Volume Change Of Unsaturated Soils Research Articles

A Unified Two Independent Stress Variable Approach to Moisture-Change-Induced Unsaturated Soil Volume Change

In 1968, Matyas and Radhakrishna introduced the concept of the state surface, demonstrating that unsaturated soil volume change is dependent on two independent stress state variables, net total stress and suction. For decades the basic theory of unsaturated soils has been known, and a holistic view of the elastoplastic response of unsaturated soils, based on a modified state surface approach (MSSA), makes it clear that a method accounting for independent roles of net total stress and suction is required to quantify volume change of unsaturated soils. Still, reliance on pre-unsaturated-soil-mechanics-era methods persists, particularly within the geotechnical practice community. Unsaturated soil theory forms the basis for compelling arguments for discarding long-held efforts to classify unsaturated soils as exclusively expansive or exclusively collapsible with respect to volume change response. A more fundamental and unified approach to thinking about volume change of unsaturated soils supports the use of a consistent Stress Path Method to practice-based volume change analyses. Implications of geotechnical engineers’ continued reliance on expansive and collapsible soil classifications, often based on index-based correlations and non-stress-path appropriate laboratory testing, are explored. Recommendations for laboratory testing and modeling for moisture-change induced unsaturated soil volume change are made.

Probabilistic analysis of soil-water characteristic curve based on machine learning algorithms

The soil-water characteristic curve (SWCC) is the constitutive relationship curve of soil suction and water content, which has important engineering significance for studying the soil strength, permeability coefficient and volume change of unsaturated soils. Considering the experimental measurement of the SWCC is time-consuming, many empirical methods have been suggested to estimate the SWCC. This paper proposed a machine learning algorithm to predict SWCC from limited sets of soil properties. By predicting the parameters of the Fredlund and Xing model (which is called FX model), the most probable SWCC can be obtained. Since SWCC can’t be accurately determined, the residual probability distribution functions of parameters of FX model are also derived in this paper. Finally, this paper predicted the 90% confidence interval of the SWCC. The result shows that the machine learning algorithm has high accuracy and generalization ability. The suggested method provides a practical means to estimate the SWCC and the variability of it, so that the error associated with the prediction model can be explicitly considered.

Review of expansive and collapsible soil volume change models within a unified elastoplastic framework

Numerous laboratory tests on unsaturated soils revealed complex volume-change response to reduction of soil suction, resulting in early development of state surface approaches that incorporate soil expansion or collapse due to wetting under load. Nonetheless, expansive and collapsible soils are often viewed separately in research and practice, resulting in development of numerous constitutive models specific to the direction of volume change resulting from suction decrease. In addition, several elastoplastic models, developed primarily for collapse or expansion, are modified by add-on, such as multiple yield curves/surfaces, to accommodate a broader range of soil response. Current tendency to think of unsaturated soils as either expansive or collapsible (or, sometimes, stable), has likely contributed to lack of development of a unified approach to unsaturated soil volume change. In this paper, common research and practice approaches to volume change of unsaturated soils are reviewed within a simple macro-level elastoplastic framework, the Modified State Surface Approach (MSSA). The MSSA emerges as a unifying approach that accommodates complex volume change response of unsaturated soil, whether the soil exhibits collapse, expansion, or both. Suggestions are made for minor adjustments to existing constitutive models from this review, typically resulting in simplification and/or benefit to some of the most-used constitutive models for unsaturated soil volume change. In the review of practice-based approaches, the surrogate path method (SPM), an oedometer/suction-based approach, is demonstrated to be consistent with the MSSA framework, broadly applicable for use with expansive and collapsible soils, and yielding results consistent with measured field stress-path soil response.

