Soil Constitutive Model Research Articles

Development of Constitutive Model for Simulation of Cemented Soil

Classical soil constitutive models fail to describe the strain-softening response of cemented soil at a low stress level. This paper proposes a constitutive model that incorporates the Modified Cam Clay model to describe pressure-dependent soil stiffness and strength corresponding to volume changes into (1) empirical functions to recognize the mobilized friction and initial cementation as evolutions of the deviatoric plastic strain and (2) a power-type compressibility function to track the effective stress-dependent volumetric response. In this case, the no additional complexity related to the model formulation is added. The cement-treated constitutive model is implemented to simulate the drained triaxial test. The results indicate that the initial state of isotropic stress and the cement degradation rate defines the stress–strain-volume response, whereas the deviatoric stress eventually approaches the critical state strength.

A unified thermal-hardening and thermal-softening constitutive model of soils

• Thermo-elastoplastic constitutive model of soil is adopted. • Thermal-induced equivalent stress tensor . • Effect of temperature on the pre-consolidation stress and critical stress ratio is considered. • Thermal-hardening and thermal-softening behavior of soils can be described properly. Based on the modified Cam-clay model and the concept of subloading yield surface, a thermo-elastoplastic model of normally consolidated and overconsolidated soils is presented in detail. The model is able to describe the thermal-hardening and thermal-softening mechanical behavior of soils with a unified set of parameters. A thermo-induced equivalent stress is proposed to consider the effect of temperature on yield surface and evolution of overconsolidation during shearing process. At the same time, the effect of temperature on the shear stress ratio at the critical state is also implemented in the model. Through comparing the simulated results with test results under different loading and temperature conditions, the availability and the accuracy of the proposed model are carefully verified. Finally, the generating mechanism of the thermal-hardening and thermal-softening mechanical behavior of soil under non-isothermal condition is discussed based on the proposed model.

A New Soil-Water Characteristic Curve Model for Unsaturated Loess Based on Wetting-Induced Pore Deformation

The soil-water characteristic curve (SWCC) is the basis for describing seepage, strength, and constitutive model of unsaturated soil. The existing SWCC models do not work accurately for evaluating loess, because they do not consider the pore deformation that is induced by wetting. The present study develops a new SWCC model for unsaturated loess. The model considers the effect of wetting-induced pore deformation (WIPD) on the SWCC. The new model includes 6 parameters, which could be confirmed by laboratory tests. The pore volume function (PVF) was described by the WIPD. The shift factor ξ1i and the compression factor ξ2i were introduced into the model. The relationship between the void ratio e and ξ1i and ξ2i was established using the average pore radius. The new SWCC model for saturated loess was improved based on the classical van Genuchten (V-G) model. If the WIPD had not been considered, the new model would regress into the classical V-G model. SWCC tests of unsaturated loess with different void ratios were carried out to verify the new model. The model parameters were calibrated in the original state, and the SWCCs of different void ratios were predicted by the new model and found to be in good agreement with the test results.

Effects of choice of soil constitutive model on seismic performance of moment-resisting frames experiencing foundation rocking subjected to near-field earthquakes

The current study investigated the extent to which the choice of the soil constitutive models can impact the predicted seismic performance of a 20-story reinforced concrete moment-resisting building with a mat foundation considering the Seismic Soil-Structure Interaction (SSSI). Since the soil, in general, is the weakest material, involved in the commonplace geotechnical engineering projects, a soil constitutive model would be able to rule the dynamic response of the system. In this research, the hardening plasticity-based soil constitutive model, named “hyperbolic hardening with hysteretic damping”, in conjunction with the two simple, conventional soil models, namely, the isotropic elastic with hysteretic damping model, and elastic-perfectly plastic Mohr-Coulomb with hysteretic damping model, were invoked in the three-dimensional coupled soil-structure numerical simulations using FLAC3D software. The direct method of analysis was used for analyzing the soil-foundation-structure system in one single step without a need to separately analyze each part of the domain. The cherry-picked earthquake excitations, viz, the 1999 Chi-Chi (Taiwan), and 2011 Kohriyama (Japan), were scaled by means of the widely-used response spectrum matching method as per the design response spectrum of a strong rock. The plastic moment concept was employed so as to assign the elastic-perfectly plastic model to the superstructure and its foundation. Additionally, the strain-compatible shear modulus and damping dependency on the cyclic shear strain were considered via the programmed hysteretic damping algorithm. The numerical predictions included the response spectra at the seismic bedrock and ground surface, base shear forces, shear force distributions along the building height, maximum and permanent foundation displacements, and foundation rocking, plus the flooring lateral deflections and inter-story drifts. The life safety limits for the transient and residual total inter-story drift ratios were not met whilst considering the soil pre-failure plasticity. The concluded remarks herein on the significant effects of the soil hardening plasticity on the seismic performance of the adopted 20-story moment-resisting frame would be applicable to the other structures on a mat foundation, potentially experiencing the foundation rocking amidst the earthquakes.

