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A cube of Barre Granite (approximately 22 cm on a side), free of boundary loads, was sequentially and concentrically drilled out with 5.1-, 10.1-, and 15.0-cm core bits on the top and bottom sides and with a 10.1-cm core bit on a third side. The top and bottom sides are parallel to the grain or lift plane, which is horizontal in the quarry, and the third side is parallel to the rift plane, which is vertical and strikes N. 30° E. Strains resulting from overcoring were recorded on these sides with thirty-six 45°-rosette resistance strain gages. The strain changes observed upon coring seem to have been derived from a complexly changing internal residual stress field. Initially, as the top surface was overcored, the resulting strain changes were mostly compressional; that is, the block contracted. Strain changes became more extensional as the bottom of the block was overcored. Finally, there were large net strain changes that were radially compressional and tangentially extensional, relative to each set of concentric overcores. The largest strain changes were observed adjacent to freshly cut surfaces, yet significant changes were monitored over the entire surface of the block after each overcoring. Cumulative strain changes monitored on opposing gages symmetrically located across a freshly cut surface were, much of the time, similar in sense, magnitude, and direction. The residual stress field is related to the changing geometry of the body, the induced free surfaces, and seemingly to the degree of homogeneity, orientations, and magnitudes of the stresses when they were frozen in. Cumulative strain changes at individual gage locations were large as much as 330 X 10~ 6 (extension) and 275 X 10" 6 (compression). The deformation generally appears to be elastic. At certain gages, however, there were occasional large anomalous strain changes that are interpreted to result from permanent deformations, such as intergranular movements or intragranular gliding. These mechanisms may also be partly responsible for a pronounced creep event that was observed after the top 15.0-cm coring was completed; the whole block, which had initially contracted, expanded for 48 hours after the coring event, until the net strain change was nearly zero. The extensional deformations typically occurring across a pilot borehole as a result of stress-relief overcoring techniques can be caused by at least two possible stress conditions an externally applied compressional stress field or the mobilized tensile component of the internal or residual stress field. When externally applied compressive forces are cut by overcoring, the innner and outer walls of the newly created annulus will expand. An internally balanced stress field may be partially relieved upon cutting of the outer cylindrical surface of the newly created annulus. If the tensile energy is relieved locally, then compressive strains are mobilized, causing the annulus to become thinner. The interior wall thus expands, and the exterior wall contracts. A deformation-measuring device placed in the interior hole measures extension of the interior wall in both stress conditions considered. INTRODUCTION With any deformation of a natural rock mass, whether manmade or natural, there is an accompanying disturbance of the preexisting quasistatic elastic stress field. The stresses, whether residual, gravitational, or presentday tectonic, are at least momentarily unbalanced, and energy is expended in some type of work or heat transfer. A better understanding of how released residual-elasticstrain energy is expressed at free boundaries is needed. Systematic quantitative knowledge of this phenomenon would be of great practical value for the design of mines, quarries, and engineering structures. Also, tl is knowledge may well be of value to earthquake prediction. I believe that the source of a significant quantity of the total energy released has been residual strain (stress). If the release of these stresses can be predicted quantitatively in terms of a stress and (or) strain tensor, the information will be valuable in design and contrw tion practices. In the present research, a systematic investigation of residual-strain relief was conducted to determine the magnitudes and distribution of the resulting strain changes as the geometry of a block of rock was modified and to discover the relation of these strains to fabric elements. "Prestress," "residual stress," "prestrain," "residual strain," and other uncommon terms used in this text ere defined in the glossary. This report is based on a thesis study conducted at Texas A & M University in partial fulfillment of the requirements for the Master of Science degree.

Deformations associated with relaxation of residual stresses in a sample of Barre Granite from Vermont