Hydromechanical behaviour of overconsolidated unsaturated soil in undrained conditions

Hydromechanical behaviour of an unsaturated silt with various suctions and different overconsolidated ratios (OCRs) was investigated through a series of undrained triaxial tests (constant water contents, CW). All the samples were prepared from the slurry state. Different OCRs (= 1, 2, 4, and 8 in net stress) were achieved by unloading the samples to 400, 200, 100, and 50 kPa from an initial confining net pressure of 400 kPa. Then the samples were dried to various suctions (0, 100, 200, 300, and 400 kPa). Unsaturated samples with different OCRs were then sheared at CW conditions following the conventional triaxial compression (CTC) paths. Full hydromechanical responses including the changes in deviator stress, stress ratio, volumetric strain, suction, and degree of saturation with axial strain were monitored and are presented in this paper. Some key findings include (i) the critical state for unsaturated soils with different OCRs can be well defined by Bishop’s effective stress; (ii) the peak strength in Bishop’s effective stress increases with increase of OCR, but decreases with increase of suction in the undrained condition; and (iii) the volume change of unsaturated soils in undrained conditions is related to OCRs and the volume of pore air.

A new approach to interpret the mechanical behaviour of unsaturated soil using effective stress and degree of saturation

Unsaturated soil shows complex mechanical behaviours compared to saturated soil. A number of constitutive models for unsaturated soil have been proposed. However, these models suffer limitations. In this paper, a new constitutive model has been proposed in terms of effective stress and degree of saturation based on the modelling assumption about the relationship between hardening effect and degree of saturation. The proposed volume change equation gives a new explanation for the volume change of unsaturated soil. Then, an alternative approach to simulate coupled hydraulic hysteresis and the influence of the volume change on soil–water characteristic curves is suggested. Finally, this paper presents comparisons between the model simulations and experimental data. It is shown that the proposed model represents great ability to reproduce the mechanical and hydraulic response of unsaturated soil.

Coupled infiltration model of unsaturated porous media for steady rainfall

Unsaturated soil slopes introduce complex hydro-mechanical coupled processes which greatly alter matric suction distribution of an unsaturated soil during rainfall. In order to investigate matric suction and volume change of unsaturated soils, a two-dimensional hydro-mechanical coupled infiltration model (YS-Slope) is developed by incorporating the hydraulic and mechanical characteristics of unsaturated soils, such as the soil-water characteristic curve, permeability function, shear strength, and porosity. Special attention is given to the porosity-dependent permeability function of unsaturated soils. In addition, in order to highlight effectiveness of YS-Slope and coupling effects of hydro-mechanical processes on the infiltration behavior of unsaturated soils, a series of infiltration analyses for a soil column under various soil properties is conducted and their results are compared with those of commercial software, GEO-SLOPE (2012). The results of the numerical analyses show good agreement with data from the analytical solution and laboratory tests, which indicates that the proposed model is appropriate for use in the simulatfion o the infiltration of rainwater into deformable soils. The transient seepage and rainwater flow in deformable soils are influenced by the volume change of the soil. The change in matric suction on a slope due to rainfall infiltration influences change in effective stress while the effective stress alters seepage processes according to hydraulic properties. The results indicate that hydro-mechanical coupled behavior of soils has a positive effect on the stability of unsaturated soil slops during rainfall.

A System to Measure Volume Change of Unsaturated Soils in Undrained Cyclic Triaxial Tests

It is common to employ a traditional double cell system, of which an open-ended inner cell is installed in an ordinary triaxial apparatus, to measure the volume change of unsaturated specimens. In such a system, the total apparent volumetric strain of the specimen (ev) is deduced from the water level change in the inner cell, monitored by a differential pressure transducer (DPT) considering, meanwhile, the top cap intrusion into the inner cell recorded by a vertical displacement transducer (VDT). Severe apparent volumetric strain, caused by the compliance of the double cell system (ev,SC), was observed during the undrained cyclic loading tests in a previous study. Test results on a steel-spring dummy specimen revealed that ev,SC was induced not only by such as the meniscus effect, but unexpectedly also by the asynchronous responses between the DPT and the VDT (i.e., the response of the DPT was delayed compared with that of the VDT). By doing some treatment ev,SC could be reduced to some extent, whereas the magnitude of ev,SC was still too high to be acceptable when the tested specimen approached the liquefied state. To radically solve these technical difficulties, a modified double cell system, named the linkage double cell system, was developed in this study. In this modified system, a linkage rod moving simultaneously with the loading shaft was introduced, through which the DPT could directly measure ev without considering the top cap or loading shaft intrusion. Test results for the steel-spring dummy specimen as well as for saturated and unsaturated soil specimens demonstrated that the linkage double cell system has major advantages in measuring accurately the volume change of the specimen during undrained cyclic triaxial loading tests compared with the traditional double cell system.

