Model For Unsaturated Soils Research Articles

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.

Numerical integration of the Glasgow Coupled Model (GCM)

This paper presents the derivation of the incremental constitutive relations of the Glasgow Coupled Model (GCM), an established elasto-plastic model for unsaturated soils intended to represent their mechanical and water retention behaviour. The set of incremental equations derived defines an initial value problem (IVP) that needs to be solved at each integration step once the initial (or previous) state and theincrements of suction and strain are known. To solve this IVP effectively, a small reformulation of the GCM (not involving any modification of the model, but simply presenting it differently) has been adopted withthe aim to facilitate the development of a robust algorithm capable to identify unambiguously the correct model response activated by a given stress path (i.e. elastic, mechanical yielding, water retention yielding or simultaneous mechanical and water retention yielding). Subsequent to the description of this algorithm, the paper presents an explicit substepping integration scheme with automatic error control able to integrate the incremental constitutive equations of the model for each possible response, including unsaturated and saturated conditions. The numerical capabilities of the substepping scheme in terms of accuracy and efficiency are then demonstrated in the context of the performance maps proposed originally for saturated soil models.

Calibration of BBM Parameters using the Modified State Surface Approach

The Barcelona Basic Model (BBM) developed by Alonso et al. [1] is the first and the mostwidely used elasto-plastic model for unsaturated soils. The BBM successfully explained many key features of unsaturated soils and received extensive acceptance. However, there is lack of a well-establishedmethod for selecting parameter values for the BBM from laboratory tests, although a variety of methods have been recently developed for calibrating model parameters for the BBM. Concerns still exist on the correctness and robustness of such parameter value selection procedures. The above statements were evidenced by a recent benchmark exercise on selection of parameter values for the BBM organized within a "Marie Curie" Research Training Network on "Mechanics of Unsaturated Soils for Engineering"(MUSE)[2]. Experienced constitutive modelers in unsaturated soils from 7 prestigious teams were provided with the same experimental results on an unsaturated soil to calibrate the parameter values in the BBM. Theoretically, the calibrated parameters from different teams are expected to be the same or at leastvery close. However, the selected parameter values by the 7 teams are surprisingly widely different. This paper first discussed the limitations in the existing methods to calibrate the parameter values in the BBM. A novel approach was then proposed to calibrate the parameter values for the BBM. The approach takes advantage of the close-form solution of the BBM, which is derived based upon a newly proposed Modified State Surface Approach to study the unsaturated soils [3-6]. The same experimental data, used in the MUSE benchmark exercise, were reanalysed using the proposed approach to calibrate parameters for the BBM. The results were compared with those in the MUSE benchmark exercise from which thesimplicity, effectiveness, and robustness of the proposed method were evaluated.

Physical and numerical modelling of soil-atmosphere-structure interaction

Extreme, extended wet and dry seasons increase the adverse effects that soil wetting and drying cycles have on the response of shallow geotechnical structures. In expansive soils, volumetric changes due to water content variations may result in the incompatibility of deformations at the soil-structure interface. This study proposes a physical approach and a numerical model to address the soil-atmosphere-structure interactions during soil saturation and desiccation. Experimental desiccation tests were performed on relatively thin, compacted kaolin clay samples that represent the soil-atmosphere boundary. A climatic chamber was used to impose atmospheric conditions of air relative humidity, temperature, wind velocity, and irradiance on the soil surface. Empirical mathematical expressions were obtained to estimate soil desiccation rates as a function of basic atmospheric parameters and soil properties. The experimental desiccation approach was complemented with a coupled thermo-hydro-mechanical (THM) numerical model for unsaturated soils. The coupled THM model calculates water and thermal fluxes, soil volumetric changes, vertical stresses, and the compatibility of soil-structure movements during swelling and shrinking. An example of the capabilities of the numerical model is presented for a full-scale geotechnical problem.

A nuclear magnetic resonance based quantification of pore water distribution in unsaturated soils

This study presents a novel approach to quantify the distribution characteristics of pore water in unsaturated soil by using the nuclear magnetic resonance (NMR) technology. The SWCC and NMR T2 curves under different matric suction were measured during the drying process of soil specimen, and a conversion relationship between T2 and pore radius r of was determined. The method using capillary drainage model to simulate soil drying process was feasible, and pores of unsaturated soil were divided into saturated pore, unsaturated pore, and dry pore when the suction exceeded the air entry value. Pore water distribution curve based on T2 well described the distribution characteristics of pore water in unsaturated soil; the quantification of pore water distribution was realized through a defined index named pore water damage potential (Mcp), and a good mathematical relationship was observed between the relative damage degree and the matric suction. By introducing the Mcp, a modified VG model that reflected the characteristics of pore water distribution was in a good quantitative agreement with the experimental observations. The results achieved provided a valuable reference for revealing the internal mechanism of SWCC and establishing mechanical model of unsaturated soil based on pore water distribution.

