Behaviour Of Unsaturated Soils Research Articles

Multiphase lattice Boltzmann modeling of cyclic water retention behavior in unsaturated sand based on X-ray computed tomography

The water retention curve (WRC) defines the relationship between matric suction and saturation and is a key function for determining the hydro-mechanical behavior of unsaturated soils. We investigate possible microscopic origins of the water retention behavior of granular soils using both computed tomography (CT) experiment and multiphase lattice Boltzmann Method (LBM). We conduct a CT experiment on Hamburg sand to obtain its WRC and then run LBM simulations based on the CT grain skeleton. The multiphase LBM simulations capture the hysteresis and pore-scale behaviors of WRC observed in the CT experiment. Using LBM, we observe that the spatial distribution and morphology of gas clusters varies between drainage and imbibition paths and is the underlying source of the hysteresis. During drainage, gas clusters congregate at the grain surface; the local suction increases when gas clusters enter through small pore openings and decreases when gas clusters enter through large pore openings. Whereas, during imbibition, gas clusters disperse in the liquid, the local suction decreases uniformly. Large pores empty first during drainage and small pores fill first during imbibition. The pore-based WRC shows that an increase in pore size causes a decrease in suction during drainage and imbibition, and an increase in hysteresis.

State-of-the-Art Review of Utilization of Microbial-Induced Calcite Precipitation for Improving Moisture-Dependent Properties of Unsaturated Soils

Unsaturated soil is a form of natural soil whose pores are filled by air and water. Different from saturated soil, the microstructure of unsaturated soil consists of three phases, namely, the solid phase (soil particle), vapor phase, and liquid phase. Due to the matric suction of soil pores, the hydraulic and mechanical behaviors of unsaturated soils present a significant dependence on the moisture condition, which usually results in a series of unpredictable risks, including foundation settlement, landslide, and dam collapse. Microbial-induced calcite precipitation (MICP) is a novel and environmentally friendly technology that can improve the water stability of unsaturated soft or expansive soils. This paper reviews the microscopic mechanisms of MICP and its effect on the mechanical properties of unsaturated soils. The MICP process is mainly affected by the concentration of calcium ions and urea, apart from the concentration of bacteria. The moisture-dependent properties were comparatively analyzed through mechanical models and influence factors on the experimental data among various unsaturated soils. It suggests that the variations in resilient modulus and permanent deformation are strongly related to the extent of MICP applied on unsaturated soils. Finally, the problems in the MICP application, environmental challenges, and further research directions are suggested.

Discrete element modelling of the macro/micro-mechanical behaviour of unsaturated soil in direct shear tests including wetting process

Discrete element modelling of the macro/micro-mechanical behaviour of unsaturated soil in direct shear tests including wetting process

Influence of aggregate structure on the compressibility of an unsaturated compacted silty sand

It is well recognised that the compressibility of unsaturated soils is affected by soil structure. However, the effects of soil aggregation and pore structure on the compressibility behaviour of unsaturated soil are still not well understood. In this study, the effects of soil aggregation and pore structure on the compressibility of unsaturated silty sand were investigated. Specimens were purposely prepared at two compaction water contents to similar void ratios. They were saturated after compaction and dried to suction values of 0, 50, 150, or 300 kPa following a similar hydraulic loading path, after which they were subjected to isotropic loading and unloading. Mercury intrusion porosimetry (MIP) and micro-X-ray computed tomography (μ-XCT) were used for microstructural studies. At a given suction, specimens compacted on the dry side are less aggregated (smaller aggregate sizes) showing higher compressibility, whiles specimens compacted on the wet side are highly aggregated (larger aggregate sizes) revealing lower compressibility. The low compressibility of the highly aggregated soil is attributed to the additional drying-induced intra-aggregate strength due to the suction stiffening effects from meniscus water within the aggregates.