Development of t–z curve for piles in sands under uplift force

A methodology is developed to assess the load transfer (t–z) curves of piles in sandy soils under uplift/tension force based on soil–pile interaction. Compared to current practice, the proposed approach accounts for soil and pile properties and contributes to the improvement in the construction of a nonlinear (springs) t–z curve in sandy soil which can be utilized with other computer codes to model the soil–pile response under tension forces (i.e., seismic events). The technique determines the mobilized shear stress/strain in soil associated with pile upward displacement at the soil–pile interface under progressing tension forces. A nonlinear soil constitutive model is employed to evaluate the variation of soil shear stress/strain at the soil–pile interface up to failure. The presented approach accounts for the influence of the pile length and diameter on the predicted t–z curve and has been validated through a number of comparisons with measured pile-head response and t–z curves.

Study on Effects of Water Drop for High Concrete Face Rockfill Dams in Narrow Valley

In order to analyze the influence of sudden drawdown of dam water level of high concrete face rockfill dam (CFRD), the paper took a narrow valley high CFRD project for example and conducted a comparative study of stress and deformation of panel and dam between different drop amplitude of dam water level. An elastoplastic hardening soil (HS) constitutive model was used to predict the behavior of CFRD. Then, through analyzing the calculation result of six calculation Cases, the paper summarized the effect of dam water level drawdown speed and amplitude on deformation and stress respectively of panel and rockfill zones. The results showed that owing to the arching effect of the CFRD in narrow valley, total deformation of concrete face slab occurring near the dam crest. However, the pressure and tensile stress of concrete face slab along the dam axial direction have a significant influence on the separation of the slab pieces, and potential seepage at the joints in the slab. The study indicates that the hardening soil model could better to analysis of the behavior of high CFRDs and to predict the performances of high CFRDs.

Static earth pressures on a pile supported excavation in Santiago gravel

Concrete soldier piles anchored in multiple levels are the most common method for supporting deep excavations in gravel. Their static design is based on limit equilibrium principles, although these procedures were originally developed for sheet piles or anchored walls on shallow excavations supporting medium-dense sand. The present study draws on the applicability of current design tools to model the static response of deep excavations in stiff gravels and their implications for design. For this purpose, the static response of an anchored pile system was evaluated with a detailed finite element model of a case study, and the results are compared with simplified hand-calculation procedures. The study unit was the 28 m deep excavation of the Beauchef Poniente building located in the fluvial deposits of the Mapocho River in Santiago. A plain strain model was developed in PLAXIS using the Hardening Soil constitutive model, with parameters determined from large-scale triaxial tests of local gravels, and the computed pile displacement were compared to actual displacement profiles measured with inclinometers. The analysis shows that the simplified procedures provide a reasonably good approximation to the computed earth pressures, internal forces on the piles, and stress in the anchors.

Numerical modeling of rigid strip shallow foundations overlaying geosythetics-reinforced loose fine sand deposits

This research displays the influence of geogrid inclusions on the bearing capacity of rigid strip shallow foundations overlying sand dunes. An extensive chain of settings—containing plain fill case—is verified by valuing some factors as first geogrid reinforcement depth, vertical spacing between geogrid inclusions, and geogrid extension relative to the footing width on the mobilized bearing capacity. To achieve the research aims, a group of finite element analysis is carried out to assess the studied parameters. For the purpose of validation; two-dimensional plane strain finite element model is implemented completely similar to the previously built experimental model tests using Plaxis code version 8.2. The soil is represented by Mohr-Coulomb soil constitutive model, and the geogrid reinforcement is characterized by tension elastic elements which have only a normal stiffness. Well matching is detected between the physical and numerical model test results. The results designate that geogrid insertion can severely enhance the bearing capacity of rigid strip footing overlaying sand dunes. Additionally, it is revealed that the load-settlement performance can be considerably improved. The effectiveness of the geosynthetics loose fine sand composite increases to the maximum as the optimum values of the assessed parameters are reached.

