FINITE ELEMENT ANALYSIS OF THE TIME-DEPENDENT BEHAVIOR OF SOFT CLAYS

A major consideration in the design and analysis of structures founded on soft clays is the foundation deformation that develops with time. This deformation is due to both consolidation and creep effects. At the present time, numerical analyses of time-dependent foundation deformation are restricted to either consolidation effects or undrained creep individually, without considering the combined effect of both. The objective of this thesis is to develop a numerical method that accounts for both consolidation and creep effects.;The time-independent constitutive model used in SPIN 2D is a modified Cam clay elasto-plastic strain hardening model. The yield surface is a family of ellipsoids, which also serves as the plastic potential by the associative flow rule. Creep contributions are incorporated using the associative flow rule on the same yield surface as in the time-independent model.;To investigate the validity of the constitutive model, the program was used to perform parametric studies on San Francisco Bay Mud. Soil properties and test results were taken from published information on Bay Mud. The numerical experiments included drained, undrained, and consolidation tests, creep and stress relaxation tests, and combined stress relaxation and creep tests in 'triaxial' and plane strain stress conditions. Comparison of the numerical results with the test results showed excellent agreement and affirmed the validity of the constitutive model.;To further verify the model, SPIN 2D was used to predict the long-term field behavior of the I-95 test embankment founded on a Boston Blue Clay deposit. Results of the numerical predictions showed that creep-induced deformations constitute a major fraction of the overall deformation in this problem.;To provide solutions to boundary-value problems of axisymmetric (torsionless) and plane strain configurations, a finite element program SPIN 2D was developed. SPIN 2D is a quasi-static program that has the capability to solve combined consolidation and creep problems by mixed and penalty methods and to treat incompressibility by selective reduced integration. This program requires soil parameters that can easily be obtained from conventional laboratory tests.;Recommendations are made for further improvement of the constitutive model.