Recent studies of inelastic behavior and acoustic emission in geologic materials further validate the concept that such materials may be considered to consist of an elastic media containing an array of pores and cracks. In the present paper this concept is investigated further by experiments in which the mechanical behavior of a number of geologic materials were studied in detail at low stress levels using ultrasonic pulse methods. Experiments were carried out on cylindrical test specimens of Indiana Limestone, Tennessee Sandstone and Barre Granite under relatively low confining pressures (0-5000 psi). Analysis of data from these experiments indicated that there existed a low-level critical stress at which there appeared to be a transition in the mechanical behavior of the material. Under triaxial stress, results indicated that the differential critical stress was relatively independent of confining pressure, i.e., the mechanism responsible for the transition in mechanical behavior was governed by a maximum shear stress criteria.
INTRODUCTION
From a strictly engineering point of view, the description of the mechanical behavior of a material in terms of either a purely empirical or a phenomenological model often provides a sufficient basis on which to develop a working design. In many other cases, however, a comprehensive understanding of the actual mechanism itself is required. Experimental techniques for the study of deformation and failure mechanisms in geologic materials are relatively limited. This is due in part to the inhomogeneous nature as well as other inherent characteristics of these materials, the difficulty of carrying out large scale field experiments, and the technical problems of preparing and loading the specially-shaped specimens or models often required for meaningful laboratory studies. Failure mechanisms in geologic materials have been studied.
Failure mechanisms in geologic materials have been studied directly by a number of workers (1,2,3,4). In most cases, these have been limited to studies under uniaxial and biaxial stress in thin plates and have, for the most part, involved the investigation of fracture propagation rather than failure initiation. The large scale mechanism, in this case fracture propagation, has been investigated in considerable detail, however with the exception of a few studies (5,6,7,8,9) the fundamental mechanism associated with failure initiation has been relatively neglected because of its small scale.
Geologic materials associated with mining and other near-surface activities are normally in a state of stress equivalent to low confinement. Many geologic materials under low and even moderate confining pressures (<10,000 psi) behave in a near brittle manner, exhibiting relatively little strain before failure. As a result, the study of their actual deformation, and fracture initiation mechanisms is extremely difficult. Petrofabric techniques and other direct methods of observing deformation are not practical below strains of the order of 1%. It therefore becomes necessary to resort to indirect methods for this purpose. Similarly, although optical and photographic techniques are practical for studying crack propagation under certain restricted conditions, such direct methods are not suitable for the investigation of fracture initiation, particularly in 3-dimensional specimens. As a result, indirect methods must also be employed in such cases.