Numerical study on 3D effect and practical design in shield tunneling

In practice, different design methods are used in solving geotechnical problems depending on the type of issue such as the tunneling, braced excavation, or bearing capacity of a foundation, that is, the basic mechanism of the design method differs depending on the problems even for the same ground. A numerical analysis using the finite element method has recently become familiar owing to an improved computing performance; however, it is not widely used in the design of geotechnical problems including tunnel excavation owing to the reliability of the constitutive model of the ground material. If a constitutive model of soils can properly express the properties of the ground material, a numerical analysis will play a vital role in solving the geotechnical problems. In this paper, the current state of a numerical analysis and its applicability in tunnel design are discussed. Herein, the simulation of the ground behavior during tunnel excavation is carried out using sandy and clay ground parameters for shallow and deep tunnel excavations. This paper is mainly focused on the effects of tunnel excavation under three-dimensional (3D) conditions, as well as the current design method. Non-linear 2D and 3D finite element analyses have been conducted, in which the elastoplastic sub-loading tij model has been used as a constitutive model of the soil. The performance and acceptability of the constitutive model have already been proven to reproduce the results of various model tests on different geotechnical problems such as the tunneling, braced excavation, and bearing capacity of a foundation, as well as the measured field data. It was found that a 2D finite element analysis where the rate of stress release is considered, can be used for the prediction of the ground deformation and surface settlement; however, it does not provide rational information in the prediction of tunnel lining forces such as the stress, bending moment, and axial force, which emphasize the necessity of a 3D analysis with a proper construction process in a tunnel design.

Three dimensional image of stress space geotechnical constitutive model

There are various discontinuities in geotechnical engineering problems, which can be roughly divided into two types: one is the discontinuities formed by various actions of rock and soil, such as joints, faults, fissures and shear failure surfaces in rock mass; The other is the contact surface between geotechnical structures such as various foundations, retaining structures, underground structures and geotechnical bodies. Therefore, we cannot ignore its existence in computation. At present, the finite element method and other numerical methods have been widely used to solve geotechnical engineering problems, but the solution of the above discontinuous deformation problems has not been well solved. In this paper, the constitutive model of rock and soil is established. Based on the theory of image processing technology, the common methods of dealing with discontinuous deformation problems in finite element method are pointed out, and the unreasonable points are pointed out. It is more reasonable to treat such problems as contact problems, because it can reflect the main characteristics of discontinuous deformation surface. Come on. The three-dimensional image of the constitutive model has three important functions: (1) it is helpful to enhance the theory of constitutive theory and lay a foundation for further mastery of the theory of constitutive theory; (2) Abstract constitutive theory is no longer just a bunch of boring formulas, which is helpful for beginners to improve their interest in learning the theory of constitutive theory. (3) Finally, a three-dimensional image constitutive model of rock and soil is established through experiments, and the program written by MATLAB is compared with the test results of conventional triaxial compression of rock and soil, which fully verifies the correctness and effectiveness of the constitutive model of rock and soil established in this paper.

Constitutive elasto-plastic model for fine soils in the unsaturated to saturated saturation zone

Near saturation, air bubbles and pockets can be trapped in the porous network of soils. The aim of this paper was to present a coupled model that takes into account the effect of this entrapped air on the poro-elasto-plastic behavior of the soil. The model takes into account the physical-mechanical interactions between different phases as well as the kinematics of each constituent (liquid water, dissolved air, gaseous air and solid grains). This new model was implemented in a FEM code. Some numerical simulations were performed to demonstrate its ability to reproduce a continuous transition of unsaturated to saturated states.