Modelling hydro-mechanical coupled behaviour of unsaturated soil with two-phase two-point material point method

Material point method (MPM) offers an effective approach for analysing large-deformation problems, such as landslides that often involve unsaturated soil. Several MPM formulations for unsaturated soil have been reported in the literature, but they assumed that water retention ability and permeability function are independent of soil deformation. Furthermore, most studies used a single set of material points, so they cannot simulate some processes (e.g., the infiltration of free water into unsaturated soil). To address these problems, a two-point MPM formulation has been extended from saturated to unsaturated soils, considering the influence of porosity change on the water retention curve and permeability function. The information of solid and liquid phases is carried by two individual sets of material points, assuming zero air pressure. The elastoplastic mechanical behaviour is modelled by the Drucker-Prager model using Bishop’s stress. The MPM simulations were compared with the results of physical model tests (for large-deformation problems) and finite element analysis (for small-deformation problems). It is evident that the proposed MPM formulation is able to capture various hydro-mechanical problems well. In particular, the computed failure processes of unsaturated soil slope under rainfall are consistent with results from physical model tests.

Enhancement of a hydro-mechanical hypoplastic model for unsaturated fine-grained soils accounting for small strain stiffness

Due to global climate change, larger extreme seasonal and daily moisture variations have been occurring more frequently in the last decades. This phenomenon can affect geotechnical structures by inducing cyclic coupled hydro-mechanical loads. However, reproducing this behaviour from a numerical point of view requires robust constitutive models that can predict the coupling between the hydraulic and mechanical behaviour of fine-grained soils, combined with predictions of history-dependent stiffness evolution at small strains. For this reason, in the present work the hypoplastic model for unsaturated finegrained soils was further modified to better predict the water retention behaviour of unsaturated soils incorporating a smoothed hysteretic Water Retention Curve (WRC). In addition, the constitutive model wascalibrated using experimental data available on the literature of a completely decomposed tuff (CDT) from Hong Kong. At the end, the capabilities of the extended model to predict cyclic behaviour of unsaturated soils were evaluated using cyclic constant water triaxial tests at different suctions The results indicate that the extended model is able to describe with more accuracy the cyclic hydro-mechanical behaviour of the decomposed tuff if additional suction-dependency of one of its small-strain parameters is considered. Without this, the model can be calibrated to data at a given suction but its cyclic predictions for different suctions are not reasonably accurate.

Extending modified Cam-Clay to unsaturated soils by incorporating a scaled stress variable

This paper extends the modified Cam-Clay model to describe the behaviour of unsaturated soils by replacing the effective stress with the scaled stress. The scaled stress accounts for the mechanical effect of capillarity by factoring Bishop’s stress with a power function of degree of saturation. The extendedmodified Cam-Clay model postulates the existence of a yield curve in the mean scaled stress 𝑝̅– deviatoric scaled stress 𝑞̅ plane. Coherently with the modified Cam-Clay model, the stress-strain behaviour is predicted by means of an elastic law inside the yield curve. As the scaled stress state evolves onto the yield curve, the behaviour becomes elasto-plastic and this is modelled by a plastic flow rule combined with a volumetric hardening law. The extended modified Cam-Clay model is then validated against experimental data published in the literature. Results show that the proposed approach is capable of well predicting the mechanical behaviour of unsaturated soils during triaxial loading. Future work will be directed at further validating the present formulation along different stress paths as well as accounting for the combined effect of partial saturation and cementation within a critical state framework based on bounding surface plasticity.

Finite element modelling of unsaturated soils using a modified version of the Barcelona Basic Model

In this paper, unsaturated soil behaviour is numerically simulated using a modified version of the Barcelona Basic Model. The model has been implemented as a User-Defined Soil Model (UDSM) in the finite element code PLAXIS. Unlike the original formulation based on net stress ([1]), this model isformulated based on the well-known Bishop’s effective stress, with suction as an additional stress variable. Furthermore, a non-associated flow rule and the dependency on Lode’s angle proposed by [2] are incorporated. The Soil Water Characteristic Curve (SWCC) follows the Van Genuchten model ([3]). First, the simulation of laboratory tests along different stress paths and levels of suction are compared againstexperimental data. Then, an unsaturated soil embankment subject to a time-dependent precipitation is simulated by means of a fully coupled flow-deformation analysis. The results show the capability of the model to reproduce the main features of unsaturated soil behaviour both at material point level and inboundary value problems.

