In order that the reader may not be misled, I feel it necessary to comment upon the title of this paper. "Laws of Rock Behavior," one may take as being part of the all-embracing "Law of Nature" and this, according to the Oxford Dictionary, relates to the principles of conduct recognized as being pleasing to God. I feel it would be presumptuous to erect a code of rock behavior on this basis, so turn to a secondary definition, namely that a "Law" is that which is intrinsically reasonable. It is extremely unlikely that any group of geologists, or workers in rock mechanics would be in complete agreement about what could be taken to be "intrinsically reasonable." Therefore, this paper does not present any deified utterances, but rather deals with a few aspects of rock behavior which I consider to be intrinsically reasonable and with which I hope the reader will concur.
The mode of behavior of rocks in the Earth's crust covers the range from wholly brittle to completely ductile; with a deformation time scale which may be measured in milliseconds, for some events such as meteoritic impact, or in millions of years for the tectonic deformation involved in mountain building. A complete survey is, of course, outside the scope of an introductory paper, so I must be both brief and selective in my treatment.
Any attempt to establish the mode of rock behavior in the Earth's crust should take into account data and relationships established in laboratory experiments and correlate these with field evidence. Consequently, the first section of this paper presents a brief discussion of the experimental aspects of rock mechanics which are pertinent to the subsequent discussion.
As the brittle behavior of rock is of interest to the engineer as well as to the geologist, this aspect of rock mechanics has been widely studied. Indeed, the volume of work is considerable, and one can demonstrate the complexity of "natural laws" merely by tracing the development of thought regarding brittle fracture during the past quarter century.
Only a couple of decades ago, most geologists were content with the Coulomb criterion of brittle failure for rocks which exhibited a linear relationship between principal stresses at failure and the Mohr concept of failure for those with a nonlinear relationship. Only a few were aware of the Griffith theory of failure. However, as interest in the study of rock failure increased, this theory became more popular and after modification1, the theory was held, for a short time, to predict accurately the breaking strength of brittle rock. However, from experiments in which measurements of both lateral and longitudinal strain in the specimen were determined, coupled at the same time with acoustic and pulse velocity studies2,3 it soon became apparent that a rock subjected to compression began to "crack-up" at about one-half its ultimate short-term failure load. Moreover, it had become apparent that the data obtained for a single rock type tested 1) in compression, 2) extension, and 3) torsion could not be compared in a direct and simple manner.4
