Unsaturated Soil Mechanics Research Papers (original) (raw)

Suction-water content relationship (SWR) is important for mathematical modelling of unsaturated soil behaviour. It is the graphical relationship between soil suction and soil-water content. Particle morphology has the influence on SWR. However, limited studies are available in the literature on the influence of particle shape on the SWR. Therefore, the present study attempts to investigate the influence of the particle shape on the SWR. The paper describes the effect of particle shape on the SWR of a locally available plastic soil by mixing two cohesionless soils, sand and rock quarry dust.

- by and +1
- •
- Unsaturated Soil Mechanics

The Intergovernmental Panel on Climate Change (IPCC) provides convincing evidence of global warming as a result of increased greenhouse gas production. There has been a greater occurrence of extreme climate events in recent decades. We need to ensure that our buildings and infrastructure can cope with such events and possibly more extreme events in the future. A good grounding in unsaturated soil mechanics will be necessary to understand future changes involving the drying and desiccation of soils that will occur in dry seasons and the wetting and infiltration processes that prevail during wet seasons. To predict the impacts of climate change will require the use of robust numerical modelling of climate/soil interactions that can be used to model the effects of future climate regimes. To achieve this we need high quality field observations involving climate/soil interaction that can be used to validate the models. This paper reports on a study in the UK to acquire such data.

- by Domenico Gallipoli
- •
- Earth Sciences, Environmental Science, Geophysics, Remote Sensing

Spatial variability of material properties is inherent in both natural soil deposits and earth structures, yet it is often ignored during geotechnical design. With the objective of developing novel methods for assessing the effects of soil variability on groundwater flow, this study presents a stochastic finite element model of seepage through a flood defense embankment with randomly heterogeneous material properties. Stochastic modeling is undertaken by means of a Monte Carlo simulation which involves a large number of finite element analyses, each with randomly varied porosity at element level, which leads to a corresponding random variation of both permeability and water retention properties across the embankment domain. This provides a statistical distribution of responses, such as total flow rate and time to reach steady state, instead of a single deterministic result as in conventional studies of seepage through unsaturated heterogeneous soils. As the degree of heterogeneity increases, water tends to flow along the most permeable paths inside the soil mass, resulting in an irregular shape of the predicted wetting fronts and pore pressure contours. The mean and standard deviation of the computed quantities strongly depend on the statistics of the input porosity field. Simulations are also conducted to compare the statistical variation of flow rate with and without dependency of the water retention curve on porosity. With recent growth in computer speed, stochastic finite element models based on the Monte Carlo approach can become a powerful design tool, especially if a quantitative assessment of geotechnical risks is required. Copyright © 2011 John Wiley & Sons, Ltd.

- by Marcelo Sánchez and +1
- •
- Finite Element Methods, Numerical Modeling, Unsaturated soil, Geotechnical Engineering

Most Federal roads in Nigeria constructed in about the last 6years have experienced failure within the first 6 months to 2 years. The pavement design method used in Nigeria and some other tropical and subtropical countries (Road Note31) was investigated to find out whether it is part of the cause of these roads. Material characteristics were taken from back calculation data of Nigerian federal roads by Claris et al (1986). Mechanistic analyses were done and the values were compared with that provided by Road Note 31. The results showed that design thickness provided by Road Note 31 were 30% to 60% of the calculated values and the provided pavement Equivalent Single Axle Load (ESAL) were 10% to 20% of the design ESAL which represents between just below one year to one and a half years instead of the 5 to 10 years most of the roads were designed for.

An invited lecture on slope stability and landslides delivered at the University of Cyprus in Nicosia on the 20th February 2020. The lecture goes through basic slope stability and landslide analysis based on the limit equilibrium method with comments on particular points of interest or caution from examples of real landslides investigated, analysed and stabilised. Starting from general considerations, the lecture then focuses on the effect of the climatic conditions in Greece and Cyprus on the mechanical behaviour of soils near the ground surface (unsaturated soils, their behaviour and the effect of changes in partial saturation) and the consequences of increasing building activity related to tourism with examples from Cyprus.

- by Michael Bardanis
- •
- Slope Stability, Landslides, Tourism, Unsaturated Soil Mechanics

