Since 1964, the Dept. of Mining at The Pennsylvania State University has been carrying out extensive studies associated with the phenomena of microseismic activity and inelastic behavior in geologic materials. Briefly, the program to date has involved the following stages:
Study of the microseismic noise and energy rates for selected Geologic materials under incremental, uniaxial tension.1,2
Investigation of the frequency spectra of microseismic activity of selected geologic materials under incremental, uniaxial tension.3,4
Investigation of the time-dependent inelastic deformation of geologic materials and the analysis of such deformation in terms of mechanical models.5,6
Simultaneous study of microseismic activity and time-dependent inelastic deformation in geologic materials under incremental, uniaxial compression.7,8
Details of the research involved in stage 4 of the program constitute the subject of this chapter. It should be noted that the majority of the experimental data quoted in this chapter forms part of a thesis to be presented by one of the authors (Kim) in partial fulfillment of the requirements for the Ph.D. degree in mining engineering at The Pennsylvania State University.
During the last two or three years, a number of important papers have been published which deal primarily with the fundamental structure and mechanical behavior of geologic materials. Recent papers 9, xo have provided research workers in the rock mechanics field with a meaningful overall picture of rock as a structural material. The concept that many geologic materials may be considered basically brittle, elastic solids containing various distributions of cracks and pores appears to provide a sound model for the structure of such materials.
Bieniawski11-14 has developed a very meaningful scheme for considering rock fracture in compression. This scheme, shown in Fig. 1, covers all significant processes taking place in such materials from initial load application to complete failure. The authors suggest that the general concepts developed by Brace, Bieniawski, Walsh, and others provide an extremely useful framework in which the results of current as well as earlier research should be examined.
This chapter describes an experimental study carried out to investigate creep and microseismic activity in a number of geologic materials under uniaxial compression and to ascertain what relationships, if any, exist between these two phenomena. For the most part, the chapter deals with the experimental aspects of the problem under study. A detailed consideration of the results of the current study in terms of fundamental rock structure and fracture processes will be delayed, however, until a comparison study being carried out under triaxial compression is completed.
Fig. 1--Bieniawski's scheme for considering rock fracture in compression.14 (Available in full paper)
Many materials emit transient vibrations in the audible and subaudible range when stressed. In rocks, this phenomenon is referred to as microseismic activity,? and each individual microseismic disturbance referred to as a "microseismim" (e.g., see Obert and Duvall15). Quantitatively, such transients may be described in either the amplitude-time or the amplitude-frequancy domain. In this chapter, the analysis will be restricted to the former. In order to monitor microseismic activity, a sensitive transducer (e.g., an accelerometer) is attached to the test specimen;