A simple numerical simulation method for unsaturated stratum under the water environmental load

A simple numerical simulation method is proposed to investigate the mechanical response of the unsaturated stratum under the water environmental load. The proposed numerical simulation method applies the water environmental load to the unsaturated stratum as the load condition, and adopts the developed non-orthogonal elastoplastic constitutive model for unsaturated soil to describe the mechanical behaviors at the material scale. The mechanical response of unsaturated stratum is obtained by solving the discretized governing equation of the mechanical equilibrium. The proposed numerical simulation method is implemented based on the commercial finite element software ABAQUS. The effectiveness of the proposed method is verified through the element tests on the Botkin silt, the element tests on the Pearl clay, and a model test of the rising water level on a sand column. The numerical simulations of the rising and decreasing groundwater level in the unsaturated stratum are conducted. During the rising groundwater level, the non-collapsible stratum produces rebound deformation, the collapsible stratum below the groundwater level occurs rebound deformation, and the collapsible stratum above the groundwater level mainly produces settlement deformation. During the decreasing groundwater level, both the collapsible stratum and the non-collapsible stratum produce the settlement deformation.

Experimental basis for determining contact angle in the ideal model of unsaturated soils

In an ideal unsaturated soil model, the contact angle (CA) is often determined by the sessile drop method (SDM), or assumed without any test. However, both approaches can lead to errors in the model calculation results. In this study, methods that combine models of unsaturated soil with CA measurement methods is presented. The characteristics of soil CA were studied using the sessile drop method (SDM) and the direct observation method (DOB). The results demonstrated that the CA values of soils and CA variation characteristics measured with SDM were significantly different from those measured with DOB. SDM measures the CA of a surface, which is a “plane” formed by many particles, whereas DOB measures the CA between soil particles on a smaller scale. The measurement results of SDM can determine the degree of soil hydrophobicity but may cause large errors when used as a parameter of an ideal unsaturated soil model. The DOB can simulate the natural drying-wetting cycles of soils, and its measurement results are closer to the natural conditions. Using the mean CA as a model parameter is reasonable, except when the CA value is 0. The hysteresis phenomenon caused by using CA hysteresis for the soil-water characteristic curve (SWCC) is likely to overestimate the effect of CA hysteresis on soil water retention behavior.

Memory repositioning in soil plasticity models used in contact problems

We aim to enhance the stability of finite element models of dynamic structural contact with multiphase granular soils which are described by advanced soil plasticity models that can simulate monotonic and cyclic behaviour of multiphase soils. Often, numerical oscillations cannot be avoided in these contact models and can cause advanced soil models to significantly overshoot stress, leading to unrealistic discontinuities in the stress paths. This situation can challenge the stability of the stress integration scheme and the global finite element solver and lead to the early termination of the analysis. We specifically address the issue of stress overshooting by presenting novel solutions and the corresponding stress integration schemes for a representative soil model for unsaturated granular soils. Also, several examples are provided to evaluate the integration scheme and show the advantages and limitations of the proposed overshooting solutions in solving a contact-impact problem involving unsaturated granular soils.

Korelasi Air Entry Value (AEV) Dengan Kuat Geser Tanah Jenuh Sebagian

The characteristics of unsaturated soil seen from the Groundwater Characteristics Curve (SWCC). One of main variables determine the SWCC of soil are Air Entry Value (AEV). The AEV is the initial point of air entering the soil void. SWCC was obtained from the fitting of unsaturated soil model from Soil Vision. Although statistically a fitting model can match the point of laboratory result, the AEV resulted different point from the curve shaped by laboratory points. Therefore, this study aims to understand further the influence of AEV on unsaturated particularly on the change of soil share strength parameters. This studi used field investigation, a series of laboratory and numerical analysis to obtain Fredlund bimodal and Van Genutchen fitting model. Both models, then used to calculate the shear strength parameter (φ’, c’) change affected by different water content. On the other hand, the AEV consider as the major aspect on the calculation. The result shows that Van Genutchen model demonstrated the higher similarity with the laboratory test shear strength test comparing to Fredlund Bmodal’s. for silt sand oil in this study. This study is applicable for back filled and landslide case.