Soil Moisture Active Passive (SMAP) Satellite Data and Unsaturated Soil Response

The importance of degree of soil saturation in geotechnical problems resulted in inclusion of unsaturated soil mechanics in various applications for several decades. However, in spite of recent progress in remotely sensed soil moisture measurement, geotechnical community has not yet taken advantage of these advances in analysis of unsaturated soils. NASA launched its Soil Moisture Active Passive (SMAP) satellite in 2015 with the aim of providing surface and root zone soil moisture content over the global land surface at 3-day average intervals. SMAP, as a widely validated and near-real-time database, offers a rich soil moisture database at a global scale that can be used in studies considering unsaturated soil behaviour. A study of the relationship between soil water content and seismic ground response is presented in this paper using SMAP, which includes the tracking of the variations in Earth's surface soil moisture caused by earthquakes.

Effects of thermal cycles on soil behaviour: theoretical and experimental studies

Fundamental understanding and proper modelling of soil behaviour under thermal cycles are increasingly important and essential for the analysis and design of many emerging infrastructures, such as geothermal structures and embankment-atmosphere interactions under a changing climate. Previous studies mainly focus on monotonic thermal loading of thermo-mechanical behaviour of soils. Based on a unified, state-dependent theoretical framework in the form of compliance matrix, a new constitutive model is developed to simulate the cyclic thermo-mechanical behaviour of saturated and unsaturated soils. This new bounding surface model is formulated in terms of Bishop’s stress and suction. Apart from the loading and bounding surfaces, a memory surface is incorporated in the model to simulate cyclic thermal behaviour of soils. To verify the new model, computed results are compared with measured data from cyclic heating-cooling tests on saturated and unsaturated soils at various suctions. Based on this new model, two engineering applications are analysed including cyclic thermally loaded floating energy pile foundations and a deep excavation in the unsaturated ground. Consistent results are obtained between computed and measured data.

Pore scale investigation of unsaturated granular soil behaviour by means of in situ CT experiments

With continuing evolution of imaging techniques from medical applications and materials science, non-destructive imaging experiments have also become an important method to investigate soil specimens. Amongst other methods, computed tomography (CT) has developed to a tool to visualise and better understand the microstructure of different soils based on 3D image data. Furthermore, the acquisition of a temporal series of CT images allows to study processes in soils on the microscale, e. g., during mechanical loading. In order to study the hydraulic and mechanical behaviour of unsaturated granular soils, we combine different custom-built miniaturised experimental set-ups with geomechanics background with computed tomography, yielding so-called in situ CT experiments. By means of image reconstruction and further image analysis based on segmented CT images acquired during different hydraulic and mechanical experiments, we study the drainage and imbibition process as well as the shear process of unsaturated sand and glass bead specimens on the pore or grain scale. The analysis of data on the microscopic level, including the phase distribution, interfacial areas, contact lines as well as radii of curvature of capillary menisci, allows to obtain insights into the macroscopic water retention behaviour and shear behaviour of granular soils.

A constitutive framework for the chemo-mechanical behaviour of unsaturated non-expansive clays

Both osmotic and matric suction changes have a significant influence on the mechanical behaviour of clays. Despite the different types of interactions at the microstructural level, both suctions havea relevant effect on the fabric of non-expansive clays. Starting from experimental observations at the laboratory scale, it is possible to identify some common features characterizing the mechanical response of non-expansive clays to salinity and degree of saturation changes. This paper presents an elastoplasticframework to reproduce the behaviour of unsaturated clayey soils upon changes in the salt concentration of the pore fluid. In particular, it presents a strategy to include osmotic suction induced by pore fluid salinity in BBM-like models [1]. The model was implemented in the Thebes code and it was calibrated on experimental data performed on Boom clay [2] and remoulded loess [3].