Au cours des dernières décennies, un intérêt croissant est porté à la terre crue en tant qu'éco matériau de construction pour son caractère local, sa capacité à réguler l'hygrométrie des espaces intérieurs et la faible consommation d'énergie qui caractérise son cycle de vie global. Cependant, le développement de la construction en terre crue compactée est limitée par certains freins comme une faible résistance mécanique ou encore une sensibilité à l'infiltration de l'eau. Le principal but de cette étude est d'améliorer les propriétés mécaniques de briques de terre crue par l'emploi d'une nouvelle procédure de compactage : si la pratique commune est d'appliquer des charges de compactage relativement faibles (environ 15 MPa) durant quelques secondes, la méthode ici proposée se caractérise par l'application de pressions de compactage élevées (jusqu'à 100 MPa) pour une durée correspondant à la dissipation de la surpression de l'eau interstitielle. Pour cela, l'équipement mis au point est muni d'un système de drainage de l'eau interstitielle. Dans le cadre de cette étude, trois niveaux de contrainte de compactage ont été testés : 25, 50 et 100 MPa. Après compactage, les échantillons (une dizaine pour chaque niveau de contrainte) ont été placés dans une enceinte climatique (température de 25°C et humidité relative de 62 %) jusqu'à équilibre hydrique. Une fois cet équilibre atteint, les échantillons ont été testés en compression simple non confinés afin d'évaluer leur module d'Young et leur résistance mécanique en compression. Afin d'analyser et d'appréhender comment l'augmentation de la contrainte de compactage modifie le réseau poreux et la distribution de la taille des pores dans le matériau, des essais par porosimétrie d'intrusion au mercure ont été effectués sur des éprouvettes compactées respectivement à 25 et à 100 MPa. Cette nouvelle méthode de compactage à haute pression permettant l'atteinte de performances mécaniques élevées pourrait contribuer à la fois à changer les pratiques courantes de production de briques en terre crue comprimée, mais aussi orienter différemment la conception des presses. Ainsi, par l'atteinte de meilleurs performances mécaniques, un des principaux freins à l'utilisation de la terre crue comme matériau de construction serait levé.

One of the most severe failure causes for earthen dams is “seepage and piping,” which, due to chemical and physical effects, causes the soil particles to weaken and lose the soil’s property. Piping has been the primary cause of failure in most of the world’s earth dams. This paper covers the definition and various types of piping and piping causes and effects in earth dams. I will explain the difficulties in dealing with piping, and currently available solutions to prevent piping in earth dams during the design period and construction stage.

- by Zana Taher
- •
- Photogrammetry, Expansive Soils, Unsaturated Soil Mechanics
- by Mostafa Alyousif
- •
- Unsaturated soil, Unsaturated Soil (Engineering), Testing, Unsaturated Soils

The accurate estimation of soil bearing capacity plays an important role in the design of a stable foundation for road pavement and airfields projects. Plate load test is one of the quick, direct and economic methods for soil bearing capacity determination. This method is useful to solve the design problems for subgrade and subbase layers, where application of high wheel loads is expected due to heavy traffic during construction and service life of the pavement. This study is interesting to investigate, analyze and discuss the geotechnical behavior of soil layers selected from two locations south of Iraq. On the other hand, the comparison is made between the values of bearing capacity, modulus of sub-grade reaction and elasticity of soil determined by carrying out field plate load tests and conventional laboratory tests. Furthermore, the effect of stress distribution and test condition on the shape of elasticity-stress curve and modulus of subgrade reaction are discussed in detail. areas (Coduto, 2001). On the other hand, the output of the plate load test is commonly relative to direct measurement of modulus of subgrade reaction (Ks) where the Ks parameter is used as a primary input for rigid pavement design. Typically, the modulus of subgrade reaction is determined from field plate load test where there is no direct laboratory method to measure it. However, the Ks value can be estimated from some stiffness tests as indirect procedure. Some studies in Iraq investigated the bearing capacity of the soil calculated from laboratory and field tests and tried to find an empirical relationships between them such as: Al-Obaidi (2008) investigated the effect of sub-base layer thickness and relative compaction on the values of bearing capacity and modulus of sub-grade reaction. The study found that the increases of the layer thickness and relative compaction lead to smoothly increase in both of bearing capacity and modulus of subgrade reaction. Ahmad (2009) concluded that the bearing capacity values obtained from laboratory test using Terzaghi equation and from field plate load test seems to be similar for shallow soil layer. 2 TEST PROGRAMME AND METHODOLOGY The test program of this work is divided into two parts: Laboratory tests and Field plate load test.

- by Qasim Al-Obaidi
- •
- Collapsible Soils, Unsaturated Soil Mechanics, Soil Suction, Plate Loading Test
- by Yanxin Mao
- •
- Unsaturated Soil Mechanics

