ABSTRACT

ABSTRACT

Numerical modeling techniques such as finite element models have proven to be very effective in the analysis of geo-mechanical problems if realistic data about the stress-strain characteristics of each structural element is provided. But the development of the finite element method into an acceptable design code has been hampered by uncertainties in extra-polating laboratory measured data to field conditions. Holographic interferometry has been applied to solid mechanics in measuring the displacement and the deformation of diffusely reflecting objects. The surface of the objects does not need to be smooth or coated with photoelastic material. The high sensitivity and the full-field view of the method are both desired for sophisticated measurement. Models simulating the components of geological structures created by underground mining activity were considered. Using holographic interferometry and finite element technique the interactions between roof-pillar-floor were examined in order to understand the behaviors of in-situ, layered rock strata. Different types of layered models of cylindrical shape were studied. Comparison of the results obtained from holographic and finite element methods were made. The results show that holographic interferometry is not only useful in analysis of interaction of layered discontinuous structures but it can also be utilized as a method of providing realistic input data and mechanical properties for analytical models without using the standard tests.

INTRODUCTION

Holographic interferometry is a technique of measuring surface displacement. It is a photographic technique which needs no surface instrumentation, but measures displacement by superimposing a picture (hologram) of the surface in a disturbed condition over a picture of this surface in a undisturbed condition thus creating an interference pattern which describes the way that the surface has moved. This pattern is made up of contour lines of equal displacement. Holographic interferometry, holometry, is capable of measuring movements remotely as small as 12 micro inches (305 x 10-6 mm) on the surface of any material including granular materials [1]. This is a very attractive feature which makes physical modeling more feasible since no instrumentation is required. By observation of the fringes, not only can the amount of displacement be determined but also the fringes describe how a load disturbs the surface, specifically, uniformly, or non-uniformly or the way adjacent layers of strata with different stiffness interact, which demonstrates that this technique can be very valuable in stress and strain analysis. Holometry was discovered quite by accident when two University of Michigan holographers noticed that a pattern of lines appeared across the surface of a model that accidently moved while being hologramed [2]. This technique, which requires a hologram or pictures made with a monochromatic light source became practical only after the intervention of the laser in the early 1960's, and already has a widespread acceptance in the various sciences. The primary acceptance has been in quality control where construction irregularities can be detected without the need to test the material to destruction. For example, a tire can be holographed at two different pressures and the resultant hologram would show if the tire expanded uniformly [2]. It was not long before investigators were able to determine the surface deflection required to generate a fringe. Wilson et al [3] have shown that the displacement is given by the relationship (mathematical equation)(available in full paper) where D is displacement between fringes, ¿ is the incident angle of the laser light on the model and ¿ is the wa

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