Experimental Investigation of Capillary Rise in Unsaturated Gypseous Soils

Capillary rise is a phenomenon that must be considered an inevitable factor when dealing with unsaturated gypseous soil. This study investigates the behavior of gypseous unsaturated soil under capillary action, where different gypsum content and soil densities are experimentally tested in a developed laboratory model consisting of Transparent pipes 1 m long and 0.05 m in diameter. All the pipes are placed inside a rectangular basin with a dimension of (1500×500×500 mm). The soil was taken from Tikrit city in Iraq at a depth of (1.5 to 2 m) under the natural soil surface; the laboratory tests clarified that the soil is poorly graded sand with a gypsum percent of 59%. The three other gypseous soil samples were prepared by dissolving with HCl acid to gain the required percentages of gypsum (22, 37, and 6%). The results show that the presence of gypsum in the soil slows down the rise of water in the soil after rapid rising in the first stages of the test. The low and moderately gypseous soils required a shorter time to reach the maximum capillary rise than highly gypseous soils. Also, the existence of gypsum increased the max. The height of the capillary rise and the degree of saturation in the capillary zone were also noticed for all soil densities examined. The effective stress will be low in the fully saturated parts while it’s high in the low saturated parts.

Soil-atmosphere interaction: cracking of a compacted soil under the effect of a thermo-hydric stress

Reusing excavated material in geotechnical engineering reduces the carbon impact of a project. Such materials are usually placed in a compacted state in order to achieve the mechanical and hydric characteristics required to guarantee the safety of the structures. A good geotechnical knowledge of the materials is therefore necessary as well as a good anticipation of their behaviour over time. Indeed, in some situations, as in the case of waste storage, a low hydraulic conductivity is required. The use of crushed rocks rich in clays (argillite), possibly improved by adding bentonite, could be interesting. However, this addition, beneficial in terms of hydraulic conductivity, could be damaging from a mechanical point of view by the development of cracks at the interface atmosphere-compacted soil. For this purpose, samples compacted at the normal Proctor optimum are exposed to a relative humidity of 46% and a temperature of 22.5°C. The thickness, mass and surface condition (cracking) were monitored during the drying process, and measurements were taken in the thickness of the specimen after 29 hours of exposure. The results make it possible to compare the two materials at the same compaction energy. The argillite sample shows a significant shrinkage but no cracks at this scale. On the different hand, with the addition of bentonite, a significant cracking was observed and analysed. These results provide information on the hydromechanical behaviour of unsaturated fine soils at the atmosphere-compacted soil interface.

One-dimensional cyclic loading tests on saturated and unsaturated sand-clay mixtures

A series of one-dimensional cyclic loading tests are carried out on three different proportions of Sydney sand – kaolin mixtures, at various initial degrees of saturation. The settlement of the samples throughout the tests is accurately monitored. The effects of the degree of saturation and frequency of the cyclic load on the evolution of the settlement during the tests are thoroughly investigated for each mixture. The settlement patterns are compared to those of saturated samples to highlight the effect of soil suction on the behaviour. The results show that soil settlement decreases while the time required for the completion of the primary consolidation increases with the decrease in the degree of saturation of the soil. This trend is more pronounced in soils with higher kaolin content. The loading frequency, however, seems to have negligible effects on the settlement behaviour of unsaturated soils under cyclic loading. The result of the study provides valuable data for future simulations and validations of various constitutive models.

Stability analysis of unsaturated soil slope considering softening and non-softening approach

In conventional geotechnical engineering practice, peak shear strength parameters are widely used in the design of geo-structures constructed with or within unsaturated soils. However, a reduction inshear strength from the peak to the residual value is typically observed during the shear deformation in typical fine-grained unsaturated soils. Several geo-structures in unsaturated soils undergo a large shear deformation prior to reaching the failure condition. Thus, the factor of safety of such geo-structures will gradually but significantly decrease with the development of the shear deformation. For this reason, the strain-softening behaviour of unsaturated soils should be considered for reliable design of the geo-structuresin unsaturated soils. In this study, an unsaturated clay slope under 10 years rainfall infiltration were modelled using softening and non-softening approach using commercial finite element software SIGMA/W. The responses of the studied slope to the long-term rainfall infiltration were analysed. The results of the softening and non-softening analysis were compared. This study provides valuable information with respect to the significance of the strain-softening that are useful in the rational design of slopes in unsaturated soils.

