On Photoelastic Techniques, in Situ Stress and Strain Measurement, and the Field Geologist

Abstract

Geologists should be aware of field applications of photoelastic techniques for measuring in situ stress and strain in rocks. The biaxial strain gage may be the most flexible of techniques now available for field utilization. This device simply entails a flat disk with a centered hole that acts as a stress raiser. It can be attached around its periphery to exposed surfaces of rock or to the back of boreholes. Overcoring releases in situ strain; the strain within the gage area is integrated to produce a symmetrical isochromatic figure pattern when viewed under polarized light. The symmetry of the pattern identifies the principal directions of stress and strain. The type of pattern identifies the principal strain ratio. Both kinds of information are obtained with a single measurement, and with repeated measurements the distribution and character of the stress field can be deduced. A specialist's knowledge of the principles of photoelasticity is not required in order to interpret the gages. A uniaxial gage...