The influence of tensile strength on the behaviour of cohesionless soils is typically ignored in geotechnical engineering practice. However, the tensile cracking and subsequent failure characteristics of earth structures, such as dams, slopes and embankments, are significantly influenced by the tensile strength. For this reason, a semi-empirical model is proposed for predicting the variation of the tensile strength of unsaturated cohesionless soils with the degree of saturation, using the soil-water characteristic curve (SWCC) as a tool. The proposed model is capable of predicting the tensile strength arising from matric suction and surface tension, which are related to saturated pores and to the air-water interface associated with water bridges around interparticle contacts in unsaturated pores, respectively. Information about (i) the matric suction (u a-u w), the capillary degree of saturation (S c), and the residual degree of saturation (S r) derived from the SWCC; (ii) the mean particle size (d 50) and the coefficient of uniformity (C u) from the grain-size distribution curve; (iii) the void ratio (e); and (iv) the friction angle (t) at low normal stress level is required to employ this model. The proposed model is validated by comparing the prediction results with measured tensile strength of 10 different unsaturated cohesionless soils (including five sandy soils and five silty soils). The proposed model is promising for use in engineering practice applications as it only requires conventional soil properties, alleviating the need for cumbersome experimental studies for the determination of tensile strength of unsaturated cohesionless soils. Résumé : L'influence de la résistance à la traction sur le comportement des sols non cohésifs est généralement ignorée dans la pratique de l'ingénierie géotechnique. Cependant, les caractéristiques de fissuration à la traction et de rupture subséquente des structures terrestres telles que les barrages, l'inclinaison et les digues sont influencées de manière significative par la résistance à la traction. Pour cette raison, un modèle semi-empirique est proposé pour prédire la variation de la résistance à la traction des sols non cohésifs insaturés avec le degré de saturation, en utilisant la courbe caractéristique sol-eau (SWCC) comme un outil. Le modèle proposé est capable de prédire la résistance à la traction résultant de la succion matricielle et de la tension superficielle qui sont liées à des interstitiels saturés et à l'interface air-eau associée, aux ponts d'eau autour des contacts interparticulaires dans les interstitiels insaturés, respectivement. L'information de (i) la succion matricielle (u a-u w), degré de saturation capillaire (S c), et le degré de saturation résiduelle (S r) dérivé du SWCC; (ii) la taille moyenne des particules (d 50) et le facteur d'uniformité (C u) de la courbe de distribution granulométrique; (iii) le taux de vide (e); et (iv) l'angle de frottement (t) à un niveau de contrainte normal est requis pour utiliser ce modèle. Le modèle proposé est validé en comparant les résultats de prédiction avec la résistance à la traction de 10 sols non cohésifs insaturés (y compris cinq sols sableux et cinq sols limoneux). Le modèle proposé est prometteur pour une utilisation dans les applications de la pratique d'ingénierie, car il ne nécessite que des propriétés de sol conventionnelles, atténuant le besoin d'études expérimentales encombrantes pour la détermination de la résistance à la traction des sols non cohésifs insaturés. [Traduit par la Rédaction] Mots-clés : résistance à la traction, sol non cohésif insaturé, succion matricielle, tension superficielle, courbe caractéristique de l'eau souterraine.

- by Penghai Yin
- •
- Crack, Unsaturated Soil Mechanics, Desiccation, Tensile Strength

Spatial and temporal variations of pore water pressure within slopes in response to rainfall that lead to slope failure, are one of the major uncertainties in evaluating slope stability. This paper reports on the study of slope stability with respect to pore water pressure variation with rainfall during actual failure in September 2011. The studied slope, situated near the peak of Doi-Inthanon national park, Northern Thailand, consisted of granitic residual soil fill that suffered from soil erosion and shallow failure. The KU-tensiometers were installed to monitor both pore water pressure and suction in the slope. Various laboratory and field tests were conducted, namely, direct shear tests on both fully saturated and unsaturated soils, soil water characteristic curve, and field infiltration tests. Two-dimensional (2-D) Back analysis of slope stability, for failure event in 2011 suggested that the critical pore water pressure distribution can be assigned to the ru value of about 0.43 or u=0.43H. Based on three dimensional (3-D) stability analysis, the pore water pressure can be 30% higher when compared with the 2-D analysis. 2-D Finite Element seepage analysis appeared to capture general trend of pore-water pressure change reasonably well. However, it seemed to underpredict the pore-water pressure at failure especially for depth greater than 2 meters.

- by Apiniti Jotisankasa and +1
- •
- Slope Stability, Landslides, Geotechnical Properties of Residual Soils, Unsaturated Soil Mechanics

The paper describes and evaluates an incremental plasticity constitutive model for unsaturated, anisotropic, nonexpansive soils (CMUA). It is based on the modified Cam‐Clay (MCC) model for saturated soils and enhances it by introducing anisotropy (via rotation of the MCC yield surface) and an unsaturated compressibility framework describing a double dependence of compressibility on suction and on the degree of saturation of macroporosity. As the anisotropic and unsaturated features can be activated independently, the model is downwards compatible with the MCC model. The CMUA model can simulate effectively: the dependence of compressibility on the level of developed anisotropy, uniqueness of critical state independent of the initial anisotropy, an evolving compressibility during constant suction compression, and a maximum of collapse. The model uses Bishop's average skeleton stress as its first constitutive variable, favouring its numerical implementation in commercial numerical analysis codes (eg, finite element codes) and a unified treatment of saturated and unsaturated material states.

