic sludge blanket reactor and mixed it with sand to test the strength of a poorly-graded sand in a wide range of saturations, S (between 0.02 S and 0.74 S). We found that biofilm treatment of sand increases soil strength through cementation over a wide range of saturations and through adsorptive forces among sand, biofilm surfaces, and water molecules at low saturations (S < ~0.3). Our results suggested that homogenized biofilms mixed with sand can be used to improve the strength of sand over a wide range of saturations.

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.

A temperature-dependent model for tensile strength characteristic curve of unsaturated soils

Tensile strength is a key parameter controlling the formation of desiccation cracks in soils. Desiccation cracks are triggered in unsaturated soils due to drying imposed by natural processes or engineering applications mainly involving elevated temperatures. However, there is no closed-form model in the literature to capture the effect of temperature on tensile strength. This study presents a temperature-dependent model for the tensile strength characteristic curve (TSCC) of unsaturated soils. The model employs the suction stress characteristic curve (SSCC) to represent the uniaxial tensile strength of unsaturated soils at different water contents and temperatures. The model incorporates the effects of temperature into adsorptive and capillary suction stress components. The temperature-dependent adsorptive suction stress is obtained by accounting for thermal induced changes in suction stress at dry state through the Hamaker constant and the density of water. The temperature-dependent form of capillary suction stress is derived by employing temperature-dependent forms of surface tension, contact angle and enthalpy of immersion. Upon comparison, results from the proposed TSCC exhibited a very good agreement against laboratory-measured tensile strength data for two clayey soils tested at different temperatures ranging from 20 to 60 °C. The presented model can improve the analysis of desiccation cracking in unsaturated soils.

Obtaining Suction Distribution Within Vadose Zone of Highway Pavement System in Southwestern Nigeria Using Physico-Empirical Approach

The stability of the pavement system on old Ogbomoso-Ilorin road in southwestern Nigeria basement complex was investigated. This was carried out by estimating the suction, degree of saturation and effective stress of the soils within vadose zone from soil–water characteristic curve (SWCC) and suction stress characteristic curves (SSCCs) employing physico-empirical approach. For this purpo se, the grain-size distribution (GSD) curves of forty two disturbed soil samples, scooped from borings at anomalously low resistivity points delineated by 2-D Electrical resistivity imaging were fitted using empirical equation. Permeability and Atterberg limits tests were also carried out for engineering classification of the subsoil. The GSD curves indicated the soils are uniformly-, poorly- and gap-graded. The permeability of the soils ranged within 1.13 × 10–6 − 9.35 × 10–5 m/s suggests very fine sands, silts/clay-silt laminate of poor sub-drainage. The intermediate suction range values obtained from SWCCs were less than 150 kPa indicate matric suction. The soils suction; 1.053–79.529 kPa, varied with air entry values; computed (1.350–3.907 kPa) and estimated (0.406–3.672 kPa), indicating the soils could shrink. The high suction gradient; 0.223–12.403%/kPa, indicates decreased evaporation flux, the ability of the soils to generate flow and suggests the soils are deformable. The SSCCs indicated the degree of saturation at which minimal soils deformation could occur varied between 0.230 and 0.560 with maximum suction stress of 1.9–8.0 kPa. The presumed equilibrium suction of the area within the range 17.365–79.529 kPa was relatively high compared with suctions of all other soils. This indicates decreasing suction and hence significance shrinkage of the soils. The pavement materials properties would therefore have undergone significant volume change and impaired the stability of the pavement.

Measurement of Suction Stress and Soil Deformation at High Suction Range

Abstract Suction stress is the effective stress independent of mechanical boundary conditions but is due to water content variation. It occurs on or near particle contacts and does not directly transfer through the soil skeleton and hence can only be and has been measured indirectly and deduced from soil’s strength or deformation. In this work, a new technology to measure the soil-water retention curve, soil shrinkage curve, and suction stress characteristic curve in a high suction environment is established. A humidity-controlled device to measure suction stress was developed based on a previously established drying cake method. Nitrogen gas and molecular sieves were introduced as desiccant in addition to saturated salt solutions to generate an extreme dry environment up to 850 MPa of matric suction for the first time. A focus on the effect of adsorptive soil-water interaction on hydromechanical properties of expansive soils provides enriched information on elastic modulus variation, soil deformation evolution, and matric suction development of soil in very low water contents, making the suction stress measurement possible for matric suction as high as 850 MPa.

