The strength of a jointed-rock mass is not well-understood. It is believed, however, that the behavior of a rock mass is governed by both the intact rock properties and the properties of the discontinuities. Rock research in the past has been concentrated on the determination of intact rock properties. Several studies are presently underway which should provide more insight into the properties of the discontinuities. Few research programs, however, have been undertaken with the intent of studying the composite rock mass behavior where either the intact rock strength or the properties of the discontinuities may control failure.
There are three approaches available for studying the behavior of a rock mass, namely, field studies, mathematical models (finite element), and geomechanical models. A geomechanical model has been developed in this study where the strength and failure modes of a model rock mass are studied as a function of intact block geometry, joint orientation, joint properties, and confining pressure. The objective of this study is to gain some insight into the parameters that govern the behavior of a rock mass, which parameters are most important, and when these parameters control.
This chapter describes only one phase of the study the development of equipment for use with the rock-blocks model. The models being tested are approximately 22 x 22 x 6-in. The size varies slightly depending upon the joint orientation used. The equipment developed includes model material molds, reaction frames, loading apparatus, measuring and recording instrumentation, and other accessory equipment pertinent to the operation of a successful study.
The usual problems in equipment development, such as uniformity of load application, loading capabilities, and reaction capacities are all present. A jointed specimen presents additional problems however, since it is composed of numerous individual intact blocks. In a jointed model, each individual block must be allowed to move as desired. Its freedom of movement must not be hindered or restricted by the load application source. Since each block is an individual unit, it is important that an instrumentation and data acquisition system be used that will yield data for intact properties of these individual blocks and the relative movements of the blocks. These major problems, among others, were dealt with in the development of this equipment, and a workable solution has been obtained.
In a previous paper,1 the development of a rocklike model material for use in this study is described. This material is composed of 76% river sand, 10% Hydrocal B-11 gypsum cement, and 14% water by weight. It is a high-density, low-void-ratio material that is vibrated on a vibrating table. The material simulates rock in general and a schistose gneiss rock in particular. In Fig. 1, normalized Mohr envelopes are shown for both materials. The slopes of the initial and secondary straight-line portions of the curves for both materials are very close. The only difference in the curves is the normalized normal stress where the change in slope occurs.
This material has advantages over previously developed materials reported in the literature in that it simulates rock in many of its physical properties, not just one or two.