Multiscale modeling of large deformation in geomechanics

SummaryLarge deformation soil behavior underpins the operation and performance for a wide range of key geotechnical structures and needs to be properly considered in their modeling, analysis, and design. The material point method (MPM) has gained increasing popularity recently over conventional numerical methods such as finite element method (FEM) in tackling large deformation problems. In this study, we present a novel hierarchical coupling scheme to integrate MPM with discrete element method (DEM) for multiscale modeling of large deformation in geomechanics. The MPM is employed to treat a typical boundary value problem that may experience large deformation, and the DEM is used to derive the nonlinear material response from small strain to finite strain required by MPM for each of its material points. The proposed coupling framework not only inherits the advantages of MPM in tackling large deformation engineering problems over the use of FEM (eg, no need for remeshing to avoid mesh distortion in FEM), but also helps avoid the need for complicated, phenomenological assumptions on constitutive material models for soil exhibiting high nonlinearity at finite strain. The proposed framework lends great convenience for us to relate rich grain‐scale information and key micromechanical mechanisms to macroscopic observations of granular soils over all deformation levels, from initial small‐strain stage en route to large deformation regime before failure. Several classic geomechanics examples are used to demonstrate the key features the new MPM/DEM framework can offer on large deformation simulations, including biaxial compression test, rigid footing, soil‐pipe interaction, and soil column collapse.

Nonlinear collapse simulation of Daikai Subway in the 1995 Kobe earthquake: Necessity of dynamic analysis for a shallow tunnel

This research conducts dynamic time-history analysis and a modified cross-section racking deformation (MCSRD) method to analyze the collapse mechanism of the Daikai Subway during the 1995 Kobe earthquake using FLAC2D. A simplified approach to consider the nonlinear behavior of a structure element is imported and combined with the FLAC2D built-in nonlinear soil constitutive models to accurately consider the nonlinear interaction between the soil material and structure. Based on the proposed framework, the damage evolution process of the Daikai Subway station can be traced by the dynamic time-history analyses. The collapse pattern roughly agrees with field observation. Regarding the MCSRD method, the critical member of the “central column” is identified but the internal forces did not reach its limit state; this may be because inertial force is neglected, which is a characteristic of the pseudo-static method. Therefore, it is recommended to conduct dynamic time-history analysis for a shallow buried tunnel in soft soil because the inertial force may largely influence the seismic characteristics.

Predicted and observed settlements induced by the mechanized tunnel excavation of metro line C near S. Giovanni station in Rome

This paper deals with the effects induced by the mechanized excavation of Rome metro line C in the area of an old masonry building, the Carducci school. Class A settlements predictions are obtained performing full 3D soil-tunnel-structure interaction numerical analyses, using a simple elastic perfectly plastic soil constitutive model. The developed model realistically simulates the main excavation and construction features influencing the induced settlements, such as tunnel advancement, front pressure, TBM-EPB design (shield’s weight, overcut and conicity), tail void grouting and grout hardening over time. The measured settlements are reported and compared with the results of numerical analyses performed before (class A prediction) and after tunnelling; the latter carried out to implement in the model the front pressure and TBM conicity actual values, both higher than assumed in the design. Since before the excavation the foundations were reinforced with micropiles, and these were not modelled, the comparison between monitoring data and numerical predictions is limited to the settlements outside the building. Monitoring data are also compared with further analyses conducted using small-strain soil stiffness and using a constitutive model able to reproduce the non-linearity of soil behavior (Hardening Soil). The different predictions of the two models are investigated analyzing the vertical strains distributions and the stress paths around the tunnel. Finally, a reasonable interpretation for the remaining differences between numerical results and field data is proposed and used to back-analyze the settlements, obtaining a satisfactory agreement. The results confirm the effectiveness of the proposed 3D numerical approach, associated with relatively simple soil constitutive models, as a tool to predict tunnelling-induced settlements both in the design and the construction phase, independently of the geotechnical context.

A simple 3D elastoplastic constitutive model for soils based on the characteristic stress

This paper presents a direct method to establish a 3D elastoplastic constitutive model for soils based on the characteristic stress. In the established model, a new yield function is first formulated through an interpolation function in the characteristic stress space. A hardening parameter that suits for monotonic and cyclic loading conditions is established by introducing the concept of the subloading surface. Then, a 3D constitutive model for soils is established by combining the previously proposed plastic potential function. Only seven material parameters with clear physical meanings are included in the proposed 3D elastoplastic model. The deformation behaviours of soils under monotonic and cyclic loading conditions and the critical state properties under true triaxial stress conditions are analysed on the basis of the presented model. The capability of the model is verified by the comparisons between the model responses and the test results of soils under the 3D monotonic and cyclic loading conditions.