Most of the recently postulated unsaturated shear strength models have been calibrated only for a short variety
of soils. In addition, these models are yet to be extended and calibrated over a wider range of matric and
total suction states. The present work focuses on further refinements of previously proposed shear strength equations
in light of newly obtained experimental evidence of shear strength behavior of compacted silty sand at a
critical state from suction-controlled triaxial tests conducted between 0.05 MPa to 300 MPa suction range. A refined
and rather simple equation comprising two independent functions, is introduced and validated, including
a thorough parametric investigation, to predict the unsaturated shear strength of compacted silty sand at a critical
state for a wide range of matric and total suction states. The experimental program included a total of 21
consolidated drained (CD) triaxial tests conducted on statically-compacted specimens of silty sand under strainand
suction-controlled conditions. Experimental results show that the angle of internal friction (ϕ′) remained
virtually constant over the entire range of induced suction states; however, the shear strength increased while
the angle of internal friction with respect to suction (ϕ⁠b) decreased with increasing suction, with both varying
non-linearly. Finally, a gradual increase in brittleness of the test soil at peak-failure condition, as well as an increasingly
marked strain-softening post-failure, was observed with increasing suction.

- by Ujwalkumar Patil
- •
- Unsaturated Soil Mechanics

“Raw earth” (“terre crue” in French) is an ancient building material consisting of a mixture of moist clay and sand which is compacted to a more or less high density depending on the chosen building technique. A raw earth structure could in fact be described as a “soil fill in the shape of a building”. Despite the very nature of this material, which makes it particularly suitable to a geotechnical analysis, raw earth construction has so far been the almost exclusive domain of structural engineers and still remains a niche market in current building practice. A multitude of manufacturing techniques have already been developed over the centuries but, recently, this construction method has attracted fresh interest due to its eco-friendly characteristics and the potential savings of embodied, operational and end-of-life energy that it can offer during the life cycle of a structure. This paper starts by introducing the advantages of raw earth over other conventional building materials followed by a description of modern earthen construction techniques. The largest part of the manuscript is devoted to the presentation of recent studies about the hydro-mechanical properties of earthen materials and their dependency on suction, water content, particle size distribution and relative humidity.

The paper gives a brief description of double wall cell triaxial equipment for testing soil samples under unsaturated conditions and presents results of some of the calibrations necessary to achieve accurate measurements of inner cell water volume, pore water volume, pressure, load and displacement. The calibration of the measurement devices showed linear relationships between the raw readings and applied values with the regression constants differ from 1 and 0. The result of apparent volume change calibrations showed significant correlation between cell water volume fluctuation and temperature fluctuation inside testing room. A correction to the measured volume was therefore required to reduce the effects of long-term temperature fluctuation on the measured volume. Investigation of the accuracy of volume change measurement with the double wall cell showed excellent matching between the inner cell volume change and pore water volume change under saturated conditions, indicating the high reliability of the double wall system for apparent volume change measurements.

Au cours des dernières décennies, un intérêt croissant est porté à la terre crue en tant qu'éco-matériau de construction pour son caractère local et la faible consommation d’énergie qui caractérise son cycle de vie global. Toutefois, sa résistance mécanique relativement faible et sa sensibilité à l'eau freinent le développement de ce matériau. Le principal but de cette étude est d’améliorer les propriétés mécaniques de briques de terre crue par l’emploi d’une nouvelle procédure de compactage à pression élevée maintenue pendant une durée correspondante au temps de consolidation. Un système de compactage a été élaboré afin de compacter le sol à haute pression en permettant le drainage de l’eau interstitielle. Dans un premier temps, les caractéristiques géotechniques du matériau d'étude ont été déterminées. Après compactage à 25, 50 et 100 MPa, les échantillons ont été placés dans une enceinte climatique (25°C, humidité relative de 62 %) jusqu’à équilibre hydrique, puis testés afin d’évaluer leur module d’Young et leur résistance mécanique en compression. Cette nouvelle méthode de compactage à haute pression a permis d'atteindre des performances mécaniques élevées. Ainsi, un des principaux freins à l'utilisation de la terre crue comme matériau de construction serait levé.

The idea that one of these areas of study, either Geomechanics, or Rock Mechanics, is included entirely into the other, or that it is a division of the other is mistaken. Both share common units of study (chapters, sections and topics), but they also have distinct sections, found in only one of them. This technical note presents the author's views about what should be included into the two areas (disciplines and sciences), both in their shared area of study, and in the specific sections, distinct to each of them. From a mechanical point of view - in general, the rocks are subject to various natural phenomena and /or technological processes taking place on an industrial scale or as an experiment, which may cause mechanical or mechanical effects. Therefore, phenomena and processes of this kind can be classified into two categories: - mechanical, the displacements, deflections and / or modification of the properties of rocks caused by the action of forces and / or pressures (eg, compaction under the weight of the rock layers above); - non-mechanical (physical, chemical), but with mechanical consequences that have as a result either the emergence of new tension states or the modification of the existing mechanical properties and parameters. An example of this is the phenomenon of swelling that increases the volume of shale layers and, under in situ conditions, generates additional pressure on surrounding rock layers.

