Behaviour Of Unsaturated Soils Research Articles

A constitutive model for hydromechanically coupled behavior of unsaturated soils with hydraulic hysteresis

There are several constitutive models developed for understanding coupled hydromechanical behavior of three phase medium of unsaturated soils as well as models for explaining hydraulic hysteresis in water retention. However, very few attempts that merge the two aspects of behavior are available. This study develops a one-way coupled model for understanding the hydromechanical behavior of unsaturated soils. In addition to the hysteresis between main drying and wetting retention curves, the model considers non-uniqueness of retention behavior resulting from void ratio changes due to compression under the stress application. As for the elastoplastic stress strain relationship of soil skeleton, the model is based on the formulation of classical plasticity relying on the critical state concept. Consequently, volumetric deformation due to wetting-drying cycles and its effect on elastoplastic behavior through simultaneously changing matric suction is modeled. Model results are calibrated with the results of isotropic compression stages of triaxial tests at both constant suction and constant water content conditions.

Estimation of percolation of water balance cover using field scale unsaturated soil parameter

Water balance covers for landfill closure are used as a barrier which act with the natural processes to reduce percolation. The ideal performance of water balance cover is characterized by the minimal quantity of percolation. The rate of percolation of water balance cover largely depends on unsaturated soil behavior. In this study, percolation was evaluated through unsaturated soil parameters of six instrumented lysimeters. The field instrumentation included moisture sensors, tensiometers, rain gauge, dosing siphon, and pressure transducer. Soil water storage (SWS) capacity (SA) was quantified from the soil water characteristic curves (SWCC) which were developed based on laboratory experiments and field instrumentation data. Required SWS (SR) was also measured from the field monitoring results. Based on analysis, the relative storage ratio (SR/SA) was observed to be greater than unity (1) in most of the cases, indicating potential percolation. The SR/SA was also found competent to identify the lysimeter with higher quantity of percolations. The estimated percolation from the laboratory experimented and field generated SWCCs fairly resembled with the actual field measured percolation. The analyzed results also developed a framework to estimate the thickness of the cover storage layer required to manage percolation for the specific region of the study area.

Numerical modeling of unsaturated soil behavior considering different constitutive models

Recent advances, not only in fluid flow but also in soil mechanics, have allowed the understanding and forecasting of common engineering problems such as slope stability, soil shrinkage and soil collapse. However, owing to limited access to data or more sophisticated numerical tools, the modeling of soil behavior is usually carried out considering simpler constitutive models which cannot predict some important features of unsaturated soils. This study is focused on the numerical modeling of unsaturated soils, adopting four constitutive models based on theories of elasticity and plasticity. For each model, a numerical simulation of a circular footing resting over a soil that is subject to drying and wetting processes is analyzed. Through the comparison of results, it is possible to highlight the use of more sophisticated constitutive models for unsaturated soil behavior, particularly forecasting the phenomenon of pore collapse during wetting processes.

Experimental Study on Swelling Behavior and Its Anisotropic Evaluation of Unsaturated Expansive Soil

Evaluation of swelling behavior is important for designing structures in expansive soils areas, especially for highway that the swelling pressure generated upon pavement and retaining structures in both vertical and horizontal directions due to infiltration. In this study, modification was made on unsaturated consolidation oedometer to provide synchronized measurement of vertical swelling strain (VSS) and lateral pressure (LP) of expansive soil under constant net normal stress and controlled matric suction. Vertical swelling (VS) test and lateral swelling (LS) test were conducted to investigate the anisotropic swelling behavior. The influence of mean net stress and net stress ratio on VSS was investigated, and the anisotropic swelling behavior of unsaturated expansive soil was characterized using anisotropic swelling ratio. The results show that the VSS nonlinearly decreased as the mean net stress increased and increased as the net stress ratio increased. The expansive soil would rapidly enter the passive state due to lateral swelling pressure under relatively low surcharge, with major principal axis rotating from vertical direction to lateral direction, which advances the possibility of passive failure for light retaining structures. The anisotropic swelling behavior objectively exists and varies with matric suction and net normal stress, which should not be ignored for engineering application.

