INTRODUCTION

Numerous studies have been done relating to long-wall and shortwall mining subsidence, but very little criteria is available on subsidence pertaining to room and pillar mining, which is the common mining practice in the United States. Field study data on subsidence resulting from room-and-pillar mining is difficult to obtain because complicated geometric and physical parameters need be employed and long term monitoring is required.

In recent years, a new method of model simulation of subsidence has been developed (1). The technique of holographic interferometry has been successfully applied to a self-loading, simple-cavity model simulating subsidence resulting from longwall mining. Holographic interferometry has the capability of detecting full-field movement on the surface of any material, even granular material, by means of generating displacement contour lines with a separation increment of less than 13 microinches.

A similar method has been adopted for analyzing subsidence above room-and-pillar mines. This paper presents the modeling method and analysis of the results taken from that model.

HOLOGRAPHIC INTERFEROMETRY

In 1948, Gabor invented holography, which is the technique of reviving three demensional images using a monochromatic light source. A full demonstration was not made then because a clean monochromatic light source was not available. Later, in the early 1960's, the laser was adopted as a monochromatic and coherent light source. Since then, applications have been made in many different fields ranging from crime prevention to three dimensional television.

In this technique, an image is usually recorded by means of constructing a hologram which is a record of the interference pattern formed on a high resolution photographic system as shown in Figure 1

.

A beam splitter divides the laser beam into two beams: (1) an object beam which is expanded while passing through a beam expander and illuminates the object, and (2) a reference beam which is expanded and illuminates the holographic plate. Subsequent to an exposure, the holographic plate is developed and illuminated by the expanded reference beam. The original scene of the object is revived in a three-dimensional image.

The wave fields reflected by the object in two slightly different configurations can be superimposed to form an interference fringe patten1. Thus, it is possible to measure the displacement that took place between the two configurations. This method is called holographic interferometry. There are two methods of achieving this phenomena. The first is the double exposure method, wherein two exposures of the object are made prior to developing the film. The first exposure represents the original condition and the second, the disturbed condition. The second method is the real time holographic method. In this case, only the first exposure is made of the initial configuration and developed in place, or later elsewhere, and repositioned in the original location.

When the object is viewed through this hologram, the original image is revived by means of the hologram and subsequent movement of the model is detected by viewing interference fringe lines. In both cases the surface deflection between the adjacent two fringes can be calculated.

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