Dissertation/Thesis Abstract

Hypoplastic Constitutive Law Adapted to Simulate Excavations in Chicago Glacial Clays
by Sarabia, Fernando, Ph.D., Northwestern University, 2012, 251; 3508541
Abstract (Summary)

Economic considerations drive the more effective use of space in urban areas, promoting the construction of taller buildings with deeper basement structures. The crowded nature of urban environments imposes strict restrictions to the tolerable performance of these new constructions.

These restrictions are translated in the need for the development of more precise tools that can be used by engineering practitioners to predict construction induced deformations. Geotechnical finite element simulations are a common technique to estimate construction performance. This methodology can be enhanced by the use of optimization routines to calibrate the constitutive model parameters with existing data.

This thesis introduces enhancements to existing techniques that improve the agreement between model predictions and both, laboratory test data and field performance observations. Specifically, the finite element simulation strategy adopted in this research incorporated the use of an advanced soil model that is conceptually capable of capturing the nonlinear nature of soil stiffness from the very small to large strain levels.

The techniques incorporated in this research were developed to be used for the prediction of deformations of deep excavations in the Chicago area. The parameters of the selected model were either directly estimated from existing laboratory test data or computed using optimization techniques that used sophisticated triaxial test results as target observations. The procedures used to estimate the model parameters incorporated techniques to account for natural variation in degree of soil plasticity, density and degree of overconsolidation of the different Chicago glacial till layers.

The validity of the estimated parameters is evaluated by simulating a top-down excavation performed at the One Museum Park West project in Chicago and comparing the model predictions with the collected performance monitoring data. The project site used as a test bed for this research was heavily instrumented during construction with inclinometers, optical survey points and strain gauges. The collected information was compared to model predictions at a section along the north side of the excavation. This research also developed finite element modelling strategies to incorporate the nonlinear behavior of the structural elements in a basement construction; specifically, the creep and shrinkage effects of the supporting slabs in top-down excavation and the nonlinear bending stiffness of the retaining walls.

The results of this comparison showed reasonable agreement between the computed and observed lateral wall movements at all stages of the excavation. The lack of agreement between the computed and measured ground surface settlements is attributed to the heterogeneous nature of a surficial granular fill encountered at the site.

The relative importance of utilizing a soil model able to capture small strain stiffness nonlinearity, slab creep and shrinkage effects and wall nonlinear bending stiffness was demonstrated by performing parametric evaluations with and without incorporating these effects into the finite element model and comparing the results with the observed performance. Specifically, these parametric evaluations showed that when at the OMPW site small strain stiffness nonlinearity is ignored, the computed ground movements in the hard glacial layers next to the toe of the wall are overpredicted; if the time dependant slab deformations (i.e., creep and shrinkage) are ignored, the lateral wall movements are underpredicted; and not accounting for the nonlinear bending stiffness of the retaining does not significantly change the computed ground movements.

Indexing (document details)
Advisor: Finno, Richard J.
Commitee: Buscarnera, Giuseppe, Dowding, Charles H.
School: Northwestern University
Department: Civil and Environmental Engineering
School Location: United States -- Illinois
Source: DAI-B 73/09(E), Dissertation Abstracts International
Subjects: Civil engineering
Keywords: Chicago clays, Excavations, Finite element analysis, Hypoplastic constitutive law, Hypoplasticity
Publication Number: 3508541
ISBN: 9781267342720
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