Swelling and Collapse of Unsaturated Soils Due to Inundation Under One-Dimensional Loading

Volume change of unsaturated soils is controlled by two stress state variables which are net normal stress and matric suction. Volume change phenomenon due to a change in matric suction or water content can be either shrinkage, swelling or collapse, and depends on the level of net normal stress and the initial void ratio of the soil. In this paper, consolidated kaolin specimens were air dried to various water contents and then inundated with water under different net normal stresses in order to study the wetting-induced volume change. The oedometer apparatus was used to carry out the swell test while the constant-rate-of-strain apparatus was used to carry out the constant volume test. The objective of this paper is to investigate the transition of swelling and collapse phenomenon due to different net normal stresses and to derive a method to estimate the wetting induced volume and net normal stress change of unsaturated soil. Based on the tests results, the pre-consolidation void ratio for the same soil before and after inundation is the same, the slope of the normal and unsaturated compression lines are the same under constant degree of saturation conditions and slopes of the unloading line under saturated and unsaturated condition are different. Based on the test results, a method to determine the wetting-induced swelling and collapse of unsaturated soils was proposed. The proposed method was verified using data from the literature.

Introduction to the Special Issue of Geotechnical and Geological Engineering Entitled: “Thermo-Hydro-Mechanical Behavior of Soils and Energy Geostructures”

This special issue of Geotechnical and Geological Engineering highlights state-of-the-art research on the thermo-hydro-mechanical behavior of soils, rocks and energy geostructures. Although the impetus of the special issue was to collect journal papers on subjects related to an NSF-funded international workshop on ‘‘Thermally Active Geotechnical Systems for Near Surface Geothermal Energy’’ held in March 2013 at Ecole Polytechnique Federale de Lausanne in Switzerland, the special issue expanded to collect a broader set of papers from the field of Energy Geotechnics that involve thermal soil behavior. A total of 15 journal papers were selected to be published as part of the special issue, and went through the standard peer review process for the journal. Two papers focused on the thermal properties of unsaturated soils. Likos (2014) presented a pore-scale model for predicting the thermal conductivity of unsaturated sands using basic soil properties. Dong et al. (2014) presented an evaluation of various approaches to predict the thermal conductivity of unsaturated soils, and compared them with experimental data for sand, silt and clay. One paper was presented on the thermal volume change of unsaturated soils, and the impact of these volume changes on the response of a heated shallow foundation (Houston et al. 2014). Three papers were published on different aspects of geothermal heat exchangers in conventional applications (boreholes and horizontal trenches). Wu et al. (2014) studied relationships between the thermal conductivity dryout curve for unsaturated soils and the performance of geothermal heat exchangers in horizontal trenches, Ozudogru et al. (2014a) compared the performance of different numerical simulations to evaluate the response of geothermal heat exchangers in vertical boreholes, and Walker et al. (2014) studied the impact of different stratigraphic units on the overall thermal response of a geothermal heat exchanger in a vertical borehole. Seven papers were focused on the thermal and thermo-mechanical response of energy piles. Loveridge et al. (2014) analyzed the response of different types of auger cast piles that were converted into energy piles. Yu et al. (2015) used numerical modeling to evaluate the thermal conductivity of different soil layers from the analysis of a thermal response test on an energy pile. Kramer et al. (2014) evaluated the thermal response of an energy pile in sand constructed within a laboratory container. Murphy and McCartney (2014) studied the J. S. McCartney (&) Department of Structural Engineering, University of California San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0085, USA e-mail: mccartney@ucsd.edu

