Micromechanics of steady state, collapse and stress-strain modeling of soils

Balasingam Muhunthan, Purdue University

Abstract

Increased evidence of the influence of their discrete nature on the observed behavior of soils has prompted a number of investigators to look at these materials at the microstructural level of these materials. This study follows along this line of approach. The soil material behavior and response is considered herein as the resultant of the behavior of microsystems. It is shown that a comprehensive description of particulate media can be achieved via two measures: a scalar measure giving the mean value of a fabric descriptor, and a tensor measure giving its distribution. The development allows the fabric descriptors based on the solid phase or the void phase to be treated in a unified manner. The description based on void phase is shown to be more powerful because its parameters are easier to determine experimentally. A technique based on an extension to the classical stereological principle is presented to determine the parameters from observations made with an image analyzer or scanning electron microscope. Examples illustrating its use in soil mechanics are highlighted. A simplified fabric parameter derived from the tensor measure is shown to be powerful in studying the evolution of fabric of soils under shear deformation. Based on the evolution of this fabric parameter, it is proposed that irrespective of their insitu nature of fabric, soils do tend to attain an ultimate structural arrangement specific to an imposed shear loading path. These developments on the mathematical characterization of fabric are used to evaluate some important concepts in soil mechanics in detail, including the question of the uniqueness of the critical state line and the state boundary surface. A generalized concept of an ultimate state surface for soils is proposed. It is shown that this surface reduces to the classical critical state line when fabric effects are neglected. Finally, the application of thermomechanics as an alternative to plasticity theory in developing simple constitutive models for soils is discussed. This theory allows the systematic incorporation of fabric concept and leads to the development of a simple stress-strain model for soils. The new model is a simple extension to the Modified Cam-Clay model with the consideration of fabric change. For simplicity in presentation the model is developed for axi-symmetric conditions. It could however be extended to a general three-dimensional space and incorporated in numerical codes.

Degree

Ph.D.

Advisors

Chameau, Purdue University.

Subject Area

Civil engineering

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