Victor N. Kaliakin

FINITE ELEMENT ANALYSES OF ANISOTROPIC POROELASTICITY: A GENERALIZED MANDEL'S PROBLEM AND AN INCLINED BOREHOLE PROBLEM

The finite element equations for non-linear, anisotropic poroelasticity are cast in the form of measurable engineering constants. Two problems of importance to the rock and petroleum industry are analysed by the FEM. First, the classical Mandels problem with an extension to transversely isotropic case is investigated. Second, the problem of an inclined borehole is explored. In particular, the effect of material anisotropy on stress concentration near the wall with implication to borehole instability is examined in detail.

Insight into deficiencies associated with commonly used zero-thickness interface elements

Abstract Although they are widely used in the finite element analysis of geologic structures, zero-thickness interface elements possess certain fundamental deficiencies. In particular, spurious stress oscillations attributed to inappropriate quadrature schemes, lack of accuracy due to excessively large stiffness parameters, and inaccurate interface stress predictions due to insufficient mesh fineness have been cited in the past. In this paper the performance of two commonly used linear zero-thickness interface elements is examined in detail. Behavioral deficiencies that have not appeared in the literature are identified. Although some of these deficiencies are similar to phenomena previously noted for such elements, in the present case they are attributed to different factors. An improved linear zero-thickness interface element that overcomes the aforementioned deficiencies is developed and its predictive capabilities critically assessed.

Analysis of concrete beams reinforced with externally bonded woven composite fabrics

Abstract With the deterioration of this nations infrastructure comes the growing need for effective means of rehabilitating structures. Possibly one of the most challenging tasks is to upgrade the overall capacity of concrete structures. The utilization of composite materials in rehabilitating such structures represents an innovative use of new technology. Laboratory experiments have recently been conducted on a series of reinforced concrete T-beams to study the effectiveness of using externally applied composite fabrics as a method of increasing a beams shear capacity. In the experiments, woven composite fabrics made of aramid, E-glass, and graphite fibers were bonded to the webs of T-beams using a two-component epoxy. The beams were loaded in flexure and tested to failure. All beams failed in shear. The ultimate strengths of the externally reinforced beams were 60–150% higher than the strengths of beams without external reinforcement. This paper presents analyses of the externally reinforced concrete beams using finite element models. The quality of numerical simulations are assessed by comparing them with experimental results. To gain better insight into the behavior of externally reinforced concrete T-beams, several numerical parametric analyses were also performed. The significance of the results of these analyses with respect to potential retrofits of existing concrete beams in the civil infrastructure is addressed.

Modeling the Behavior of Geosynthetic Encased Columns: Influence of Granular Soil Constitutive Model

AbstractUsing a high-strength geosynthetic for encasement of granular columns increases the strength of a given column and improves its stress-displacement response. This paper describes the results from a series of three-dimensional finite-element analyses that were performed to simulate the behavior of a single geosynthetic-encased column in soft clay. To examine the sensitivity of the results to the constitutive model that was used to simulate the behavior of the encased granular soil, analyses were performed using models for the encased soil possessing various levels of sophistication. For each model that was studied, comparative analyses were performed to simulate the behavior of a dense and a loose granular soil within the encasement. The results demonstrate the importance of selecting a constitutive model that accurately captures the shear-induced volume-change behavior of the encased granular soil. Additional findings provide guidance for other researchers seeking to model the behavior of geosynth...

Finite element formulation and application of poroelastic generalized plane strain problems

Abstract A generalized plane strain finite element is developed for the analysis of poroelasticity problems. The validity and accuracy of the special element is demonstrated by analyzing inclined borehole problems in isotropic and transversely isotropic poroelastic materials. For the former, comparison is made with an analytical solution and the latter, with a three-dimensional finite element solution. A substantial reduction in computational effort is realized for the generalized plane strain finite element, as compared to the three-dimensional finite element. This is achieved without sacrificing the accuracy and the ability to account for the three-dimensional material anisotropy and far field stress components.

