Unsaturated Expansive Soil Research Articles

Modelling the effects of pore-water chemistry on the behaviour of unsaturated clays

Due to their various applications in geo-environmental engineering, such as in landfill and nuclear waste disposals, the coupled chemo-hydro-mechanical analysis of expansive soils has gained more and more attention recently. These expansive soils are usually unsaturated under field conditions; therefore the capillary effects need to be taken into account appropriately. For this purpose, based on a rigorous thermodynamic framework (Lei et al., 2014), the authors have extended the chemo-mechanical model of Loret el al. (2002) for saturated homoionic expansive soils to the unsaturated case (Lei, 2015). In this paper, this chemo-mechanical unsaturated model is adopted to simulate the chemo-elastic-plastic consolidation process of an unsaturated expansive soil layer. Logical tendencies of changes in the chemical, mechanical and hydraulic field quantities are obtained.

Hydro-mechanical coupling effect on surficial layer stability of unsaturated expansive soil slopes

Rain-induced surficial slope failures in expansive soils are frequently reported in the literature. These failures are primarily attributed to the shear strength reduction due to the loss of suction associated with the water infiltration. For this reason, slope stability is conventionally evaluated taking account of the hydraulic response of the soil to infiltration of water. The unsaturated expansive soil swells significantly upon wetting; the associated hydro-mechanical coupling effect on the slope stability of expansive soils is not considered in conventional geotechnical engineering practice. This paper aims to numerically evaluate the coupling effect or swelling on the hydraulic response as well as the stability of surficial layer of a typical expansive soil slope. Both hydro-mechanical (coupled) and hydraulic (uncoupled) responses of the slope to a low intensity prolonged rainfall are modeled using the commercial software SIGMA/W and SEEP/W, respectively. Subsequently, the infinite slope formulation is used to compute the factors of safety (FS) profiles extending both coupled (SIGMA/W) and uncoupled (SEEP/W) analysis. The results of the study presented in this paper show that coupled analysis (considering swelling) leads to different suction (negative pore water pressure (PWP)) and FS profiles within the surficial layer from those resulting from uncoupled analysis at the same elapsed time. The wetting fronts in the PWP profile from coupled analysis advance at a relatively faster rate in comparison to uncoupled analysis contributing to more critical FS values or failure conditions. The study highlights the coupled hydro-mechanical behavior of expansive soil has an adverse effect on the slope stability and hence should be taken into account in practice for estimating reliable values of FS.

Soil water characteristic curve and improvement in lime treated expansive soil

Methods commonly used to evaluate the improvement of lime-treated expansive soil include swelling characteristics and unconfined compressive strength. In the field, lime-treated expansive soils are in compacted unsaturated state. Soil water characteristic curves (SWCCs) represent a key parameter to interpret and describe the behavior of unsaturated expansive soil. This paper investigates the use of SWCC as a technique to evaluate improvements acquired by expansive soil after lime treatment. Three different lime contents were considered 2%, 4% and 6% by dry weight of clay. Series of tests were performed to determine the SWCC for the different lime content under curing periods of 7 and 28 day. Correlations between key features of the soil water characteristic curves of lime treated expansive soils and basic engineering behavior such as swelling characteristics and unconfined compression strength were established. Test results revealed that initial slope (<TEX>$S_1$</TEX>), saturated water content (<TEX>$w_{sat}$</TEX>), and air entry value (AEV) play an important role in reflecting improvement in engineering behavior achieved by lime treatment.

Stability Analysis of Unsaturated Expansive Soil Slope Covered by Geo-Membrane

In view of the holes appearing in different area of geo-membrane when the geo-membrane technology is applied to the unsaturated expansive soil slope, the VADOSE/W is used to analyze the wetting-drying cycles caused by rainfall and evaporation on slopes covered by geo-membrane. The influence on the pore-water pressure and volume water content were discussed just caused by the holes. The results show that the hole is nearer to the toe of slope, its impact on the whole seepage field is greater. In addition, the hole appears on the top of slope that the wetting-drying cycle effect is remarkable. It is ensured that the integrity of the geo-membrane which in the lower slope and take some drainage measures in the construction process.

Numerical Study of Unsaturated Expansive Soil Canal Slope Covered by Geo-Membrane

In view of the holes appearing in different areas of geo-membrane when the geo-membrane technology is applied to the unsaturated expansive soil canal slope, the VADOSE/W is used to analyze the pore-water pressure of the internal canal slope by changing the falling water level. The results show that the hole is nearer to the toe of slope, its effect on the whole seepage field is greater. The greater the rate is, the soil water content is greater.