Mesoscale analysis of the suction stress characteristic curve for unsaturated granular materials

There is still no theoretical framework accounting for linkage between seepage and deformation of unsaturated granular materials. Using a mesoscale liquid bridge model, we propose the first approach for deriving the suction stress characteristic curve (SSCC). Then, we verify the method by obtaining both the soil–water characteristic curve and SSCC for cubic and tetrahedral granular packing. The approach is further validated by generating the SSCCs of granular packings with different particle size distributions. On this basis, a new two-parameter model is suggested that satisfactorily predicts the SSCCs of various real granular materials. The nonlinear variation of strength versus suction is also properly described by a new formula for three kinds of soil. We believe that this SSCC model can help resolve solid–fluid coupling in seepage and deformation problems in unsaturated granular engineering.

Evaluation of capillary water retention effects on the development of the suction stress characteristic curve

The suction stress characteristic framework is a practical approach for relating the suction and the water-filled pore volume to the stress state of unsaturated soils. It predicts the effective stress by developing the suction stress characteristic curve from the soil-water retention curve. In this framework, the effective degree of saturation is usually calculated by the empirical water retention model of van Genuchten (published in 1980). In this paper, the use of a generalized soil-water retention model proposed by Lu in 2016, which differentiates the role of capillary and adsorption mechanisms, in the suction stress characteristic framework is studied. A redefinition of the effective degree of saturation is suggested, by choosing the retention state where capillarity approaches zero instead of the residual retention state. The validity of this assumption is examined using experimental data obtained by unsaturated shear strength and retention tests and datasets collected from the literature. The proposed definition is applicable for a variety of soils where capillarity is the dominant mechanism in producing suction stress within the range of suction 0–1500 kPa. In addition, it is observed that the generalized soil-water retention model presents a more realistic prediction of unsaturated shear strength compared with empirical water retention models.

Suction Stress Characteristic Curves of Cohesive-Frictional Soils from Multiple Suction-Controlled Testing Methods

Abstract In most geotechnical construction works, compacted soils of cohesive-frictional nature are expected to remain mostly under partially saturated conditions and hence experience continuous ch...

Effect of clay minerals on the suction stress of unsaturated soils

The primary objective of this study is to investigate the characteristics of suction stress in two types of unsaturated weathered soils with different clay minerals. To obtain the mineralogical properties of the two samples, X-ray diffraction and scanning electron microscope analyses were conducted, and a large amount of kaolinite and montmorillonite was subsequently contained in the granite soil and mudstone soil, respectively. The granite soil was first remolded to obtain the same relative density and particle size distribution as the mudstone soil, and an automated soil-water characteristic curve (SWCC) apparatus was used to measure the water content and matric suction. The weathered granite soil had a higher air-entry value, and the volumetric water content was significantly changed by small changes in matric suction. The weathered mudstone soil showed a large hysteresis in the SWCCs between drying and wetting processes because of the ink-bottle effect, contact angle effect, and trapped air. The suction stress characteristic curve (SSCC) demonstrates that weathered mudstone soil had a larger magnitude of suction stress. The SSCCs of the weathered granite and mudstone soils display similar tendencies to the SSCCs of silt and clay, respectively. Therefore, the unsaturated characteristics considerably depend on the clay mineral composition and particle size distribution.

