A Laboratory Study Of The Fracturing Of Rocks By Hydraulic Pressure

As with most rheologically defined materials, rocks react differently in different environments to similar forces. Physical changes and deformations depend as much on the rate of change of stress as the magnitude of stress. In many instances the rate of change is more important. Under load, all real materials will flow if given time. If there is no time for flow, materials tend to react in a brittle manner. Particular characteristics are developed in rocks during their periods of formation and postformative geologic history. Whatever their mode of occurrence, rocks are a record of their formation and changes. Where analyses are possible, advantage should be taken of the natural inherent characteristics to help design economical functional structures. A number of techniques of measurement and analysis have been developed. The fracturing of rock by hydraulic pressure has been examined by many authorities but few within the scope of available published literature have used the actual rocks concerned in tests to examine directional characteristics or preferred strain propagation rates and directions. In a number of investigations of the properties of rocks for mine design purposes and stress analysis problems in which the writer has been associated, it has been demonstrated that there are definable preferred shear planes in most rocks. Preferred shear planes can be identified by a photoelastic method of analysis, and the information can be used to improve design of optimum drilling and breaking directions for mining purposes. Definable principal strain directions can be determined to facilitate design directions of optimum stability for permanent mine openings. In field fracturing operations the same directional characteristics as outlined above should be used to define preferred fracturing directions as in the laboratory from oriented samples. It is mainly with this aim that