A thermo-chemo-electro-mechanical framework of unsaturated expansive clays

Prediction of the coupled chemo-mechanical behaviours of porous media is an important problem in many areas, i.e., expansive clays in geotechnical and petroleum engineering, engineered barriers in the underground storage of nuclear wastes, and biological tissues in biological engineering. A further complex condition occurs when the voids are not fully saturated by liquid, such that capillary effects cannot be neglected a priori and might play an important role. Focusing on the modelling of expansive clays in the geotechnical field and based on the modified mixture theory, the work presented in this paper contributes to the construction of a theoretical framework used to model such complex coupling behaviours. The Clausius–Duhem inequality, which governs the dissipation associated with mechanical work, phase transformation, mass transport and thermal transport, is rigorously derived. Based on this theoretical framework, a chemo-poro-elastic unsaturated model is developed. The model is subsequently used to simulate the salt solution infiltration process through an unsaturated expansive clayey soil that induces changes in the mechanical and hydraulic field quantities. The logical tendencies are obtained and provide a preliminary demonstration of the capabilities of the newly developed theoretical framework.

Analyses of Anti-Seepage Measure for Expansive Soil Subgrade in Hefei

Deformation of expansive soil subgrade depends on moisture content. Anti-seepage measure of unsaturated expansive soil was carried out using ABAQUS as for the influence of atmosphere. The results show that there is large change in water content using anti-seepage measure measure. After using anti-seepage measure, range of water content decreases. At the same time, range of water content value obviously decreases with the increase of the depth of seepage control. Therefore, damage of pavement caused the deformation can decrease by seepage control measure. The results could be used for engineering design and numerical calculation.

Design and Construction Guidelines for Deep Soil Mixing to Stabilize Expansive Soils

AbstractThis paper discusses both design methodology and construction procedures for stabilizing expansive subsoils of moderate active depths, using deep soil mixing (DSM) technology/construction. These procedures were derived as a part of the research focusing on the evaluation of effectiveness of DSM technology in mitigating shrink-and-swell behaviors of expansive subsoils under actual field conditions. The design methodology formulated was based on an analytical model proposed for a DSM-treated composite section by modifying the existing heave prediction model for untreated and unsaturated expansive soils. The required area treatment ratio was determined based on the target heave magnitude for the composite section. Design charts were developed depicting estimated heave for increasing treatment area ratios and for various initial swell pressures. Based on this design methodology, DSM construction was implemented under actual field conditions at two test sites. Upon construction, both test sections were...

Elasticity moduli of expansive soils from dimensional analysis

In this paper, a dimensionless model is proposed to estimate the modulus of elasticity of unsaturated expansive soils extending the dimensional analysis (DA). Experimental results of conventional and suction-controlled triaxial tests for three different compacted expansive soils from the literature are used in this study to apply the DA. The state of hydration of soil expressed in terms of the matrix suction and the degree of saturation, the level of compaction and the confinement described by the initial void ratio and the confining stress, respectively, are considered to form the dimensionless parameters towards reliably estimating the soil modulus of elasticity. Validation of the proposed dimensionless model is achieved by comparing its estimations of the soil modulus of elasticity with the experimental values of the modulus of elasticity obtained from the results of triaxial shear tests. In addition, the estimated values of the modulus of elasticity from another semi-empirical model presented by the present authors, for the three unsaturated expansive soils used in this study, are used to verify the proposed dimensionless model. The dimensionless model is rigorous and reliable for estimating the modulus of elasticity of unsaturated expansive soils.

Prediction of the modulus of elasticity of compacted unsaturated expansive soils

A semi-empirical model was proposed by for predicting the variation of the modulus of elasticity with respect to matric suction for soils with plasticity index Ip values lower than 16%. This model has been extended in this paper to predict the modulus of elasticity of unsaturated expansive soils. The information required for using the model include the soil–water characteristic curve (SWCC) and the modulus of elasticity of soil under saturated condition along with two fitting parameters. Experimental data of triaxial tests for three different expansive soils from the literature were used to examine the validity of the model. The results of the study suggest that the model can reasonably predict the modulus of elasticity of unsaturated expansive soils. The study results are encouraging in reliably using the modulus of elasticity based method proposed by for estimation of the volume change behavior of expansive soils.

Expansive soil crack depth under cumulative damage.

The crack developing depth is a key problem to slope stability of the expansive soil and its project governance and the crack appears under the roles of dry-wet cycle and gradually develops. It is believed from the analysis that, because of its own cohesion, the expansive soil will have a certain amount of deformation under pulling stress but without cracks. The soil body will crack only when the deformation exceeds the ultimate tensile strain that causes cracks. And it is also believed that, due to the combined effect of various environmental factors, particularly changes of the internal water content, the inherent basic physical properties of expansive soil are weakened, and irreversible cumulative damages are eventually formed, resulting in the development of expansive soil cracks in depth. Starting from the perspective of volumetric strain that is caused by water loss, considering the influences of water loss rate and dry-wet cycle on crack developing depth, the crack developing depth calculation model which considers the water loss rate and the cumulative damages is established. Both the proposal of water loss rate and the application of cumulative damage theory to the expansive soil crack development problems try to avoid difficulties in matrix suction measurement, which will surely play a good role in promoting and improving the research of unsaturated expansive soil.

