International Journal of Geomechanics

Constitutive models for geologic materials and interfaces involve a number of parameters that need to be determined from appropriate laboratory tests. Because the test behavior is influenced by a number of factors such as material variability in test specimens, initial density, mean pressure, and stress paths, the parameters determined from such tests need to be averaged or optimized. The averaging procedure is often used. However, in view of the importance of the parameters in analysis and design, it is desirable and necessary to use advanced procedures such as optimization methods so as to find their improved and realistic values. This paper presents an optimization procedure for the determination of parameters in the unified disturbed state concept constitutive models. A series of multiaxial laboratory tests on a sand under different initial mean pressures, density, and stress paths are used to evaluate the optimized parameters. The stress-strain and volume change behavior is then back-predicted using the parameters from the conventional averaging procedure and the proposed optimization procedure. The results show that the optimized parameters provide improved predictions of the test data. The optimized parameters are used in a finite element procedure to predict cyclic behavior in a boundary value problem involving a shake table test. The proposed procedure can provide a useful methodology for the optimization of parameters for a wide range of available (plasticity, creep, damage, etc.) constitutive models. It can lead to improved analysis and design of geotechnical problems, particularly while using computer (finite element) procedures.

Piles driven into clay are often subjected to indirect loading as a result of the surcharge applied on the surrounding area. During the drained period, both the piles and the soil undergo downward movements caused by the axial and the surcharge loading, respectively. Depending on the relative movement of the pile–soil system, positive and negative skin friction develop on the pile’s shaft. Negative skin friction is the drag force that may be large enough to reduce the pile capacity and/or to overstress the pile’s material causing fractures or perhaps structural failure of the pile, and/or possibly pulling out the pile from the cap. A numerical model that uses the finite element technique combined with the soil responses according to Mohr–Coulomb criteria was developed for case simulation. The computer program CRISP (developed by Cambridge University) was used in this study. The numerical model was first tested against the results predicted by the bearing capacity theories for pile foundations in clay subjected to axial loading. Upon achieving satisfactory results, the numerical model was then used to generate data for piles subjected to surcharge loading. The predicted values were compared well with the field data and the empirical formulae available in the literature. Based on the results of the present investigation, design charts and procedures are presented to predict the location of the neutral plane and to estimate the drag force acting on the pile’s shaft for a given pile–soil–loading conditions. In the case of excessive drag force, coating the pile’s shaft with a thin layer of bitumen is advisable to eliminate or minimize the drag force. The design procedure presented herein would provide the means to establish the need and the extent of the pile coating. Furthermore, it demonstrates the role of the factor of safety on both pile capacity and the depth of the neutral plane.

Abstract: In this paper, an interface or joint subject to large shear deformation is modeled. In the proposed algorithm, continuous interface elements with a finite thickness are reconstructed at every load step based on current interface configuration, by employing the concept of contact band element. Special strain expressions for the continuous interface elements are derived with regard to the characteristics of shear strain concentration along the interface. The elastic cross-anisotropic model with the special Mohr–Coulomb criterion is applied for the continuous interface elements in view of the anisotropy of interface materials. Simulation of a pullout test has shown that large pullout displacement and realistic structure configuration might be effectively modeled and smooth distributions of mobilized shear stresses along the interface and axial forces in the reinforcement can be obtained without any fluctuation for different interface element thicknesses. However, the stress and axial force distributions along the interfaces and the reinforcement, especially near left end of the reinforcement, vary with the interface thickness. It strongly implies that the continuous interface element with an appropriate thickness should be a good choice for a rock interface or joint with fillings in.

Changes in soil pore volume and shape in response to internal and external mechanical stresses alter key soil hydrologic and transport properties. The extent of these changes is dependent on details of pore shape and size evolution. We present a model for quantifying rates of deformation and shape evolution of idealized spheroidal pores as functions of macroscopic stresses and soil rheological properties. Previous solutions for shrinkage of spherical pores embedded in a viscoplastic matrix under isotropic stress were extended to spheroidal pore shapes and biaxial stresses using Eshelby’s classical theory. Bulk soil behavior was obtained from upscaling of detailed single pore deformation. Results show that pore closure rates increase with decreasing initial aspect ratio (i.e., oblate pores close faster than spherical pores), and with higher deviatoric stress. Incomplete pore closure is attributed to soil hardening due to pore shape accommodation under biaxial stresses. The model provides a means for approximating pore deformation as input to predictive models for soil hydraulic properties.

This paper is devoted to coupled hydromechanical modeling of an underground cavity subjected to excavation, drying, and resaturation. An elastoplastic damage model is proposed for the description of poromechanical behavior of semiductile rocks in saturated and unsaturated conditions. The poroelastic behavior of material is nonlinear and dependent on water saturation degree. The plastic deformation is described using a curved yield surface and a nonassociated flow rule, in order to describe the pressure sensitivity and transition from volumetric compressibility to dilatancy. The evolution of material damage is related to tensile plastic strains. The coupling between plastic deformation and damage is taken into account. The capillary effects on plastic deformation are considered through a generalized effective stress concept. The proposed model is implemented in a finite element code for fully coupled thermohydromechanical problems. An example of application is presented for a cavity subjected to excavation drying and resaturation. The evolutions of plastic and damaged zone, and saturation degree around the cavity are studied.

A method is presented for the analysis of coupled consolidation in unsaturated soils due to loading under conditions of plane strain as well as axial symmetry. The method is based on the transformation of the governing differential equations by the Fourier transform, when the soil system is deformed under plane strain conditions, or Hankel transform for problems exhibiting axial symmetry. The effect of such transformations is to simplify considerably the solution from a computational point of view. In addition, using these transformations the same differential equations can be used to analyze consolidation under both the above conditions. Results are presented to point out some aspects of the consolidation in unsaturated soils generated by the application of strip as well as circular loads.

A boundary-element method for solving the fully coupled equations of thermoelastic consolidation is developed for multilayered media. Conditions of temperature, heat flux, pore pressure, hydraulic flux, displacement, and traction are enforced at the interface between layers. In addition, a criterion for making an optimal choice between two efficient Laplace inversion schemes is developed, significantly reducing computation time. The improved scope and numerical efficiency of the method are illustrated by applying it to the analysis of a decaying radioactive waste repository buried in an engineered clay tunnel, itself embedded in granite. The difference in hydraulic conductivity between clay and granite is found to increase the buildup of pore pressure by a factor of 5 in some places.