Date of Award

Spring 1996

Document Type


Degree Name

Doctor of Philosophy (PhD)


Civil/Environmental Engineering

Committee Director

Isao Ishibashi

Committee Member

A. Osman Akan

Committee Member

Duc T. Nguyen

Committee Member

Yozo Mikata


The failure phenomena in earth structures such as slopes and embankments on clay foundations are a consequence of the formation of failure surfaces.

A finite difference computer code, FLAC (Itasca Consulting Group, 1993), with the isotropic Mohr-Coulomb plasticity model, is used to analyze the undrained progressive failure of a strain softening soil during a slope excavation process. The focus of the study is on the short-term stability of slopes. In the first series of analyses, which are aimed at the evaluation of anisotropic strength effect on slope stability, the isotropic Mohr-Coulomb plasticity model is slightly modified to implement the anisotropic nature of undrained strength.

In the second series of analyses, which are aimed at the evaluation of progressive failure effect on slope stability, the stress-strain-strength relations determined based on existing laboratory data from the literature are closely curve-fitted as a function of the plastic shear strain. In order to define the stress-strain-strength relations for the entire elements with varying i angles, the typical laboratory soil testings are considered as variations of a testing method in which i angles at the incipient failure differ. In addition, the resulting stress-strain-strength relations are assumed to systematically vary in their patterns. Consequently, for a given i angle distribution in the slope, individual element has its own unique stress-strain-strength relations such that the relative pattern of stress-strain relations for varying stress paths are explicitly implemented to evaluate the progressive failure effect.

Since the first series of analyses are focused on the anisotropic nature of undrained strength alone, while the second ones are concerned about the anisotropic stress-strain-strength relations, the difference in their results is attributed to the effect of progressive failure. It was explicitly shown that the anisotropic stress-strain-strength relations are always accompanied by the sequential mobilization of peak strengths of the elements along the failure surface. Consequently, it was confirmed that the summation of the peak strengths mobilized along the failure surface is significantly less than that of the isotropic compression strengths. However, the strength reduction due to the anisotropic stress-strain relations (progressive failure) was found to be not as critical as that caused by the anisotropic undrained strength. In addition, several anisotropic strength variations along with varying degree of strain softenings are parametrically implemented. It was found that the effect of strength anisotropy is more critical than that of strain softening. Based on this research, the strain compatibility technique in the slope stability analysis proposed by Koutsoftas and Ladd (1985) is critically reassessed.