Wetting-induced softening behavior of an unsaturated expansive clay

Studying the wetting-induced softening behavior of unsaturated soils is important for understanding rain-induced slope failures in unsaturated soils. However, experimental studies on expansive soils are limited. This study investigates the wetting-induced softening characteristics of an unsaturated expansive clay via suction-controlled triaxial tests on recompacted and natural specimens. It was found that the wetting-induced swelling of the soil included a significant component of plastic strain and its magnitude decreased with the net confining stress. When a soil specimen was subjected to wetting at a constant deviator stress, a threshold value of soil suction was identified after which the axial strain increased greatly. This value tended to increase with the applied net stress ratio. After the threshold suction, the wetting process for recompacted specimens could be divided into a yielding process associated with volumetric strain hardening/softening and a failure process in which the volumetric behavior depended on the applied net stress ratio. The natural specimens exhibited a brittle failure like a heavily overconsolidated soil during wetting at a constant deviator stress and continuous volumetric dilation occurred after the threshold suction. The wetting-induced softening soil behavior revealed from the present study could interpret the rain-induced progressive slope failure in unsaturated expansive soils.

The input work expression and the thermodynamics-based modelling framework for unsaturated expansive soils with double porosity

In this paper, based on the porous media theory, the specific expressions of the total stress and the input work of unsaturated expansive soils with double porosity are proposed, and then the energy-conjugate variables are further derived. We make distinctions between the effects of capillary water and adsorbed water on the soil behaviour, as well as between the deformations of micro- and macrostructure. According to the derived input work expression and the work-energy-dissipation relations for an open multiphase thermodynamic system, we establish a thermodynamics-based modelling framework for unsaturated expansive soils with double porosity.

An insight into the unloading/reloading loops on the compression curve of natural stiff clays

Oedometer tests were carried out with loading/unloading/reloading on natural stiff clays, Ypresian clays (YPClay), taken from several depths. Common unloading/reloading loops were identified. Further examination of the unloading or reloading curves shows that each path can be satisfactorily considered as bi-linear with a small and a larger slope separated by a threshold vertical stress. This threshold stress can be considered as the swelling pressure corresponding to the void ratio just before the unloading or reloading. Indeed, upon unloading, when the applied stress is higher than the threshold stress or swelling pressure, the mechanical effect is dominant and only small mechanical rebound is observed, corresponding to a small microstructure change; by contrast, when the applied stress is lower than the swelling pressure, physico-chemical effect becomes prevailing and soil swelling occurs with a larger microstructure change. Upon reloading, when the applied stress is lower than the swelling pressure, the microstructure is not significantly affected thanks to the contribution of the physico-chemical repulsive force, leading to a small volume change; on the contrary, beyond the swelling pressure, the mechanical effect becomes dominant giving rise to larger volume changes corresponding to the microstructure collapse. Like unsaturated expansive soils, it is found that there is a good relationship between the swelling pressure (threshold stress) and the void ratio just before the unloading or reloading. This is confirmed by the results from the data reported in the literature about other stiff clays such as Boom and London clays. It can be then deduced that the unloading/reloading loop is rather due to the competition between the mechanical and physico-chemical effects on the microstructure changes than the viscosity effect as commonly admitted.

Hysteretic soil water characteristics and cyclic swell–shrink paths of compacted expansive soils

The investigation of the hydromechanical behavior of unsaturated expansive soil depends on the determination of hysteretic soil water characteristic curves (HSWCCs). This research investigates the HSWCCs of two natural expansive soils initially compacted at optimum moisture content (OMC) and maximum dry density (MDD) and then saturated to their maximum expansion state to ensure identical capillary history. Filter paper, pressure plate and chilled mirror hygrometer suction measurement techniques were utilized, and corresponding measurements of three-dimensional volume change made. The cyclic swell–shrink paths were deduced in terms of volumetric strain versus either water content or suction. The cyclic three-dimensional volumetric measurements showed a much higher amount of swelling and shrinkage than those determined from one-dimensional measurements. Hydraulic hysteresis rapidly decreased with the swell–shrink cycles as a result of macro-structural stabilization. Under the no/low confining conditions, the swell–shrink cycles generally caused a decrease of global expansion and an increase of global shrinkage. Moreover, a special phenomenon showing a reduction of global shrinkage on significant drying was discovered in the less plastic soil sample.