- by Petru CIOBANU
- •
- Geology, Rock Mechanics, Geotechnical Engineering, Geomechanics

The soil water characteristic curve (SWCC) indicates the relationship between the matric suction and the water content. In recent years, determination of the soil water characteristic curves (SWCCs) of soils has become an important tool in interpreting the engineering behaviour of unsaturated soils. This is because such a curve allows the prediction of the engineering behaviour of an unsaturated soil Permeability, shear strength, volume change and deformability of unsaturated soils are mainly affected due to water content and suction. This paper presents the results of an experimental investigation of the general nature of the SWCC for soils with different gradations in terms of moisture content and volumetric water content versus soil matric suction obtained from the mining of the Kuala Trong in Taiping, Perak. Pressure plate testing methods were used to establish the SWCC to evaluate the soil water characteristic curves of mine tailing sand. The study found that higher pressure leads to higher saturation and air-entry values. The results also show that the initial water content and the sizes of the sand particles have a great influence on the SWCC of the studied sand.

- by Muhammad Hezmi and +2
- •
- Unsaturated Soil Mechanics, Soil-Water Characteristic Curve

Mounding often occurs beneath engineering structures designed to infiltrate reuse water. AQTESOLV software and a spreadsheet solution for Hantush, together with soil moisture water balance (SWAGMAN farm model), were used for quantitatively predicting the height and extent of groundwater mounding underground to assess the groundwater-flow simulations of infiltration from a hypothetical irrigation site. Horizontal and vertical permeability, aquifer thickness, specific yield, and basin geometry are among the aquifer and recharge properties inputs. For 2.2 ha sites, the maximum heights of the simulated groundwater mound ranges up to 0.29 m. The maximum areal extent of groundwater mounding measured from the edge of the infiltration basins of 0.24 m ranges from 0 to 75 m. Additionally, the simulated height and extent of the groundwater mounding associated with a hypothetical irrigation infiltration basin for 2.2 ha development may be applicable to sites of different sizes, using the recharge rate estimated from the SWAGMAN farm model. For example, for a 2.2 ha site with a 0.0002 m/day recharge rate, the irrigation infiltration basin design capacity (and associated groundwater mound) would be the same as for a 1.1 ha site with a 0.0004 m/day recharge rate if the physical characteristics of the aquifer are unchanged. The study claimed that the present modelling approach overcomes the complications of solving the Hantush equation for transient flow. The approach utilised in this study can be applied for other purposes such as measuring the feasibility of infiltrating water, attenuation zone, risk mitigation essential for decision-makers and planning regulators in terms of environmental effects and water use efficiency.

Contaminants flow through compacted soil liners and coversare usually considered under saturated conditions during design stage. Whereas, the compacted soil barriers in reality are neither completely saturated nor completely dry, rather they are in an unsaturated state. Thus, unsaturated flow principles need to be properly simulated to determine the contaminant flow through compacted soil barriers in order to protect groundwater. Soil water characteristic curve (SWCC) which is the relationship between soil suction and water content is the main modeling parameter of unsaturated soil. This paper evaluates the SWCC data of a tropical laterite soil used as a liner material in sanitary landfill. The effect of gradation with respect to fines content on the dry of optimum, optimum and wet of optimum moisture contents on SWCC were investigated. Laboratory tests using pressure plate apparatus were conducted to determine the variation of volumetric water content with soil suction for a pressure range of 1 kPa to 1000 kPa. The experimental SWCC data were fit to the Brooks and Corey parametric equation to compute the unsaturated hydraulic conductivity. Based on the SWCC data, greater air entry suctions were obtained for specimens with higher fines contents when compacted wet of optimum moisture content and the water retention capacity increased with increase in fines content. Using the SWCC, the unsaturated hydraulic conductivities of the soil liner used in simulating leachate migration were computed.

- by Muhammad Hezmi
- •
- Unsaturated Soil Mechanics, Swcc

A new elasto-plastic model for unsaturated soils incorporating the effect of evolving fabric anisotropy is presented. The model is formulated in terms of mean net stress, deviator stress and suction and reduces to the Barcelona Basic Model for an isotropic material. Model simulations are performed and compared against experimentally observed behaviour from constant suction triaxial tests on isotropically and anisotropically compacted kaolin samples along a variety of stress paths. Simulations indicate an improvement in the prediction of the size and shape of the yield surface compared to the Barcelona Basic Model. In addition, the deformation generated during shearing are better predicted than in the Barcelona Basic Model, even though experimentally observed strains are still grossly over-estimated. Further refinement to model assumptions are required to improve predictions of experimental data and the reformulation of the model in terms of average skeleton stresses, instead of net stresses, might help in this respect as outlined in Al-Sharrad (2013).