Behavior of open steel grid decks for bridges

Abstract The life cycle of grid decks has come full circle from their introduction in the 1920s and 1930s, through their maturity in 1950s and 1960s, to their reintroduction in the 1980s. Many of these decks have been performing satisfactorily for 50 or more years of service. Open grid decks offer a lightweight deck alternative to reinforced concrete decks. Despite the good performance history of grid decks, some bridge owners are hesitant to utilize them, even in situations where weight savings is at a high premium. With a better understanding of grid deck behavior, the manufacturing process can be optimized, and design methods improved. Hence, poor details that may lead to fatigue problems can be avoided and design efficiency can be achieved. This paper presents results of research conducted with the goal of providing a better understanding of open steel grid deck behavior through experimental testing and numerical and analytical analyses. Four full-scale open grid decks were tested to experimentally quantify their structural behavior. Three-dimensional finite element models were developed for the grid decks and calibrated using the experimental results. Classic orthotropic thin plate theory and the theory for beams on elastic foundation were applied to the open decks and compared with the finite element (FE) results. Finally, parametric studies were conducted and used to quantify the effect of variations in the significant design parameters. The results of the parametric studies can be applied to optimize future grid deck designs.

Column Supported Embankments with Geosynthetic Encased Columns: Validity of the Unit Cell Concept

Column supported embankments (CSEs) are used to overcome common problems associated with the construction of embankments over soft compressible soils. The use of granular columns as deep foundation elements for CSEs can be problematic in soft soils due to the lack of adequate lateral confining pressure, particularly in the upper portion of the column. Using a high-strength geosynthetic for granular column confinement forms geosynthetic encased columns (GECs); the confinement imposed by the geosynthetic increases the strength of the column, and also prevents its lateral displacement into the soft surrounding soil. This paper presents the results of finite element analyses of a hypothetical geosynthetic reinforced column supported embankment (GRCSE) (i.e., a CSE underlain by geosynthetic reinforcement) that is constructed with GECs as the deep foundation elements. Full three-dimensional (3-d), 3-d unit cell, and axisymmetric unit cell analyses of the GRCSE were carried out to investigate the validity of the unit cell concept. The effect of the degree of nodal constraint along the bottom boundary when numerically modeling GRCSEs was also studied in this paper. Numerical results show that a full 3-d idealization is required to more precisely determine the tension forces that are produced in the geosynthetic reinforcement that underlies the GRCSE. A number of design parameters such as the average vertical stresses carried by the GECs, lateral displacement of the GECs, and the maximum settlement of the soft foundation soil, however, can be successfully calculated using unit cell analyses.

Numerical Study of Effect of Encasement on Stone Column Performance

Encasing a stone column with a high-strength geosynthetic provides the column material with significant lateral confinement, which prevents lateral displacement of the column into potentially soft surrounding soil and consequently increases the bearing capacity of the column. Although this technique has been successfully applied in practice, the load transfer mechanism of encased stone columns and their performance in comparison with conventional stone columns have not been studied in detail. This paper describes three-dimensional finite element analyses that were carried out to simulate the behavior of a single stone column with and without encasement in a very soft clay soil using the computer program ABAQUS. A comprehensive study was performed to better understand the mechanism of load transfer in conventional stone columns and geosynthetic encased stone columns. The performance of partially encased columns was then compared to that of fully encased columns and conventional stone columns.

Stress-Strain and Strength Characteristics of Silt-Clay Transition Soils

AbstractThe influence of nonplastic silt content on the stress-strain, volume change, and strength characteristics of anisotropic silt-clay transition soils is investigated. A series of one-dimensional compression tests, isotropic compression tests, and undrained and drained triaxial tests on three soils of similar base clay but different nonplastic silt contents are performed on cross-anisotropic specimens prepared from consolidation of slurries. Tests with major loading directions both parallel and perpendicular to the specimens’ axis of deposition are performed. It is observed that the tested silt-clay soils are less compressible with increasing silt content during one-dimensional and isotropic compression tests. During drained and undrained triaxial tests, the normally consolidated soils at the same consolidation stress show larger values of stiffness and drained and undrained shear strengths with increasing silt content. The silt-clay soils also show larger pore pressure changes during undrained tria...

Numerical Simulation of Column Supported Embankments with Geosynthetic Encased Columns: Influence of Soft Soil Constitutive Model

This paper describes the results of three-dimensional finite element analyses that were carried out using the commercial computer program ABAQUS to simulate the behavior of column supported embankments (CSEs) that utilize geosynthetic encased columns (GECs) as the deep foundation elements. To examine the influence of the soft soil constitutive model on the numerical results, analyses were performed using modified Cam Clay, Mohr Coulomb, and linear elastic models for the soft soil. Existing laboratory results for soft Bangkok clay were used to calibrate the model parameters that were used in this paper. Numerical results demonstrate the influence of the soft clay constitutive model on the stresses that are transferred to the GECs, the values of maximum settlement, the lateral displacement of the GECs, and the hoop forces in the geosynthetic encasement.