Fast Determination of Soil Behavior in the Capillary Zone Using Simple Laboratory Tests

INE/AUTC 13.1

A constitutive model for unsaturated cemented soils under cyclic loading

On the basis of plastic bounding surface model, the damage theory for structured soils and unsaturated soil mechanics, an elastoplastic model for unsaturated loessic soils under cyclic loading has been elaborated. Firstly, the description of bond degradation in a damage framework is given, linking the damage of soil's structure to the accumulated strain. The Barcelona Basic Model (BBM) was considered for the suction effects. The elastoplastic model is then integrated into a bounding surface plasticity framework in order to model strain accumulation along cyclic loading, even under small stress levels. The validation of the proposed model is conducted by comparing its predictions with the experimental results from multi-level cyclic triaxial tests performed on a natural loess sampled beside the Northern French railway for high speed train and about 140 km far from Paris. The comparisons show the capabilities of the model to describe the behaviour of unsaturated cemented soils under cyclic loading

Contribution to the Non-Lagrangian Formulation of Geotechnical and Geomechanical Processes

Numerical simulations of geomechanical and geotechnical processes, such as vibro-injection pile installation, require suitable algorithms and sufficiently realistic models. These models have to account for large deformations, the evolution of material interfaces including free surfaces and contact interfaces, for granular material behavior in different flow regimes as well as for the interaction of the different materials and phases. Although the traditional Lagrangian formulation is well-suited to handling complex material behavior and maintaining material interfaces, it generally cannot represent large deformation, shear and vorticity. This is because in Lagrangian numerical methods the storage points (nodes resp. material points) move with the local material velocity, which may cause mesh tangling resp. clustering of points. The present contribution addresses the development of models for geotechnical and geomechanical processes by utilizing Eulerian and Arbitrary Lagrangian-Eulerian (ALE) formulations. Such non-Lagrangian viewpoints introduce additional difficulties which are discussed in detail. In particular, we investigate how to track interfaces and to model interaction of different materials with respect to an arbitrarily moving control volume, and how to validate non-Lagrangian numerical models by small-scale experimental tests

Mechanics of dilatancy and its application to liquefaction problems

A novel conceptual model of the mechanics of sands is developed within an elastic-plastic framework. Central to this model is the realization that volume changes in anisotropic granular materials occur as a result of two fundamentally different mechanisms. The first is purely kinematic, dilative, and is the result of the changes in anisotropic fabric. There is also a second volume change in granular media that occurs as a direct response to changes in stress as in a standard elastic-plastic continuum. Inclusion of the two sources of volume change into the modified Cam Clay dissipation function results in a new anisotropic model which is suitable for sands with pronounced anisotropic granular arrangement. The conditions that lead to features such as phase transition line and ultimate state line that dense sands exhibit are predicted theoretically by the new anisotropic sand model and confirmed with experimental results. The conventional volumetric-shear strain relation obtained from triaxial experiment is used to determine the evolution of fabric anisotropic parameter.; The new anisotropic sand model is generalized to 3-D cases. Bounding surface plasticity theory is used to capture plastic deformation at small strain levels as well as during unloading/reloading. This enables the robust modeling of the accumulation of plastic strains as well as the buildup of excess pore pressure under cyclic loading of sands. The bounding surface formulation is implemented to the numerical code FLAC3D and used to simulate drained and undrained triaxial tests on Ottawa sand. The FLAC3D model is also used to simulate undrained cyclic triaxial test and predict the liquefaction behavior of Nevada sand observed in centrifuge tests. The analysis shows that the stress induced volumetric strain is the main cause for pore pressure build up leading to initialization of liquefaction whilst the fabric induced volumetric strain influences the post liquefaction behavior of sands

Application of the Elastoplastic-Viscoplastic Bounding Surface Model to Cyclic Loading

The predictive capabilities of the elastoplastic-viscoplastic bonding surface mode, with emphasis response of cohesive soils subjected to cyclic loading, are discussed herein. This model, which represents a generalized three-dimensional constitutive formulation for isotropic cohesive soils, is developed within the framework coupled elastoplasticity-viscoplasticity and critical state soil mechanics

Development of a Computer Vision-Based Three-Dimensional Reconstruction Method for Volume-Change Measurement of Unsaturated Soils during Triaxial Testing

Problems associated with unsaturated soils are ubiquitous in the U.S., where expansive and collapsible soils are some of the most widely distributed and costly geologic hazards. Solving these widespread geohazards requires a fundamental understanding of the constitutive behavior of unsaturated soils. In the past six decades, the suction-controlled triaxial test has been established as a standard approach to characterizing constitutive behavior for unsaturated soils. However, this type of test requires costly test equipment and time-consuming testing processes. To overcome these limitations, a photogrammetry-based method has been developed recently to measure the global and localized volume-changes of unsaturated soils during triaxial test. However, this method relies on software to detect coded targets, which often requires tedious manual correction of incorrectly coded target detection information. To address the limitation of the photogrammetry-based method, this study developed a photogrammetric computer vision-based approach for automatic target recognition and 3D reconstruction for volume-changes measurement of unsaturated soils in triaxial tests. Deep learning method was used to improve the accuracy and efficiency of coded target recognition. A photogrammetric computer vision method and ray tracing technique were then developed and validated to reconstruct the three-dimensional models of soil specimen

A Practical Model for Advanced Nonlinear Analysis of Earthquake Effects in Clay Slopes

Presented in this paper is an effort in providing an advanced yet practical tool with a reasonable level of complexity for modeling of clays in realistic geotechnical engineering problems. SANICLAY model is a Simple ANIsotropic CLAY plasticity model that has been developed by Dafalias et al. (2006). The SANICLAY model provides successful simulation of both undrained and drained rateindependent behavior of normally consolidated clays, and to a satisfactory degree of accuracy of overconsolidated clays. An associated flow rule extension of the SANICLAY model has been employed in the present study, trading simplicity for some accuracy in simulations. The model requires just three constants more than those of the Modified Cam-Clay model, all of which can easily be calibrated from well-established laboratory tests. In order to make the model applicable to practical problems in geotechnical engineering, this simple version of SANICLAY model has been efficiently integrated in FLAC3D program. An illustrative example describing earthquake behavior of saturated clayey slope using the simple form of the SANICLAY model is presented and discussed

Cyclic Loading Response of Cohesive Soils Using A Bounding Surface Plasticity Model

The concept of the bounding surface in plasticity theory has been used to develop a general three-dimensional constitutive model for cohesive soils within the framework of critical state soil mechanics. The present work focuses on the response of the above model under cyclic loading conditions. It is shown mainly qualitatively and partially quantitatively, that the model predicts in detailed form a material response which does agree with observed experimental behavior under undrained and drained loading conditions at any overconsolidation ratio and for different cyclic deviatoric stress amplitudes