The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code “Compass” for coupled flow-deformation analysis involving unsaturated soils and application of the improved code to the simulation of pressuremeter tests in unsaturated soils.
On the constitutive side, a unique relationship is proposed between degree of saturation, suction and specific volume by introducing dependency on specific volume in the simplified van Genuchten equation. This is a significant improvement over the common assumption of a state surface expression for degree of saturation. If combined with an elasto-plastic stress-strain model predicting the variation of specific volume, the proposed relationship is capable of reproducing irreversible changes of degree of saturation and changes of degree of saturation experimentally observed during shearing. Predictions show very good agreement with experimental results from tests on compacted Speswhite Kaolin published in the literature.
On the numerical side, a number of changes to the code “Compass” have been performed. The new relationship for degree of saturation is implemented in the code and the implementation is validated against three benchmark problems. Use of the new relationship for degree of saturation results in significantly different predictions to those obtained if a conventional state surface expression for degree of saturation is used (as present in the original code).
Implementation of the water and air continuity equations in “Compass” has been corrected by expressing these equations in terms of flux velocities relative to the soil skeleton. This is the form in which the equations should be expressed if they are to be combined with Darcy’s law for liquid and gas flows. The simulation of a notional laboratory test shows that the incorrect combination of Darcy’s law with absolute flux velocities, as present in the original code, causes significant errors.
The convergency algorithm at constitutive level employed in the code has been corrected by introducing residual flux terms in the two flow equations, analogous to residual forces in the equilibrium equation. These terms must be taken into account if a convergency algorithm for an elasto-plastic stress-strain model is used and the relationship assumed for variation of degree of saturation involves any dependency on net stresses. A numerical study of a notional laboratory test shows that omission of residual flux terms results in substantial errors and may cause failure to converge.
The plane_strain formulation of code “Compass” has been corrected by imposing the condition of nullity only on the out-of-plane component of the total strain rate vector instead of the out-of-plane component of each single contribution to the total strain rate, as was done in the original code. Such inconsistency, due to the history of development of finite element programs, also appears in other examples published in the literature. Numerical simulations of two types of bi-axial tests show that significantly different results are generally predicted by the correct and incorrect formulations, and also provide an explanation why this type of error was difficult to detect in codes implementing traditional models for saturated soils.
The potential of the enhanced version of code “Compass” for analysing boundary value problems is demonstrated by simulations of pressuremeter tests in unsaturated soil. This study also provides some initial insight into the interpretation of pressuremeter tests in unsaturated soil by simulating tests at different loading rates in a normally consolidated soil. The mechanical behaviour of the soil is represented by the elasto-plastic Barcelona Basic Model of Alonso, Gens and Josa (1990) while the variation of degree of saturation is modelled by the new relationship proposed in the thesis. The entire range of loading rates, from undrained to fully drained (with respect to liquid), is simulated. Relatively small changes of suction are predicted even in the fastest test and the computed cavity pressure-cavity strain relationships are all very similar regardless of loading rate. It may therefore be possible to model even rapid pressuremeter tests in unsaturated soils as a drained (constant suction) process. Further work is required to investigate the generality of this conclusion.

The paper presents a bounding surface model that describes the gradual yielding of unsaturated soils subjected to isotropic loads. The model originates from consideration of the capillary bonding between soil grains, which leads to the definition of a “unified normal compression line” that is valid in both saturated and unsaturated conditions. This line has the same slope and intercept of the saturated normal compression line but is formulated in terms of a “scaled stress” variable, which takes into account the mechanical effect of capillarity by factoring the average skeleton stress (also known as Bishop’s stress) with a power function of degree of saturation. The normal compression behaviour of unsaturated soils is therefore described by only one additional parameter, which is the exponent of the degree of saturation in the scaled stress expression. For over-consolidated soils, the occurrence of gradual yielding is introduced by assuming that, as the soil state moves towards the unified normal compression line, the slope of the loading curve tends towards the slope of the unified normal compression line according to an expression requiring only one extra parameter. Interestingly, this expression can be integrated in a closed form to provide a general equation for all loading paths in saturated and unsaturated conditions. Different loading curves are simply distinguished by the different values of the integration constants. Unloading paths are also simulated in a similar way. The proposed model requires a total of five parameters, which include the three standard parameters for saturated soils (i.e. the slope and intercept of the saturated normal compression line and the slope of the swelling line) plus one parameter to describe unsaturated behaviour and one parameter to describe the gradual yielding of over-consolidated soils subjected to loading.

- by Domenico Gallipoli
- •
- Civil Engineering, Soil Science, Unsaturated soil, Geotechnical Engineering

In this paper an Intrinsic Compressibility Framework for anisotropic clayey soils (Belokas and Kavvadas 2011), is extended, to account for the effect of partial saturation, leading to a complete volumetric compression framework for anisotropic, unsaturated clayey soils. The proposed framework incorporates compression curves that depend on both suction and degree of saturation to capture the bonding effect of interparticle forces formed when degree of saturation drops below unity. It can reproduce: a) compression lines that depend on the level of stress induced anisotropy, b) unique compression lines for saturated materials irrespectively of the level of the applied suction, c) a continuously increasing post yield compressibility for soils compressed under constant suction, and lastly d) a maximum of collapse.