Examining the predictive capabilities of a bounding surface plasticity-based hyperelastic constitutive model for unsaturated granular soils

The performance of a recently developed state-dependent constitutive model for unsaturated granular soils is evaluated. The model employs the Bounding Surface plasticity framework and evaluates elastic strains assuming hyperelastic behavior. To realistically simulate the deformation of unsaturated granular soils, the mechanical behavior was modeled without a purely elastic component. The inherent hydro-mechanical coupling was realized by introducing a Bishop-type effective stress, an appropriate work-conjugate variable, and a soil-water characteristic curve function. Relevant details about the model development, parameter estimation, and the assessment of the model’s predictive capabilities are presented. The model performance is evaluated with experimental data obtained for drained and constant-water stress paths. With a given a set of parameter values, the model realistically simulates the main features that characterize the shear and volumetric behavior of unsaturated granular soils over a wide range of matric suction, density, and net confining pressure. This is found to be true for both drained and constant-water stress paths.

Thermal properties of a tropical unsaturated soil

Ground thermal properties, especially the thermal conductivity, are of paramount importance for the design of ground source heat pump systems (GSHP), used for space heating and cooling. However, very little information, if any, are available from the thermal characteristics of tropical unsaturated soils related to the GSHP application. To evaluate the thermal behaviour of a typical Brazilian tropical unsaturated soil, an extensive experimental investigation was conducted at the test site of the University of Sao Paulo at São EESC/USP) comprising Carlos (a detailed soil characterization; field monitoring of the seasonal groundwater table variation; soil and ambient temperatures, and matric suction of the top soil. This paper describes the investigation program and compares the thermal soil properties as measured in laboratory and field thermal response tests. The results were variable depending on the testing techniques; however, all results showed that the soil thermal conductivity is strongly influenced by the degree of saturation of the soil.

A general mathematical framework for modelling soil-water retention behaviour

A new constitutive model for the soil-water retention behaviour of unsaturated soils is proposed, able to reproduce the main drying and wetting paths, the cyclic retention behaviour and its dependence on the specific volume. The most significant aspect is the inclusion of the evolution, with the specific volume, of the degree of saturation when suction tends to zero in wetting paths considering the presence of entrapped air bubbles. The model is used to reproduce with success the drying/wetting cycles of two Pearl clay samples.

Effect of osmotic suction on the stability of microaggregations and aggregations in a lateritic soil

It has been established that the matric suction is the portion of the total suction that affects, more directly, the mechanical behaviour of unsaturated soils, being less considered the effects of the osmotic suction. Considering that in the deeply weathered soils will occur, depending on its chemical-mineralogical characteristics, more or less ionic exchange between the soil particles and the chemical products to which the soil has been exposed to, the study of the osmotic suction and its impact on the soil textural and structural stability becomes relevant. In natural terrain and slopes, the chemical products come mostly from agricultural inputs, while in road constructions they come from additives such as lime and cement that are incorporated to the soil with the objective of enhancing its mechanical behaviour. In the case of cast in situ concrete foundations, part of the concrete’s chemical compounds migrates to the surrounding soil. In all of the above mentioned cases, the matric suction acts on the chemical compounds’ migration process, affecting soil’s osmotic suction. Aiming at assessing the impacts of these chemical compounds on the soil properties and behaviour, soil-water characteristics curves of a lateritic soil mixed with a dolomitic limestone and a fertilizer were evaluated. The results show that, in the case of the fertilizer mixture, the variation of osmotic suction contributed to the disaggregation of the soil.

Soil water characteristic curve parameters of collapsible sand in Lambayeque, Peru

Several structures are built on collapsible soils in the mining and petroleum industries and on civil sites. In order to analyze the stability of such structures, one must properly study the unsaturated soil behavior. Collapsible soils are frequent sand soil that are susceptible to a significant and sudden reduction in volume upon wetting. An important factor is matric suction, which es related to moisture content through the soil water Characteristic Curve (SWCC), the SWCC is obtained through a filter paper technique and provides a valuable relationship between suction and water content unique to soils. This measures the influence of parameters on the behavior of the structure of collapsible soil. Interactions between structure and soil must be properly evaluated as the bearing capacities of shallow and deep foundations are linked to properties of soil suction, moisture, and grade saturation. This work has experimentally measured the parameters of suction and moisture on the behavior of collapsible sands, where an oil storage tank will be built in the city of Lambayeque in Peru. Undisturbed soil specimens obtained from geotechnical exploration campaigns were used. The filter paper method used in this study was that proposed by models of Brooks-Corey and Van Genuchten. Results show consistent values near reported values from literature.