Uncertainty Quantification in Water Retention Characteristic Curve of Fly Ash Using Copulas

Abstract Water retention characteristic curve (WRCC) is a primary input for modeling the unsaturated behavior of geomaterials. The inadequacy of measurement methodologies in capturing the measurable range of the suction of geomaterials along with material variability induce uncertainties in the quantification of WRCC. The goal of this study is to demonstrate the usefulness of copulas for quantifying the uncertainties in WRCC attributed to the aforementioned reasons. Four instruments measuring different ranges of suction of four fly ashes were used to generate 54 combinations of measured data that would represent possible cases of data-induced uncertainties. van Genuchten WRCC equation parameters (a and n) were determined for these combinations. Statistical analysis of the parameters determined from the 54 combinations indicates a negative correlation with each other; therefore, they represent a bivariate random vector. Four commonly used copulas, Gaussian, Frank, Plackett, and No. 16, were utilized to construct the bivariate density function of the parameters, among which the Gaussian copula was found to be the best fit. The impact of the copula-based model developed in this study on two important unsaturated functions, the suction stress characteristic curve (SSCC) and hydraulic conductivity function (HCF) of fly ash, was investigated and found to be significant. The dispersion in SSCC ranges from monotonically increasing to sharp post peak reduction in the suction range of 20–50 kPa. The limiting state of suction required to reach the theoretical minimum permeability was found to vary widely in the range of 300–9,000 kPa. The study shows that a Gaussian copula with lognormal marginals can adequately capture the significant range of uncertainties associated with WRCC.

Determination of Consistency Limits Based on Suction Stress Characteristic Curve

Consistency limits, especially the liquid and plastic limits, are prominent parameters to be determined in geotechnical investigations for classification and identification of finegrained soil. An investigation of the soil suction stress characteristic curve (SSCC) for determination of consistency limits is conducted in this work. The typical SSCC can be divided into five zones according to the state of pore water retention and the mechanisms of solid-liquid interactions. The correspondences between consistency limits of fine-grained soils and characteristic points of every zone in the SSCC are investigated. A detailed discussion of the intrinsic relationships between zones in the SSCC and soil consistency reveals the physical nature of each zone and the critical moisture content. Using experimental data from soil-water characteristic tests of undisturbed loess and disturbed loess, and van Genuchten's soil water characteristic curve (SWCC) model, consistency limits are determined for SSCCs with different void ratios. The feasibility of the presented approach for determining consistency limits of fine-grained soils based on SSCC is demonstrated.

Theoretical and experimental investigations on the role of transient effects in the water retention behaviour of unsaturated granular soils

In the present study, an experimental setup is presented and employed to examine the influence of rate of change of saturation on the transient water retention curves of granular soils with a low suction range. The results are evaluated and compared to existing theoretical approaches to model the non-equilibrium soil–water retention behaviour as well as to experimental findings from other researchers. Furthermore, suction stress characteristic curves under non-equilibrium conditions are obtained. Finally, the importance and the application of the results in hydro-mechanical modelling of unsaturated soils are discussed.

Measurement of Suction-Stress Characteristic Curve Under Drying and Wetting Conditions

Suction-stress characteristic curve (SSCC) is an important constitutive relation defining variation of effective stress because of changes in soil water content. It can be intrinsically related to soil–water-retention curve. SSCC reflects the amount of mechanical work done to a unit volume of soil by its pore water at a particular state of matric potential or water content. A new testing system, based on a drying cake method was established to measure SSCC under both drying and wetting conditions. A previously developed theory of incremental linear elasticity allows the determination of suction stress by monitoring elastic modulus and deformation of a soil cake under varying water contents. Changes in suction stress and elastic modulus result in radial displacement field of a soil cake. A twin-cake testing procedure was established to independently measure: the elastic modulus change with water content on one cake by using a miniature-loading system, and the displacement field on the other cake by using a digital still camera throughout the drying and wetting processes. A particle image velocimetry technique was used to analyze a series of sequential images to calculate the evolution of radial displacement field and its center. The test results on four different soils, covering from sand to silt and clay, were compared and validated with the results obtained independently by a transient water release and imbibition method. It is demonstrated that this new testing system provides a simple, fast, and non-destructive way to measure SSCC under varying drying and wetting conditions.