- by Panagiotis (Panos) Sitarenios and +1
- •
- Anisotropy, Constitutive Modeling, Unsaturated Soil Mechanics

Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

- by Domenico Gallipoli and +3
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code "Compass" for coupled flow-deformation analysis involving unsaturated soils and application of the improved code to the simulation of pressuremeter tests in unsaturated soils. On the constitutive side, a unique relationship is proposed between degree of saturation, suction and specific volume, by introducing dependency on specific volume in the simplified van Genuchten [48] equation. This is a significant improvement over the common assumption of a state surface expression for degree of saturation. If combined with an elasto-plastic stress-strain model predicting the variation of specific volume, the proposed relationship is capable of reproducing irreversible changes of degree of saturation and changes of degree of saturation experimentally observed during shearing. Predictions show very good agreement with experimental results from...

The problems that arise from land in Indonesia are one of them related to soft soil. One of the soft soils that often arises is on cohesive soils. The soft cohesive soil causes the bearing capacity of the soil to be low. Therefore, various methods to improve soft soils are needed to increase soil bearing capacity. One method that can be applied is the vacuum consolidation method. However, problems will arise if there are construction buildings, such as bridges, buildings, roads, and foundations near areas on the ground that will be vacuum consolidated. In this study, the building construction is modeled as a mini pile. Mini piles are modeled after 5, 10, 15, 20, 25, 30, and 35 meters from the vacuum area. The purpose of this research is to know the effect of vacuum consolidation method on the mini pile. From the results of this study, the effect of the consolidation vacuum on mini pile is that there is a tensile force on the ground which causes lateral deformation of the mini pile.

- by rinda karlinasari
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- Civil Engineering, Unsaturated Soil Mechanics

The mechanical behaviour of fine-grained soil materials to be used as impermeable cores for earth dams has been extensively studied by numerous researchers. The required properties of these materials have also been very well described by standards and specifications. Yet, more than often it is required to assess their swelling/collapse potential, especially at various vertical stresses, as a means to estimate their volume changes upon inundation which is going to be caused by filling the dam reservoir. In the paper, experimental results of five different soils are presented. The soils tested ranged from non-plastic silty sands with clay to medium plasticity clayey silts that were compacted in conditions dry, at and wet of optimum moisture content as derived from standard compaction energy Proctor tests, then subjected to one-dimensional loading conditions and then inundated. The vertical stress was up to 7.6 MPa. The experimental results are categorized according to initial moisture content relative to Proctor optimum and indicate expected magnitudes of strains due to inundation for various grain-size distributions and plasticities.

This paper presents a series of consolidated drained triaxial tests performed on weakly bonded soil. Samples were artificially prepared by mixing sand and kaolin (87% sand: 13% kaolin) and firing at 500ºC in a furnace. The weak bond strength is provided by the fired kaolin. The results are explained according to the critical state concept in order to interpret the behaviour of the studied soil. Destructured soil was also prepared and tested in the same manner as the bonded soil but without firing. The bonded samples showed more pronounced peaks in stress-strain behaviour compared to the destructured samples. The critical state line (CSL) could not be easily identified as some of the drained tests show changes in deviator stress and volumetric strain up to the end of tests. Therefore, careful assessment of stress-strain behaviour, pore-water pressure responses, stress paths and volumetric changes was made to identify the critical state line. Samples sheared at low and medium stress levels displayed contraction prior to dilatant behaviour whilst those samples sheared at higher stresses showed contraction up to the end of shearing. A " discontinuity " approach was applied to position the critical state on dilatant 2 paths in v-ln p space after considering other stress-strain and volumetric strain curves. The critical state line (CSL) in v-ln p space for bonded samples plotted above the CSL for destructured samples but bent down toward the destructured line at higher stress levels. This clearly indicates the effect of cementation bonds on the CSL for the bonded samples, as the influence of cementation remains until the stress levels increase beyond the yield stress of the bonded material. In addition, the effect of bonding can be visualized from the normalised stress paths of destructured and bonded samples.

- by Domenico Gallipoli and +2
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- Civil Engineering, Soil Science, Unsaturated soil, Geotechnical Engineering

Bonded soils are often thought about as problematic soils as they do not fit into
the frameworks of behaviour that have been developed for remoulded or unstructured soils. In reality, the role of bonding enhances the strength and stiffness of the soil. A comprehensive programme of triaxial testing has been carried out on artificially bonded sand in order to develop a framework of behaviour for bonded soils. The results of the tests are interpreted to investigate the conditions under which yielding of the bonds will occur. It is shown that bond breakdown can be predicted from a knowledge of the yield loci that define conditions under which yield will occur. It is found that a unique Critical State Line is not achieved, but that the Critical State is defined by a band of void ratios. Therefore, the ultimate state is difficult to predict for such a material. Nevertheless, normalisation of the data is possible, which is helpful in interpreting the behaviour. It is shown that, although complex, bonded soil behaviour can be explained, at least in part, by the frameworks of behaviour used for unstructured soils.

