Date of Award

Spring 1994

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

First Advisor

Pedro DeAlba

Abstract

The shear modulus is a direct indicator of sand fabric stiffness. It is a primary soil parameter affecting the response of sand deposits to cyclic shearing stresses and strains. Among the various factors which influence the shear modulus, the age at testing has been least studied. Typically, age effects have been thought of only in terms of geologic time. Recently, however, observations indicate that age effects operate over much shorter time spans (weeks to months), and that these age effects may be reversible; i.e. the sand fabric becomes stiffer as it evolves with time, but then the acquired stiffness may be reduced due to subsequent disturbance. The purpose of this research was therefore to develop instrumentation to quantify age effects in the laboratory.

A system was designed in which two types of measurements could be performed on sand specimens. Acoustical measurements were taken to quantify the rate of sand fabric evolution, and mechanical measurements were performed to compare the shear modulus vs. strain relationships of freshly formed and aged specimens, to assess the effects of disturbance upon the sand fabric, and to determine threshold shear strain levels for various specimens. The system included a special soil chamber, shear wave transducers to obtain acoustical measurements, and a direct simple shear (DSS) machine to perform mechanical measurements.

Based upon the limited data obtained to date, definitive statements cannot be made regarding the rate of sand fabric evolution, or the absolute magnitude of the increase in shear modulus in specimens subjected to seven to eight week aging periods. It has been found, however, that disturbance does reduce the shear modulus of aged specimens, with the magnitude of the reduction in shear modulus being highly dependent upon the shear strain level at testing. After disturbance, modest increases in shear modulus are observed in the specimens after 24-hour aging periods.

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