Small Strain Soil Constitutive Models HS-SMALL & RU+MCC – Calibration and Verification

The paper presents two reasonably advanced constitutive laws for soil. First one is a hybrid of the Modified Cam Clay and a new RU (reload, unload) development, and it is author’s conception. The second one is a HS-small model by Benz. In the RU+MCC model, the Modified Cam Clay model, which is an isotropic elastoplastic hardening, describes normal consolidation behaviour - plastic model originated by Burland in 1967 within the critical state soil mechanics. This model describes realistically mechanical soil behaviour in normal consolidation states. The RU part is designed to ensure more adequate soil responses to reloading paths, particularly in the range of small strains. The HS-small model is an improvement of HS model originated form Schanz and Vermeer. This concept base on cap yield surface with incorporation of two hardening mechanisms, stiffness variation at small strains, densification mechanism, Rowe’s dilatancy and some others. The author in the FEM computer code Z_SOIL.pc has implemented the RU + MCC model. The HS-small model has been implemented into the same software by Truty. To test the influence of the small strain nonlinearity on soil - structure interaction as well as to exhibit the ability of the proposed model to simulate realistically this effect, a comparative study based on the FEM solution has been carried out. As a benchmark, a trial loading test of strip footing was used. The calibration process has based on advanced laboratory and field soil tests like resonant columns, triaxial test, dilatometer test and many others.

A constitutive model for evaluation of mechanical behavior of fiber-reinforced cemented sand

The aim of this study is to present a constitutive model for prediction of the mechanical behavior of fiber-reinforced cemented sand. For this purpose, a generalized plasticity constitutive model of sandy soil is selected and the parameters of the model are determined for three types of sandy soils using the results of triaxial tests. Next, the proposed model is developed using the existing models based on the physico-mechanical characteristics of fiber-reinforced cemented sand. The elastic parameters, flow rule and hardening law of the base model are modified for fiber-reinforced cemented sand. To verify the proposed model, the predicted results are compared with those of triaxial tests performed on fiber-reinforced cemented sand. Finally, the efficiency of the proposed model is studied at different confining pressures, and cement and fiber contents.

Numerical analysis of settlement of a high-rise building using two constitutive soil models

One of the basic roles of foundations is to safely transfer loads from the structure to the subsoil in a controlled manner. Often a key parameter in deciding whether the foundation was designed correctly is the value of settlement of the building and the ground around it. This paper attempts to numerically reproduce the measured settlement of a high-rise building using geotechnical parameters already available. For this purpose, numerical calculations were carried out using two constitutive soil models: the elastic-perfectly plastic model with Mohr-Coulomb plastic criteria (MC) model and the Hardening Soil (HS) model. The resulting settlement values were compared with surveying measurements taken during and after the building’s construction. In the summary the results obtained with the use of different constitutive models, the calculation process and the adopted soil parameters are analysed and discussed.

Research Status of 1-D Nonlinear Soil Layers Seismic Response in Time Domain

The soil dynamic constitutive model has always been one of the frontier topics in soil dynamics. In the past few decades, the constitutive model of soil has been rapidly developed, and with computer technology and one-dimensional soil earthquakes. In the development of reaction analysis methods, many earthquake response models and calculation procedures for one-dimensional time domain nonlinear soil layers have emerged. In this paper, the deterministic method of theoretical analysis is used to summarize the development of soil constitutive model in soil-time seismic nonlinear method. The time-domain nonlinear soil used in different periods is introduced according to the development order of constitutive model. Layer seismic response procedures and their advantages and disadvantages, as well as the latest improvements made to them. Finally, the development direction of nonlinear method programs is prospected.

Automated Model And Parameter Selection: Incorporating Expert Input into Geotechnical Analyses

Excavation for construction of MIT Stata Center in Cambridge, MA. This complex soil-structure interaction problem required advanced numerical methods and soil constitutive models. A) PLAXIS 3D model of intermediate excavation stage; B) Model settlements at intermediate stage; C) Photo showing intermediate excavation stage; D) Model settlements at near-final state. Graphics courtesy of Zhandos Orazalin and Andrew Whittle.