- by Domenico Gallipoli
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- Civil Engineering, Unsaturated soil, Geotechnical Engineering, Soil Mechanics

This paper investigates the “initial” and “evolving” mechanical anisotropy of a compacted unsaturated soil. A wide campaign of triaxial compression and extension tests, involving different stress and suction paths, has been performed on both isotropically and anisotropically compacted samples of unsaturated Speswhite Kaolin. The first objective is the definition of the initial yield surface of the compacted soil after suction equalization and before any plastic loading/wetting path takes place. This is followed by the investigation of the evolution of the yield surface induced by plastic straining along different loading/wetting paths. Experimental results are interpreted by using two alternative stress variables, namely net stresses and Bishop’s stress. Constant suction cross-sections of the yield surface are represented as distorted ellipses not passing through the origin in the q:p plane of deviator stress versus mean net stress, and by distorted ellipses passing through the origin in the q:p* plane of deviator stress versus mean Bishop’s stress. The inclination of these distorted elliptical yield curves evolves with plastic straining but remains the same at all suction levels for a given level of plastic deformation. The critical state lines in the planes q:p and q:p*, or in the semi-logarithmic v:lnp and v:lnp* planes (v is the specific volume), are generally independent of initial anisotropy or stress history, suggesting that fabric memory tends to be erased at critical state.

- by Domenico Gallipoli and +1
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- Civil Engineering, Unsaturated soil, Geotechnical Engineering, Soil Mechanics

Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutive modeling of unsaturated soils. However, this type of tests requires sophisticated equipment and is time-consuming. This has been one of the major obstacles to the implementation and dissemination of unsaturated soil mechanics beyond the research context.
In contrast to suction-controlled triaxial tests, the suction-controlled oedometer test requires simpler equipment and a shorter testing period. Oedometer tests represent the at-rest earth pressure (K0) condition, which is an important stress state in any simulation. The major disadvantage of the oedometer test is that its lateral stress is controlled by the condition of zero lateral strain and remains unknown during the testing process. At present, no well-established, simple, and objective methods are available that take advantage of oedometer test results for constitutive modeling purposes.
This paper derives an explicit formulation of the at-rest coefficient for unsaturated soils and develops an optimization approach for simple and objective identification of material parameters in elasto-plastic models for unsaturated soils using the results from suction-controlled oedometer tests. This is achieved by combining a modified state surface approach (MSSA), recently proposed to model the elasto-plastic behavior of unsaturated soils, with the quasi-Newton method to simultaneously calibrate all parameters governing virgin behavior in elasto-plastic models. The Barcelona Basic Model (BBM) is used to demonstrate the application of the proposed explicit formulation and calibration method. Results predicted using obtained parameters are compared with laboratory test results for the same stress paths in order to evaluate the simplicity and objectivity of the proposed method.

- by Xiong Zhang and +1
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- Unsaturated Soil Mechanics

The paper presents a benchmarking study carried out within the ‘Mechanics of Unsaturated Soils for Engineering’ (MUSE) network aimed at comparing different techniques for measurement and control of suction. Techniques tested by the eight ‘Mechanics of Unsaturated Soils for Engineering’ research teams include axis-translation (pressure plate and suction-controlled oedometer), high capacity tensiometer and osmotic technique. The soil used in the exercise was a mixture of uniform sand, sodium bentonite (active clay) and kaolinite (non-active clay), which were all commercially available. Samples were prepared by one team and distributed to all other teams. They were normally consolidated from slurry under one-dimensional conditions (consolidometer) to a given vertical stress. The water retention characteristics of the initially saturated specimens were investigated along the main drying path. Specimens were de-saturated by applying suction through the liquid phase when using an axis-translation technique or osmotic method and de-saturated by air-drying, when suction was measured using high-capacity tensiometers. In general, the same technique was tested by at least two teams. The water retention curves obtained using the different techniques are compared and discrepancies are discussed in the paper.

- by Domenico Gallipoli and +2
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

The Discussers read the article by Avunduk et al. (2021) (the Authors), with special interest focused on the fall-cone (FC) penetrometer and torvane shear testing reported for the investigated fine-grained soil–anticlay foaming agent mixtures. These mixtures were prepared at the soil natural water content wn, employing the Ayvali 2, M.-Mkay and Kaolinite soil materials, with constant foam concentration and foam expansion ratios of CF = 3% and FER = 16, respectively. The Discussers would like to make some comments on the data analyses and interpretations by the Authors for the above-mentioned tests, especially regarding the FC data plotted for reducing saturation level (increasing foam injection ratio (FIR)) in their Figures 7, 10b, 11 and 12. But the Discussers would like to begin with some fundamentals of FC penetration testing for shear strength determinations of saturated fine-grained soil samples. Then, from reanalysis of digitized vane shear and FC data reported by the Authors for their conditioned soil samples, the Discussers glean some insightful observations on the variations of the cone factor K for reducing levels of saturation (increasing FIR); an aspect on which there is presently little published research. As such, this discussion article represents an important addition to the research literature.