Flexible support of a pile embedded in unsaturated soil under Rayleigh waves

AbstractUnder the action of Rayleigh waves, pile head is easy to rotate with a concrete pile cap, and pure fixed‐head condition is rarely achieved, which is a common phenomenon for it usually occurs on the precast piles with insufficient anchorage. In addition, the propagation characteristics of Rayleigh wave have been changed significantly due to the existence of capillary pressure and the coupling between phases in unsaturated soil, which significantly affects the pile‐soil interaction. In order to study the above problems, a coupled vibration model of unsaturated soil–pile system subjected to Rayleigh waves is established on the basis that the pile cap is equivalent to a rigid mass block. Meanwhile, the soil constitution is simplified to linear‐elastic and small deformations are assumed to occur during the vibration phase of soil–pile system. Then, the horizontal dynamic response of a homogeneous free‐field unsaturated soil caused by propagating Rayleigh waves is obtained by using operator decomposition theory and variable separation method. The dynamic equilibrium equation of a pile is established by using the dynamic Winkler model and the Timoshenko beam theory, and the analytical solutions of the horizontal displacement, rotation angle, bending moment and shear force of pile body are derived according to the boundary conditions of flexible constraint of pile top. Based on the present solutions, the rationality of the proposed model is verified by comparing with the previous research results. Through parametric study, the influence of rotational stiffness and yield bending moment of pile top on the horizontal dynamic characteristics of Rayleigh waves induced pile is investigated in detailed. The analysis results can be utilized for the seismic design of pile foundation under Rayleigh waves.

Coupled hydro-mechanical modelling of unsaturated soils; numerical implementation and application to large deformation problems

This paper presents coupled hydro-mechanical modelling of unsaturated soil problems by incorporating some advanced numerical and constitutive models in a general-purpose commercial software package, Abaqus. Two different strategies for the interpretation of unsaturated soil behaviour with respect to the soil volume change are considered and the relevant constitutive models are implemented through user-defined subroutines. The first approach that is commonly used in geomechanics considers suction as an additional variable and uses a constitutive model in the space of effective stress–suction assuming that soil compressibility is a function of suction. The second approach treats the constitutive model in the space of effective stress and degree of saturation assuming that the soil compressibility is a function of the degree of saturation. The hydro-mechanical behaviour regarding the change in the degree of saturation caused by both suction and net stress changes is also considered, together with the effect of hydraulic hysteresis. Validation of the implemented algorithms is presented through several benchmark problems. Finally, the application and utility of the implemented procedures are illustrated by simulations of two challenging problems of unsaturated geomechanics, including a slope failure due to seepage and rainfall infiltration as well as cone penetration tests in unsaturated soil. A suitable mesh optimisation scheme is also incorporated to handle finite deformations.

SFG modified model and particle flow simulation

Based on the unified yield surface SFG model and combined with the unified hardening parameters, the elastoplastic constitutive model of unsaturated soil is derived. The model is capable of describing the stress–-strain characteristics of over-consolidated unsaturated soils in terms of softening and shear expansion. The model can be degraded to a uniform yield surface SFG model under normal consolidation. The theoretical values of the model were calculated by MAPLE and compared with the experimental results, and the results show that the model can simulate the relevant properties of over-consolidated unsaturated soils better. Using PFC3D to simulate unsaturated triaxial tests, the process of unsaturated soil triaxial experiments was simulated by introducing gas particles and differentiating between the effective stress of the soil and the matric suction, and it was concluded that with higher matric suction, the contact force chains between particles in the specimen become more frequent and it is more difficult to form shear zones.

A critical state constitutive model for unsaturated structured soils

This paper presents an advanced approach for interpreting the hydro-mechanical response of unsaturated structured soil, including the effect of bond strength degradation and the role of the degree of saturation in unsaturated conditions. The model relies on the definition of a “structure degradation law,” which accounts for the progressive breakage of inter-particle bonding during loading. The soil compression index is assumed to be a function of the degree of structure and the effective degree of saturation. A new volume change equation is proposed to better explain the nonlinear change of compressibility of structured soils under saturated and unsaturated conditions. The proposed volume change equation is generalised to three-dimensional stress states by incorporating the modified Cam-clay model. The basic properties and performance of the proposed constitutive model are then illustrated through numerical examples with various stress paths, degrees of structure, and degrees of saturation. The proposed model is verified using the experimental data available in the literature for both structured soils and reconstituted soils under saturated and unsaturated conditions. The results indicate that the proposed model simulates the mechanical response of structured and reconstituted soils reasonably well in saturated and unsaturated conditions.