One-Dimensional Consolidation of Multi-Layered Unsaturated Soil with Impeded Drainage Boundaries

In geotechnical engineering, the consolidation of unsaturated soil is a common issue of great interest. Considering the multi-layered property and impeded drainage boundary condition of the soil stratum in real engineering, this study aimed to develop a general semi-analytical solution for assessing the one-dimensional (1D) consolidation behavior of multi-layered unsaturated soil that is subjected to a general impeded drainage boundary condition and a time-dependent loading. To achieve the final solution, the proposed consolidation system is firstly decoupled and solved in the Laplace domain. Then, the semi-analytical solutions for the excess pore-air pressure and excess pore-water pressures as well as the soil settlement are formulated. The Crump method is employed to provide their final results in the time domain. The correctness of the derived solutions was verified against the available analytical and numerical solutions, and excellent agreements were found for the two comparisons. Moreover, two studied examples are presented to illustrate the 1D consolidation behavior of multi-layered unsaturated soil and the influences stemming from the impeded drainage parameters are discussed.

A general analytical solution for two-dimensional plane strain consolidation of unsaturated soil incorporating lateral and vertical impeded drainage boundaries

In most practical consolidation problems, impeded drainage boundaries are a great concern. By simultaneously introducing the impeded drainage boundaries in the horizontal and vertical directions, this study attempts to constitute a general two-dimensional (2D) plane strain consolidation system of unsaturated soil. A newly analytical solution is developed to calculate the excess pore-air and pore-water pressures and average degree of consolidation. On the basis of the eigenfunction expansion method, the developed analytical solution is given as a product of three functions, including one variable of time, horizontal and vertical dimensions only. For the impeded drainage boundaries, the eigenfunctions and eigenvalues are derived in the horizontal and vertical dimensions. The Laplace transform technique is further adopted to solve the ordinary differential system in the time domain. The developed analytical solution is verified with the existing analytical/semi-analytical solutions and finite difference solution for several typical cases. Excellent agreements are obtained so that the correctness of the developed analytical solution is validated. Moreover, two worked examples are presented to show the superiority of the developed analytical solution and to assess the influences of the drainage parameters on the 2D plane strain consolidation behavior of unsaturated soil.

Effects of compaction and suction on the hydromechanical properties of a dam core clay

The paper presents an experimental study of the hydromechanical behavior of a compacted clay used in the design of the core of the Boughrara dam (western Algeria). The influence of compaction energy level (CEL) is explored using three different energies: Standard Proctor energy (SP), reduced Proctor energy (SP-20 blows) and enhanced Proctor energy (SP+ 20 blows). The resulting compaction curves are supplemented by measurements of suction, which is a very important parameter in the behavior of compacted unsaturated soils. In addition to the compaction curves, elastic modulus and immediate bearing capacity ratio (IBCR) are measured, and related to suction and compaction energy level (CEL). Mechanical compaction paths following the SP curve and drying-wetting paths starting from SPO are compared. Mechanical properties such as E-modulus and IBCR are related to the two independent major parameters γd and Sr. Measuring these mechanical properties in the field allows to evaluate the compaction state regardless of the CEL. The results are compared with those of the literature.

Suitability of swelling and collapse theory proposed based on virgin compression surface

This study investigates the suitability of the swelling and collapse theory proposed based on a constitutive virgin compression surface (VCS) developed within the modified Monash Peradeniya Kodikara (MPK) framework. The modified MPK approach incorporates net stress, void ratio, and moisture ratio as state variables to interpret the swelling-collapse behavior of unsaturated soils. The soil selected for this research was an expansive Quaternary age basaltic residual clay located in Victoria, Australia. The experimental program included testing on unsaturated compacted clay specimens and clay specimens stabilized with lime at the optimum lime content (OLC). The OLC was obtained based on the swelling potential. Static compaction tests were conducted on untreated and lime-treated samples to establish the VCS and to propose the relationship between moisture ratio and net stress. Next, 1-D compression and consolidation laboratory tests were carried out to investigate and compare the mechanism of collapse and swelling based on well-established suction-based theories and the moisture content-based approach of this research. The swelling and collapse response obtained through both theories were very close which verified the suitability of the moisture content-based approach, being the Modified MPK framework. This study also investigated the application of suction-controlled experimental data extracted from the relevant literature within the modified MPK framework. The novelty of this study is proposing and validating a framework for conversion of the results between two different partial theories to interpret the volumetric behavior of expansive clay. Finally, a new method is proposed to estimate the swelling-collapse state of a sample through wetting without the need to establish the VCS.

