ABSTRACT
The application of laboratory determined material properties to full scale situations has historically been a problem in the field of rock mechanics. Since laboratory tests are typically performed on small specimens and most rocks exhibit size dependent material properties, there is legitimate doubt as to whether laboratory derived material properties accurately characterize the in situ behavior of the rock mass in question. Thus, field tests have been established to measure rock mass deformability in situ. This paper presents a relatively new technique, known as the corejacking test, that could possibly be used to measure rock mass deformability. The corejacking test is a static field test originally designed to measure the creep response of salt (Van Sambeek, 1981). This test is easily adapted for use in rock masses that can be idealized as homogeneous, linear elastic continua that exhibit isotropic, transversely isotropic, or orthotropic deformation characteristics. A brief review of several static techniques used in measuring rock mass deformability is also presented.
INTRODUCTION
Scale effect is a major problem in rock mechanics (Heuze 1980). Laboratory tests on small rock specimens are generally inadequate to determine the deformability of rock masses because of a lack of anything but rule of thumb scaling laws. Typically, rock deformability (and strength) measured in a laboratory setting is overly optimistic, and can leave the engineer with a false feeling of security. This is not to say that underground engineering facilities cannot be designed using laboratory test results. The traditionally conservative nature of the design of underground openings and the adaptability of those involved in such construction has allowed the use of laboratory derived properties. However, if efforts are to be made to understand the "true" nature of rock masses, in situ tests are required. Presently, many in situ static testing methods exist for determining rock mass deformability. These include: plate bearing tests, flatjack tests, borehole jack tests, and dilatometer tests. These tests are among those most commonly used and will be discussed in the first part of this paper. Recently, a new static test called the corejacking test has been developed and used to measure the time dependent deformation of large specimens of rock salt under known and controlled boundary conditions (Blankenship and Stickney, 1982; Van Sambeek, 1981). The second part of this paper will discuss the potential applications of the existing corejacking test procedure or of any modified versions of it to characterize the deformability of rock masses that can be idealized as isotropic or anisotropic media.
REVIEW OF EXISTING PROCEDURES
The purpose of this section is to review some existing static tests used to characterize rock mass deformability and to discuss their advantages and disadvantages. More extensive reviews can be found in Goodman (1980) and in Bieniawski (1978).
The Plate Bearing Test
In the plate bearing test, rock mass deformability can be measured by loading a rock surface and monitoring the resulting deformations. This test can be conducted in underground openings or on surface outcrops. In this test, a unidirectional load is applied to the rock surface via a thick steel plate 0.5m to 1.0m in diameter.