Fractal Representation for Effective Stress of Unsaturated Soils

AbstractBishop’s effective stress equation for unsaturated soils was abandoned for some time because of the difficulties in estimating the value of parameter χ. Only recently has it been recognized that the use of Bishop’s stress equation can lead to simpler and more realistic constitutive models for unsaturated soils. Based on the analysis of the equilibrium of solid particles, it was possible to establish an analytical expression for Bishop’s parameter χ from soil pore structure. It is shown that quantitative predictions of shear strength and volume change in unsaturated soils can be made using the effective stress concept.

Introduction of Noncontact Laser Measuring System to Measure Volume Displacement of Triaxia1 Specimens

This paper introduced a measuring soil sample triaxial volume displacement system that used position sensing detector.The system course of principle and development was maken a detailed statement. The study was carried through literature [12] existing shortcomings. The shortcomings of former system was mended in the new system .This system had a high measuring precision and reasonable design. This systems triumphantly developed will be applied in triaxial specimens local sheared and port restriction .This system have greatly applied foreground to study deformation localizing , come into being cut strip and measuring volume change of unsaturated soil.

Geotechnical Parameters from VP and VS Measurements in Unsaturated Soils

This paper presents a simple procedure for evaluating the coefficients of volume change of unsaturated soils from measurements of the shear and dilatational wave velocities. The procedure is based on some analytical expressions that have been derived in the present study using the theory of linear elasticity for low-frequency excitations. The soil parameters required as input are the porosity and degree of saturation of the soil, Poisson's ratio of the soil skeleton, and three physical parameters that generally assume constant values. The results of some applications of the proposed method to a set of experimental data published in the literature are also shown.

Introduction: Dr. Delwyn Fredlund—41st Terzaghi Lecturer

Dr. Delwyn Fredlund has spent approximately 40 years conducting research on the behavior of unsaturated and expansive soils. Most of his career was spent at the University of Saskatchewan, Saskatoon, Canada, where he organized the Unsaturated Soils Group for research into unsaturated soil behavior. His research studies have involved many areas of unsaturated soil behavior, ranging from the flow of water and air through unsaturated soils to the shear strength and volume change of unsaturated soils. Dr. Fredlund is a native of Saskatchewan, Canada. In 1962 he obtained his BSc degree from the University of Saskatchewan, Saskatoon. Immediately following his BSc degree, he went to work for the Division of Building Research of the National Research Council in Saskatoon. He then went on to obtain his MSc degree in 1964 from the University of Alberta, Edmonton, Canada. Following his MSc degree, he worked as a consulting geotechnical engineer for R.M. Hardy and Associates in Edmonton, Alberta. In 1966, Fredlund accepted a position in the Dept. of Civil Engineering at the University of Saskatchewan, Saskatoon. In 1973 he earned his PhD after returning to the University of Alberta for his studies. Returning to the University of Saskatchewan, he developed a research program focused around the behavior of unsaturated soils and the numerical modeling of unsaturated soils problems. During his career he supervised over 65 MSc and PhD graduate students. He became the Head of the Department of Civil Engineering at the University of Saskatchewan from 1989 to 1994. He has also been appointed as an Adjunct Professor at Nanyang Technological University in Singapore, and the Hong Kong University of Science and Technology, Hong Kong, as well as at the University of Alberta, Edmonton. In addition, he is an Honorary Professor at the University of British Columbia in Vancouver, B.C., Canada and a Research Professor at Arizona State University in Tempe, Ariz. He has recently been appointed as an Honorary Professor at NHRI and Hohai University, Nanjing, China. Dr. Fredlund served as the Chairman of the TC6 committee on Unsaturated Soils of the International Society of Soil Mechanics and Geotechnical Engineering for 12 years. He took early retirement from the University of Saskatchewan in 2000 in order to be able to fully devote his time to do research on unsaturated soil behavior and the solution of practical geotechnical engineering problems in various parts of the world. His work on unsaturated soil mechanics has taken him to many countries of the world as a lecturer, a teacher, and a consultant. He has undertaken international programs of collaboration with countries such as China, Africa, and Vietnam. Dr. Fredlund is a member of numerous professional societies such as: the American Society of Civil Engineering, the Canadian Geotechnical Society, the Engineering Institute of Canada, and