The effects of fitting parameters in best fit equations in determination of soil-water characteristic curve and estimation of hydraulic conductivity function

It is known that the soil-water characteristic curve (SWCC) contains important information for understanding the engineering behaviors of unsaturated soils. Best fit equations (or SWCC equations) are commonly used for the representation of the SWCC. As the infiltration into the unsaturated soil is mainly governed by the hydraulic properties of soil, the SWCC equations play important roles in the assessment of the rainwater infiltration. In this paper, four types of SWCC equations were adopted and different applicable suction ranges for four best fit equations are investigated and discussed. The fitting parameters and input values in these best fit equations are also demonstrated and discussed. The residual volumetric water content (or residual saturation) as defined in the original equations was recommended to be considered as an input value for a rough estimation of the residual condition. Parametric studies were also carried out to investigate the effects of different fitting parameters on the estimation of SWCC variables and hydraulic conductivities.

Determining soil water characteristic curve of lime treated loess using multiscale structure fractal characteristic

Soil–Water characteristic Curve (SWCC) is meant to describe the mechanical behavior of unsaturated soil. The present paper focuses on the internal multi-scale microstructure of Xining untreated loess and lime-treated loess with the use of scanning electron microscopy (SEM) and image processing technique. A new SWCC model was presented based on the fractal dimension of pore size distribution. The SWCC of untreated loess was calculated from fractal dimension and fitted well with curve tested from Fredlund SWCC device. The SWCC of lime-treated loess was then calculated. Two curves of Xining untreated loess and lime-treated loess have been compared and reasons for the difference have also been discussed. The results indicate that the content of large pores in lime-treated loess decreased and the content of micro-pore increased. The bracket pores were changed into cement pores. The pore fractal dimension D of Xining untreated loess is 1.39 and the pore fractal dimension D of Xining lime-treated loess is 1.53. Air-entry value of untreated loess is 12.16 kPa, while lime-treated Loess—35.15 kPa. In transition region, matric suction of lime-treated loess was in the range of 35.15 kPa ~ 4000 kPa, while matric suction of untreated loess—12.16 kPa ~ 2600 kPa. The range of the transition region in lime-treated loess is larger than that in the loess, while in the range of saturation region, the reverse applies. Under the condition of the same matrix suction, the saturation of lime-treated loess is greater than that of untreated loess. In the residual region, the difference of SWCC of soil samples is small.

Semi-analytical solutions for two-dimensional plane strain consolidation of layered unsaturated soil

In preloading ground improvement projects, vertical drains are frequently installed to accelerate the consolidation process of a soil stratum which is generally unsaturated and layered in natural. To address such issue, this study attempts to constitute a reliable two-dimensional (2D) plane strain consolidation system of layered unsaturated soil. A novel semi-analytical solution for this system is developed using the methods of Fourier expansion, Laplace transform and their inversions. In order to obtain the final solution, the proposed system is firstly converted into a set of ordinary differential systems using the methods of Fourier expansion and Laplace transform. By defining a set of new variable vectors, these ordinary differential systems are further changed into diagonal ones and solved. Then, the semi-analytical solutions for the pore-air and pore-water pressures and the soil settlement are formulated, and numerical inversion of Laplace transform is applied to obtain the final results in temporal domain. Through comparing against the existing solution and the finite difference solution, the validity and the accuracy of the proposed solutions are verified. Moreover, a worked example incorporating a step load is presented to illustrate the 2D plane strain consolidation behavior of layered unsaturated soil and the influence from the coefficient of horizontal permeability is investigated.

A New Method to Simultaneously Measure the Soil–Water Characteristic Curve and Hydraulic Conductivity Function Using Filter Paper

Abstract The soil–water characteristic curve (SWCC) and hydraulic conductivity function (HCF) are widely used to study the hydraulic conductivity behavior of unsaturated soil. This article proposes a filter paper–based instantaneous profile method to simultaneously measure the SWCC and HCF of unsaturated soil. The method uses a small soil column that comprises several vertically stacked soil specimens sandwiched with filter papers. The water content and matric suction of the soil column are measured by weighing the soil specimens and filter papers, respectively, at regular time intervals. Therefore, the matric suction profiles and water content profiles can be obtained, and the SWCC and HCF can be calculated with the profile data. The method was applied to intact loess soil. The measured suction range is 0 to 2×105 kPa, and the measured hydraulic conductivity range is 10−5 to 10−13 m/s. Both SWCC and HCF curves display a bimodal characteristic and are consistent with previous mathematical models. This method is an inexpensive method to measure the SWCC and HCF in a large suction range.