Numerical simulation to the effect of applying rationing system on the stability of the Earth canal: Birmana canal in Iraq as a case study

Abstract Extreme conditions will cause the water level of high fill canal segment to change suddenly, which will affect the velocity and pore pressure of the slope. A 9 km irrigation earth canal in the city of Alsyahy, 15 km away from Al-Hilla city, and branching off from the left side of Shatt Al-Hilla at 57 km, was studied. The aim of this work is to study and analyze the effect of rationing system on the Birmana earthen canal during rapid drawdown case. Finite element modeling with Geo-Studio software was used in the present study to analyze the combined seepage and slope stability for three cycles. The resulting minimum safety factor obtained from the analysis using the saturated and unsaturated soil model was found to be 1.161, 1.142, 1.159, and 1.2 from the Janbu, Bishop, Morgenstern-Price, and Spencer methods, respectively. The factor of safety (FOS) values in three cycles are less than the required value. Finally, the FOC decreased by 66% from its value before the rationing system was applied, the area of canal and slip surface increased by 77 and 14%, respectively, due to the applied rationing system. These changes led to an irregular water distribution along the canal, in addition to a reduction in road width.

Updated Lagrangian unsaturated periporomechanics for extreme large deformation in unsaturated porous media

Unsaturated periporomechanics is a strong nonlocal poromechanics based on peridynamic state and effective force state concept. In the previous periporomechnics the total Lagrangian formulation is adopted for the solid skeleton of porous media. In this article as a new contribution we formulate and implement an updated Lagrangian unsaturated periporomechanics framework for modeling extreme large deformation in unsaturated soils under drained conditions. In this new framework the so-called bond-associated sub-horizon concept is utilized to enhance the stability and accuracy at extreme large deformation of the solid skeleton. The stabilized nonlocal velocity gradient in the deformed configuration is used to update the effective force state from a critical state based visco-plastic constitutive model for unsaturated soils. The updated Lagrangian periporomechanics paradigm is numerically implemented through an explicit Newmark scheme for high-performance computing. Numerical examples are presented to demonstrate the stability of the computational updated Lagrangian periporomechanics paradigm and its efficacy and robustness in modeling extreme large deformation in porous media under drained conditions.

Two-phase fully-coupled smoothed particle hydrodynamics (SPH) model for unsaturated soils and its application to rainfall-induced slope collapse

In this paper, a new fully-coupled Smoothed Particle Hydrodynamics (SPH) formulation for unsaturated soils is developed to study the influence of rainfall infiltration on slope stability. The single-layer two-phase formulation is investigated in SPH for the first time to simulate the response of unsaturated soils. The use of a single set of particles improves the computational efficiency and facilitates the implementation of infiltration boundary conditions. The Drucker–Prager strain-softening model, with the use of Bishop's effective stress, is adopted as the soil’s constitutive model. New extensions of a first-order consistent wall boundary treatment are proposed for the coupled hydro-mechanical problem to enforce non-slip/free-slip conditions for the soil phase and water phase. A novel stress diffusion algorithm for general application is introduced to smooth out the numerical noise in the stress field under large deformation. The accuracy of the formulated SPH model is examined with available analytical solutions and experimental data. The proposed numerical scheme is finally applied to the simulation of rainfall-induced slope collapse of an unsaturated slope with two different bedrock geometries. Results demonstrate that the geometry of the bedrock is shown to play an important role in the failure initiation and propagation of the collapse. It is found that the proposed model allows the investigation of both triggering and post-failure mechanism, providing a smooth stress field even at large deformations.

Analysis on Stress-Strain Model of Unsaturated Ice-Containing Soil

The relationship between the stress-strain characteristics of ice-containing unsaturated soil and its various physical properties has important implications for the design and construction of transportation and building projects. The study also reveals the varying phases of the substance that can be deposited within the pore space. During winter, the freezing-thawing cycles affect the various media that can be deposited within the soil pore space, such as water and other water. The presence of water and others in the soil’s pores can cause a freeze-thawing phenomenon and a significant change in the material’s elastic modulus. This paper presents a method that uses the representative elementary volume (REV) model to perform a representative state variable analysis on an unsaturated ice-containing soil with porosity. The three stress-strain REV models that were developed for the study were designed to analyze the properties of an unsaturated ice-containing soil under freezing-thawing conditions. Then a comprehensive analysis of the soil’s various components was performed. The effects of frozen soil on the change law of the relationship between different porous medium are studied. The study also explores the effects of the presence of porosity on the variation law of the frozen soil relationship between the stress-strain. The paper presents the study of the variation characteristics of the elastic modulus of permafrost soil. It also explores the various factors (porosity, moisture content, ice percentage, and tortuosity) that influence the stress-strain characteristics of unsaturated soil.