Pore-water pressure and water volume change of an unsaturated soil under infiltration conditions

Triaxial shearing–infiltration tests were conducted to study the pore-water pressure and volume change of unsaturated soils subjected to infiltration conditions. A modified triaxial apparatus with three Nanyang Technological University (NTU) mini suction probes along the specimen height was used for the experimental program. Elastic moduli were obtained for the soil structure with respect to changes in net confining pressure (E) and matric suction (H). Water volumetric moduli associated with changes in net confining pressure (Ew) and matric suction (Hw) were also obtained from the shearing–infiltration tests. Water volumetric strain and pore-water pressure during the shearing–infiltration tests were computed based on volume change theory. This paper presents the significance of obtaining the parameter Hwfrom an appropriate scanning curve of a soil-water characteristic curve (SWCC) for the computation of water volumetric strain and pore-water pressure changes during a shearing–infiltration test. The appropriate scanning curve should be obtained from the wetting curve of the SWCC at the matric suction where the infiltration test commences.Key words: infiltration, matric suction, triaxial, unsaturated soils, pore-water pressure, water volume change.

Effective Stress in Unsaturated Soils: Review with New Evidence

The application of the effective stress principle to unsaturated soils is critically reviewed, and the reasons underlying the difficulties in previous investigations of the effective stress in unsaturated soils are highlighted. The validity of the relationship proposed by Khalili and Khabbaz in 1998 for the determination of the effective stress parameter, χ, is examined using an extensive array of experimental data. It is shown that quantitative predictions of shear strength and volume change in unsaturated soils can be made using the effective stress concept. The uniqueness of the critical state line in the deviatoric stress–effective mean stress plane for saturated as well as unsaturated soils is investigated, and the incremental form of the effective stress parameter is derived.

Settlement rate of foundations on unsaturated soils

This paper deals with the one-dimensional consolidation of unsaturated soils due to the application of external loads. A simple equation is derived that enables one to predict the rate of settlement of shallow foundations with time. This equation uses the constitutive relationships proposed by Fredlund and Morgenstern to define the volume change of unsaturated soils, and relates the settlement rate to the average degree of consolidation for both the water and air phases. A series of examples is shown to demonstrate the feasibility and usefulness of the derived equation. Key words: one-dimensional consolidation, unsaturated soil, degree of consolidation, rate of settlement.

Constitutive relations for volume change in unsaturated soils : Fredlund, D G; Morgenstern, N R Can Geotech J, V13, N3, Aug 1976, P261–276

Constitutive relations for volume change in unsaturated soils : Fredlund, D G; Morgenstern, N R Can Geotech J, V13, N3, Aug 1976, P261–276

Constitutive relations for volume change in unsaturated soils

Volume change constitutive relations for unsaturated soils are proposed from a semi-empirical standpoint. One equation describes the deformation of the soil structure and a second equation defines the volume of water present in the element. Each equation can be viewed as a three-dimensional surface with two independent stress state variables forming the abscissas.Uniqueness is tested by measuring volume changes resulting from stress changes in two orthogonal directions and comparing predicted and measured volume changes resulting from a stress change in a third direction. Samples of undisturbed Regina Clay and compacted kaolin showed good agreement between the predicted and measured volume changes for monotonic deformation of the soil structure. The agreement was not as close for the water phase. The variation was attributed to difficulties in measuring water volume changes over a long period of time. The laboratory results indicate that the proposed constitutive equations are of the appropriate form for use in engineering practice.