Identification of Creep Model and Parameters Inverse Analysis for Nanning Expansive Soils

A series of loading - unloading consolidation creep experiments had been carried out on Nanning unsaturated expansive soils. Lots of loading - unloading creep curves under different stress levels were obtained. Through the various stages of creep deformation analyzed, it was concluded that the creep deformation of expansive soil contained elastic deformation, viscoelastic deformation and plastic deformation of components. Then model components in constitutive model of expansive soils were discussed. The rheological model of the seven components was proposed, which can describe unsaturated expansive soil elastic - viscous - plastic behavior. According to the experimental datum, all parameters of this model under different stresses were determined by the inverse analysis method. It is showed that the values of numerical calculated using this model agree well with those obtained by the creep tests. The conclusions provide a reliable theoretical basis for the rheological model of expansive soils to further study.

Unsaturated expansive soil fissure characteristics combined with engineering behaviors

The relationship among the surface fissure ratio, moisture content, seepage coefficient and deformation modulus of field unsaturated expansive soil in Nanning, Guangxi Province, China, was obtained by a direct or indirect method. Digital images of expansive soil of the surface fissure with different moisture contents were analyzed with the binarization statistic method. In addition, the fissure fractal dimension was computed with a self-compiled program. Combined with in situ seepage and loading plate tests, the relationship among the surface fissure ratio, moisture content, seepage coefficient and deformation modulus was initially established. The surface fissure ratio and moisture content show a linear relation, “y=−0.019 1x+1.028 5” for rufous expansive soil and “y=−0.071x+2.610 5” for grey expansive soil. Soil initial seepage coefficient and surface fissure ratio show a power function relation, “y=1×10−9exp(15.472x)” for rufous expansive soil and “y=5×10−7exp(4.209 6x)” for grey expansive soil. Grey expansive soil deformation modulus and surface fissure ratio show a power function relation of “y=3.935 7exp(0.993 6x)”. Based on the binarization and fractal dimension methods, the results show that the surface fissure statistics can depict the fissure distribution in the view of two dimensions. And the evolvement behaviors of permeability and the deformation modulus can indirectly describe the developing state of the fissure. The analysis reflects that the engineering behaviors of unsaturated expansive soil are objectively influenced by fissure.

Analysis of the Calculation Method of Active Earth Pressure of Unsaturated Expansive Soil

The properties of unsaturated expansive soil were analyzed, and the shortcomings of present methods which are explored to calculate the active earth pressure of unsaturated expansive soil on the retaining structure were discussed. Based on Coulomb’s earth pressure theory, a calculation formula of active earth pressure of unsaturated expansive soil was proposed taking into account of the swelling pressure parameters according to the condition of static equilibrium of sliding soil. The formula has simple composition and derivation process, is convenient for application, and the example shows that this method is reasonable.

Coupled modelling of hydro‐mechanical behaviour of unsaturated compacted expansive soils

SUMMARYThe Barcelona basic model cannot predict the mechanical behaviour of unsaturated expansive soils, whereas the Barcelona expansive model (BExM) can only predict the stress–strain behaviour of unsaturated expansive soils without the water‐retention behaviour being incorporated. Moreover, the micro‐parameters and the coupling function between micro‐structural and macro‐structural strains in the BExM are difficult to determine. Experimental data show that the compression curves for non‐expansive soils under constant suctions are shifted towards higher void ratios with increasing suction, whereas the opposite is true for expansive soils. According to the observed water‐retention behaviour of unsaturated expansive soils, the air‐entry value increases with density, and the relationship between the degree of saturation and void ratio is linear at constant suction.According to the above observation, an elastoplastic constitutive model is developed for predicting the hydraulic and mechanical behaviour of unsaturated expansive soils, based on the existing hydro‐mechanical model for non‐expansive unsaturated soil. The model takes into consideration the effect of degree of saturation on the mechanical behaviour and that of void ratio on the water‐retention behaviour. The concept of equivalent void ratio curve is introduced to distinguish the plastic potential curve from the yield curve. The model predictions are compared with the test results of an unsaturated expansive soil, including swelling tests under constant net stress, isotropic compression tests and triaxial shear tests under constant suction. The comparison indicates that the model offers great potential for quantitatively predicting the hydraulic and mechanical behaviour of unsaturated expansive soils. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Shallow foundations on expansive soils: a case study of the El Viso Geotechnical Unit, Salamanca, Spain

Construction on expansive soils can generally be resolved using piles. Where shallow foundations are used in these materials, the swelling pressure should be determined and the foundations designed to provide a greater pressure to counteract this. Clearly, it will also be necessary to determine the capacity of the ground to take this load. The paper discusses the bearing capacity of unsaturated expansive soils for a given suction value and notes the bearing capacity was increased by 26% when the effect of suction was taken into account. However, an increase in the water content would cause a decrease in suction and hence a decrease in the bearing capacity.