Engineering Disaster Investigation and Recurrence of the Covering Effect in a Petroleum and Gas Station in Seasonally Frozen Soil Region

The impermeable engineering surface layer in the cold regions blocks the water and heat exchange between the foundation soil and the atmospheric environment. Especially in cold areas with significant temperature differences, the moisture in the foundation soil accumulates at the bottom of the structure layer to form a covering effect. This will exacerbate the occurrence of engineering freeze-thaw diseases. This paper conducts a disease survey of the Guanshan oil and gas station site, which is located in a seasonally frozen soil zone. The ponding and freeze-thaw diseases under the concrete cover are analyzed. The formation process of the pot covering effect in the lower part of the concrete surface layer was reproduced through indoor tests. In addition, a water-vapor-heat coupling model of unsaturated soil was established, which quantitatively produced the formation process of silty clay covering under the concrete slab. The results show that (1) in-situ monitoring demonstrated that the water content of the roadbed soil from 0 to 50 cm below the concrete slab gathered significantly, with the water content increasing by 5 to 30%. (2) Under the action of the indoor time-varying covering effect, the moisture content of silty clay will increase with the number of freezing-thawing cycles, with a maximum migration amount of 6%. (3) Seasonal temperature changes lead to the accumulation of water in the subgrade surface, and the maximum accumulation amount is 32%. (4) During the freezing period, the liquid water and vapor water in the subgrade migrate to the surface layer of the subgrade. As a result, the moisture content of the subgrade surface layer increases, while the melting period is the opposite. The above research results can provide theoretical support and scientific countermeasures for engineering design and disease control in cold regions.

The Tension-Shear and Compression-Shear Joint Strength Model for Unsaturated Clay and Its Application to Slopes

The capillary component and adsorptive component of matric suction differently impact the soil strength. Due to the cavitation effects of pore water, the adsorption effect dominates the behavior of soil when matric suction exceeds the cavitation tension. Based on the binary medium theory, a compression-shear strength model for unsaturated soils considering both capillary effect and adsorption effect is established. Compared with test data, the proposed compression-shear strength model has better prediction performance on the compression-shear strength of soil over a range of wide suction. The soil failure depends both on tension-shear stress and compression-shear stress. The tension-shear coupling mechanism in the soil is first investigated. A concept of closed stress point is introduced to divide the two zones of tension-shear coupling stress and compression-shear stress. According to the compression-shear strength model and tension-shear failure mechanism, the tension-shear and compression-shear joint strength model applicable to plane stress conditions is then established. Compared with test data, the proposed model in this article can better predict the nonlinear strength characteristics of clays and has better applicability. Finally, using the user material subroutine (UMAT), the secondary development of the joint strength model is conducted in ABAQUS and then applied to the slope stability analysis. The calculation results show that the established strength model presents a reasonable description of the development of the tension-shear coupling plastic zone in slope and gives an accurate safety factor.

Study on the stability analysis of rainfall slope based on G-A model considering moisture content

In this paper, the moisture content on the wetting front is obtained by using the Van Genuchten (VG) model of unsaturated soil, and then the distribution of moisture content in the upper part of the wetting front is simplified as a trapezoid. The Green-Ampt (G-A) infiltration model of infinite slope with unsaturated characteristics is derived. The analytical expression of safety coefficient (FOS) of infinite slope with rainfall is solved by combining the limit equilibrium method with the unsaturated soil shear strength theory phase. The results show that: 1) compared with the traditional G-A model and the combined rectangular and 1/4 oval model, the upper part of the wetting front is simplified to a trapezoidal model, which has great advantages in infiltration rate and cumulative infiltration, especially when the slope is large or the rain intensity is heavy; 2) since the distribution of soil moisture content above the wetting front is considered, the matrix suction at the wetting front is not neglected, and the safety coefficient calculated by the method proposed in this paper is closer to the actual situation than the traditional G-A model.