A ssessment of Various Methods to Measure the Soil Suction

The foundation of the lightweight structures is commonly in unsaturated state conditions because located above the ground-water table. The matric suction governs the hydro-mechanical behaviour of unsaturated soils. Soil suction estimation is challenging both in the field and lab. The indirect and direct techniques are utilized to measure the soil suction. Several types of equipment utilized to measure the soil suction have been developed with innovative technology. However, there are constraints on reliability, suction range estimation, application, etc. The primary objective of this study is to review, describe the working principle, report limits, and benefits of various techniques utilized to measure the soil suction and select the cost-effective. A comparative study on direct and indirect technique of soil suction estimation is conducted base on recent literature, with a focus on suction range, procedure, type of suction, processing time, and application (lab/field). The apparatus utilized to measure directly or indirectly the matric suction found in the literature displays the highest range in the order of 1500 kPa except for the filter paper. The thermocouple psychrometer and the transistor psychrometer can measure a maximal total suction of 8000 kPa. The chilled-mirror hygrometer can measure a maximal total suction of 30000 kPa in the laboratory. The filter paper technique and the chilled-mirror hygrometer are cost-effective techniques. However, the filter paper technique is likely the easiest and low-cost technique to measure the matric suction and total suction for the full range with extreme care in the test procedure both in the field and lab.

Mathematical modeling of consolidation in unsaturated poroelastic soils under fluid flux boundary conditions

The spatiotemporal variability in precipitation could have a significant impact on ground subsidence. In this paper, we utilize a previously developed one-dimensional theory of consolidation for a two-fluid system, the generalization of Biot's theory, to investigate the impact of a time-dependent fluid flux across the surface of an unsaturated soil on the solid deformation and pore pressure responses. We consider three prescribed fluid flux patterns and the associated gradient of pore water pressure: constant, periodic, and exponential flux patterns. We employ a Fourier series representation and Laplace transformation in the space and time domains to derive a complete set of closed-form analytical solutions, including the complementary and particular solutions, describing both transient and steady-state excess pore water and pore air pressure responses as well as the total settlement induced by the fluid flux. Using two different types of soil, i.e. clay and sand, we demonstrate the dependence of the solution on the initial water saturation and hydraulic conductivity in the saturated and unsaturated regimes. Results of this study reveal that the fluid flux across the surface could have a significant impact on the dissipation of excess pore water pressure and the time evolution of the total settlement of the ground surface. The effect on the pore water pressure is more significant for sand than for clay irrespective of moisture content, mainly because of the much higher hydraulic conductivity of sand. Finally, we show that the hydraulic conductivity has a dominant effect on the consolidation behaviors of saturated and unsaturated soils subjected to surface fluid flux.

A hysteretic hydraulic constitutive model for unsaturated soils and application to capillary barrier systems

Unsaturated soils exhibit water retention hysteresis, with different water retention behaviour during drying and wetting paths. Water retention hysteresis has often been modelled using expressions for the main drying and main wetting water retention curves that are unsatisfactory at low values of degree of saturation. In addition, the effect of retention hysteresis on the unsaturated hydraulic conductivity behaviour has typically not been explicitly considered. This paper presents a new hysteretic hydraulic constitutive model for the water retention and hydraulic conductivity behaviour of unsaturated soils, which is effective and easy to apply. The model includes: (i) main wetting and main drying water retention curves modelled with a modified version of the van Genuchten model, improved at low degree of saturation; (ii) hysteretic scanning water retention curves modelled using a bounding surface approach; (iii) the effect of hydraulic hysteresis on a soil hydraulic conductivity curve (SHCC) model improved at low degree of saturation and including the effect of liquid film conductivity. The new hysteretic hydraulic model is then validated against experimental data. After implementation in the finite element software Code_Bright, the new hydraulic constitutive model is applied in a numerical study of the impact of hydraulic hysteresis on the behaviour of capillary barrier systems (CBSs). Water retention hysteresis, which has typically been neglected in the modelling of the hydraulic behaviour of CBSs, is shown to have a significant impact on: (i) movement and redistribution of water within the finer layer of a CBS; (ii) the phenomenon of water breakthrough across the interface between the finer and coarser layers of a CBS and the subsequent restoration of the CBS after infiltration at the ground surface ceases; (iii) the prediction of evaporation from a CBS into the atmosphere.