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MECHANICS OF CRACK PROPAGATION IN CLAYS UNDER DYNAMIC LOADING

by Vesga, Luis Fernando

Abstract (Summary)
Over-consolidated clays and shales forming part of the core section of earth dams, natural slopes, and clay deposits undergoing desiccation, can be subjected to dynamic loads from earthquakes, waves, traffic load, or machine vibrations. This research asks what happens to fissured clays under dynamic loading?. Static and dynamic compression laboratory tests were done on fissured kaolinite clay specimens. The samples were subjected to uniaxial, biaxial and triaxial stress conditions. Crack propagation was analyzed using both Linear Elastic Fracture Mechanics and Discrete Element and Finite Difference methods. Crack propagation and failure in the samples was found to vary depending on the state of water distribution in the clay pores: the saturated-funicular state, the complete pendular state, or the partial pendular state. Finding that accepted theories of unsaturated soil mechanics dont apply in latter two states, a new approach using the equivalent effective stress concept is presented and then be applied to all three states, showing that the strength of the clay is proportional to the equivalent effective stress in the soil. Since tensile stresses at the tips of a crack produce additional propagation, a new apparatus was developed to study the strength of clays under tension, and it was found that the fissured clay failed specifically were shear and tensile stresses occurred within a region of high tension. Also, subsequent dynamic compression tests revealed the crack stability thresholds (the compressive stress below which no crack propagation takes place in a fissured clay). The threshold appeared: almost constant in the complete and partial pendular states, but greatly diminished in the funicular-saturated state as water content increased. In the saturated-funicular state, the water flowed around the crack tip from zones receiving high compressive stress, to zones with highly-concentrated tensile stress. The higher the water content of the clay, the lower the tensile strength.
Bibliographical Information:

Advisor:Ian Nettleship; Julie M. Vandenbossche; Jeen-Shang Lin; Luis E. Vallejo; Rafael Quimpo

School:University of Pittsburgh

School Location:USA - Pennsylvania

Source Type:Master's Thesis

Keywords:civil and environmental engineering

ISBN:

Date of Publication:11/09/2005

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