A method for evaluating the stability of an unsaturated slope in natural terrain during rainfall

The stability of an unsaturated slope in natural terrain was evaluated based on the variation in the suction stress in the soil layer induced by rainfall. A field monitoring site where landslides have occurred in the past was selected as the study area. To apply the concept of slope stability analysis considering the suction stress in unsaturated soil, the soil-water characteristic curve (SWCC) and the suction stress characteristic curve (SSCC) of the unsaturated soil obtained from the study area were estimated using the van Genuchten (1980) model and the Lu and Likos (2006) model, respectively. The phenomenon of ground saturation induced by the infiltration of rainfall into unsaturated soil is similar to the wetting path of the SWCC. The curve-fit parameters estimated from the SWCC wetting path were therefore used to determine the matric suction and suction stress in the unsaturated soil. The amount of rainfall and the volumetric water content of the soil were measured using a rain gauge and time-domain reflectometer (TDR) sensors, respectively, at the monitoring site. An infinite slope was chosen as representative of the slope of the natural terrain in the study area because the slope length is very long but the depth of the soil layer over the rock is very shallow. The stability of this unsaturated slope in natural terrain was evaluated based on the safety factor of an infinite slope considering the suction stress in the unsaturated soil layer. The safety factor of the slope suddenly decreased during and just after a rainfall and then recovered. Notably, the safety factor of this natural slope exhibited continuous fluctuations because of changes in the suction stress caused by the evaporation and infiltration of water in the unsaturated soil layer. The variation in the suction stress in unsaturated soil induced by rainfall can be estimated from the results of laboratory tests performed to estimate the wetting process based on the SWCC and SSCC in combination with field monitoring data collected by sensors to measure the volumetric water content or matric suction in the soil. Therefore, the infinite-slope stability of a slope in natural terrain can be evaluated in real time by estimating the suction stress in the unsaturated soil due to rainfall when the volumetric water content or matric suction in the soil is being monitored in the field.

Influence of initial dry density and water content on the soil–water characteristic curve and suction stress of a reconstituted loess soil

In the northwest of China, many loess landslides have occurred without clear triggering factors (i.e., rainfall, earthquake, human activities, etc.). To better understand and analyze the hydro-mechanical properties of these slopes and then provide evidence for their stability analysis subjected to matric suction, it is essential to clarify the soil–water characteristic curve (SWCC). In this study, we conducted a set of experimental trials to examine the influences of initial dry density, water content upon the SWCCs of a loess soil taken from a loess landslide area, by using a conventional volumetric pressure plate extractor. Two common SWCC models have been investigated to evaluate which one is better for loess soil. The suction stress characteristic curves (SSCCs) were also estimated and analyzed. We found that behaviors of SWCCs would be different when the matric suction was greater than a certain value. The two SWCC equations have approximately the same performance in describing the SWCC. The rates of desorption decrease and residual water content increases with increasing the initial dry density, while the initial dry density has little, if any, influence on the air-entry value (AEV). The specimen compacted under higher initial water content would exhibit a higher AEV value and residual water content but lower rate of desorption as compared with the lower initial water content. The magnitude of suction stress had an approximately linear relationship with matric suction before the AEV value, the SSCC shapes will be markedly varied with the initial dry density and water content.

Determination of the related strength parameters of unsaturated loess with conventional triaxial test

The strength and the related parameters of unsaturated soil are the key issues in unsaturated soil mechanics. The triaxial or direct shear apparatus with suction control is used to measure the related parameters in the strength formulas proposed by Bishop and Fredlund. However, the technique is time-consuming for cohesive soils because it takes a long time to achieve a water–air equilibrium condition under a given suction state. In addition, the feasibility of the technique is also doubted. Lu et al. proposed the concept of suction stress and expressed the unsaturated strength as a function of suction stress which can be determined by the conventional triaxial test. The method can avoid measuring matric suction and is convenient in engineering application. In this paper, conventional triaxial CU tests with different moisture contents were utilized to obtain the effective steady state strength parameters and then the suction stress characteristic curves were obtained. To compare Lu et al.’s strength formula with Bishop’s and Fredlund’s, the soil–water characteristic curve of the same loess sample was also measured by the tensiometer. Theoretically, the three strength theories are different in physical mechanisms, but they can transform to each other mathematically. From the point of view of engineering applications, Lu et al.’s shear strength theory based on suction stress is more convenient to apply in engineering.