Navegando por Autor "Zornberg, Jorge Gabriel"

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Coupled analyses of excavations in saturated soil.
(2009) Nogueira, Christianne de Lyra; Azevedo, Roberto Francisco de; Zornberg, Jorge Gabriel
This paper presents finite-element analyses of excavations by using a coupled deformation and flow formulation. Specific numerical procedures were implemented into the finite-element codes to simulate the excavation construction and to solve the nonlinear coupled system. The paper discusses results of two generic excavations, with analyses conducted using different constitutive models and different excavation rates. Although the constitutive model affected the magnitude and distribution of the excess of the pore-water pressure due to the excavation process, the constitutive models only slightly influenced the dissipation rate of the excess pore-water pressure. Excavation rates that were one order of magnitude smaller than the hydraulic conductivity of the soil led to results representative of drained processes. Because of the slow rate needed for drained conditions, partially drained conditions normally prevail during excavations, highlighting the importance of coupled analyses.
FE prediction of bearing capacity over reinforced soil.
(2007) Nogueira, Christianne de Lyra; Oliveira, Rodrigo Rodrigues Vieira de; Araújo, Luiz Gonzaga de; Faria, Patrícia de Oliveira; Zornberg, Jorge Gabriel
This paper presents the numerical simulation using an elastoplastic analysis of the bearing capacity of shallow foundations. The problem involves axisymmetric conditions on reinforced soil using finite element method (FEM). The foundation soil is modeled as a nonassociative elastoplastic Mohr-Coulomb material. The reinforcement is modeled as a linear elastic material. The ultimate bearing capacity obtained in this study is compared to solutions obtained using limit equilibrium and limit analysis. A parametric study was conducted for different configurations of reinforcement for a special case of frictionless foundation soil. The numerical results show good agreement with analytical results indicating the suitability of the numerical model used in this study and implemented into the code ANLOG - Non-Linear Analysis of Geotechnical Problems.
Formation of secondary containment systems using permeation of colloidal silica.
(2011) McCartney, John S.; Nogueira, Christianne de Lyra; Homes, Daniel; Zornberg, Jorge Gabriel
U.S. Environmental Protection Agency (USEPA) regulations require the capture of spills from liquid tanks containing hazardous chemicals by using a secondary containment system. Compacted clay or geomembrane liners are commonly used in secondary containment systems, but they are cumbersome when used in conjunction with existing liquid tanks because of pipeline networks surrounding the tanks. This study evaluates the formation of hydraulic barriers for secondary containment through the permeation of colloidal silica grout. A simplified infiltration model is presented to predict the downward movement of the colloidal silica grout into a soil layer, considering the timedependent increase in dynamic viscosity of the colloidal silica for different concentrations of an electrolyte accelerator. Because the simplified infiltration model cannot predict the soil-grout interaction or the permeation of the colloidal silica by fingering, its results were calibrated by using the observations from a large-scale column test involving the permeation of colloidal silica into sand. The predicted position of the wetting front was found to match that of the experiment when the parameter governing the change in viscosity of the colloidal silica was increased by a factor of 30. The infiltration model calibrated with observations from column infiltration experiments provides a simple approach to the design of the secondary containment systems using permeation of colloidal silica.
Isotropic work softening model for frictional geomaterials : development based on lade and kim constitutive soil model.
(2008) Yang, Kuo Hsin; Nogueira, Christianne de Lyra; Zornberg, Jorge Gabriel
An isotropic softening model for predicting the post-peak behavior of frictional geomaterials is presented. The proposed softening model is a function of plastic work which can include all possible stress-strain combinations. The development of softening model is based on the Lade and Kim constitutive soil model but improves previous work by characterizing the size of decaying yield surface more realistically by assuming an inverse sigmoid function. Compared to original softening model using the exponential decay function, the benefits of using the inverse sigmoid function are highlighted as: (1) provide a smoother transition from hardening to softening occurring at the peak strength point, and (2) limit the decrease of yield surface at a residual yield surface, which is a minimum size of yield surface during softening. The proposed softening model requires three parameters; each parameter has it own physical meaning and can be easily calibrated by a triaxial compression test. Data from triaxial compression testing on Monterey No. 30 sand is applied to demonstrate the calibration procedure and examine the variation of model parameters with different loading conditions. Results show all parameters are highly correlated to confining pressures. The proposed softening model can provide a useful tool for evaluating those structures on which the post-peak behavior of frictional materials should be emphasized, e.g. earth structures under large loading or deformation conditions or the structures having an intensive soilstructures interaction, etc.
Moisture migration in geogrid reinforced expansive subgrades.
(2008) Gupta, R.; McCartney, John S.; Nogueira, Christianne de Lyra; Zornberg, Jorge Gabriel
Highways in Texas, Arkansas, Colorado, Wyoming and other parts of the United States are often constructed atop expansive clay subgrades. Considerable damage to flexible pavements has been observed in these areas in the form of longitudinal cracking. A geogrid placed between the subgrade and base layers has been used successfully in Texas as a stabilization alternative to prevent longitudinal cracking, although the mechanism of geogrid reinforcement is not well understood. This study involves measurement of the time variation in water content of an expansive clay subgrade beneath a flexible pavement to investigate if differential volume changes in the subgrade are a cause of longitudinal cracking. Two years of moisture monitoring and visual observations indicate that significant moisture fluctuations occur in the clay subgrade under the unpaved shoulder of the road, while negligible moisture fluctuations occur in the clay subgrade under the pavement. This contrast in water content changes between the shoulder and pavement indicates that both bending and stretching of the subgrade are probable causes of longitudinal cracks.
Validation of coupled simulation of excavations in saturated clay : Camboinhas case history.
(2011) Nogueira, Christianne de Lyra; Azevedo, Roberto Francisco de; Zornberg, Jorge Gabriel
This paper presents the results of an elastoplastic finite-element back analysis of an excavation made on a saturated soft organic clay deposit in Rio de Janeiro, Brazil. The excavation was conducted as part of an urbanization program, in which artificial islands were to be created in a swamp along the margins of the Camboinhas Lagoon. An extensive laboratory testing program was performed on undisturbed soil samples to characterize the stress-strain-strength behavior of the involved materials. Results from this laboratory testing program were used to calibrate a nonassociated elastoplastic constitutive model implemented in the ANLOG (Nonlinear Analysis of Geotechnical Problems) code. This code is based on the finite-element method and is capable of conducting fully coupled analyses by using a variety of constitutive models. Coupled analyses were performed to simulate the Camboinhas excavation, which was conducted under plane strain condition. Field measurements of both displacements and pore-water pressures obtained during the excavation are compared with the results of the numerical simulation. Analyses of the results show that monitored in situ and numerical results are in good agreement.