A fully coupled hydro-mechanical model for the analysis of partially saturated multiphase geo-structures using the bounding surface plasticity

This paper presents a fully coupled plasticity model for the rigorous analysis of fluid flow and solid skeleton deformation in variably saturated porous media. The flow deformation behaviour of unsaturated soils is described through the simultaneous solution of governing partial differential equations, including the equilibrium equation and the continuity equations of pore fluids. The coupling between solid and fluid phases is established based on the effective stress concept with the effective stress parameter as a function of suction. The elasto-plastic behaviour of geomaterials is formulated using the bounding surface theory. The fully coupled flow deformation framework along with the UNSW bounding surface constitutivelaw are implemented into COMSOL Multiphysics commercial software to simulate the results of several laboratory tests conducted on soils with different degrees of saturation under various drainage conditions and loading paths. The close agreement between the numerical solutions and experimental data from the literature indicates the good performance of this new elastic-plastic unsaturated model and emphasises its capability to capture the different characteristic features of response of soils with a variety of saturation degrees and over-consolidation ratios.

Effects of microstructure on THM behaviour of geomaterials

In this paper, we discuss the thermo-hydro-mechanical behaviour of geomaterials at the light of their microstructure and its changes induced by multiphysics loading. After recalling the strong links between the microstructure and the water retention properties of unsaturated soils, the relation between themicrostructure and the physical properties ruling heat and mass transfers are discussed. The mechanical behaviour of unsaturated soils is then discussed focussing on the definition of an effective stress based on a microstructure description. The experimental determination of this microstructurally-based effective stressis presented, including recent advances to identify the stress coefficient from Mercury Intrusion Porosimetry data. Opening remarks towards macroscopic modelling of unsaturated geomaterials accounting for their microstructure and its changes is finally discussed.

Consideration of microstructure in modelling the hydro-mechanical behaviour of unsaturated soils

The hydro-mechanical behaviour of unsaturated soils is often highly influenced by the microstructure; therefore, it can be beneficial to consider the effect of microstructure in a hydro-mechanical constitutive model. This paper considers the use of a microstructure-related model that adopts the effective degree of saturation as a microstructural index. The model can be used to reproduce the hydro-mechanical behaviour while the effect of the microstructure is considered. For comparison, a non-microstructuredependent model is also employed. The models are applied to simulate the behaviour of two different soilsand a comparison of the models’ performance in simulating triaxial test behaviour is made. Based on the comparison with experimental results and the non-microstructure-dependent model, it can be concluded thatthe adoption of the effective degree of saturation is beneficial to studying the hydro-mechanical behaviour of unsaturated soils affected by the microstructure.

Numerical implementation of BBM in FE package for solving unsaturated soil boundary value problems

Unsaturated soil mechanics is of keen relevance when dealing with soil above the water table orcompacted soils. Barcelona Basic Model (BBM) is one of the most widely used constitutive models for describing unsaturated soil behaviour and is available as a soil model in software like PLAXIS and CODE_BRIGHT (developed by UPC). The present study demonstrates a numerical framework to implement BBM within the ABAQUS package as a user-defined soil model using a user subroutine called UMAT (User Material). The backward Euler integration scheme coupled with Newton Raphson iterative algorithm is used to construct the model. A convergence test is conducted to check the accuracy and stability of the integration. UMAT's inability to manage pore pressure led to proxy diffusion to depict suction variation. The heat equation is used as a proxy for diffusion in three-dimensional space and time. Anoedometer test is simulated in which the soil is allowed to consolidate and then set to saturate at constant vertical stress, which leads to wetting collapse. The results demonstrate a reasonable behaviour of wettinginduced compression of soil subjected to loading with varying saturation level.