Stress Behavior Of Rock Under Drilling Loading Conditions
Authors: Somerton W.H. , Timur A. , Gray D.H. 
Published: December 2007

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Abstract
Rocks are shown to have non-linear stress-strain behavior and the behavior is dependent upon the method of loading and the past stress history of the rock. In cyclic loading tests, permanent strain was seen to increase considerably over non-cyclic loading, and there is some indication that the strength of rocks is increased by cyclic loading.
Conventional stress analysis methods were considered to be of limited value in studying stress distributions in rocks. Consequently the photostress technique was employed to study the behavior of a variety of rocks under several conditions of load. In this method strains induced in a rock sample are transmitted to a thin sheet of photostress plastic cemented to the face of the sample.
Qualitative results were obtained by the photostress method showing fringe pattern development which could be correlated with the measured stress-strain properties of the rock. Quantitative analysis showed, however, that the photostress plastic was not following strains in the rock. This was confirmed by strain-gauge measurements on the same rock sample.
The photostress method offers good possibilities for obtaining detailed stress analysis of rocks, but plastic with a higher strain-optic constant will have to be developed.

Introduction
Comparatively little is known regarding the basic mechanism of rock breakage by conventional drilling methods. Most of the past work done on the failure of rocks under loading conditions simulating rock drilling has been based on controlled laboratory drilling tests, laboratory percussion studies or theoretical analyses assuming ideal elastic or plastic behavior of rocks. The actual stress conditions existing within the rock during drilling need to be known to permit more meaningful analysis.
The system under study is extremely complex and defies analysis by conventional techniques. Consider, for example, the complex geometry of a conventional rotary rock bit. The bit consists of wedge-shaped teeth mounted on rotating cones or rollers which repeatedly contact the rock. In the case of hard rock bits, a true rolling action is approached, and the result is repeated vertical impression of the teeth into the rock face at the bottom of the hole. In drag bits, the cutting is done by scraping or gouging and the action is close to that of the machining of metals. In non-true rolling bits (the type usually used in rotary drilling), the drilling action is a combination of vertical impressions of the teeth into the rock and gouging or shearing action.