Rock is a discontinuous mass containing cracks, fissures, joints, faults and bedding planes. The engineering properties of a rock mass depend far more on these discontinuities. Thus, it is an important part of rock engineering to assess rock mass conditions.
Recently, a lot of studies have been accomplished on this subject (Barton 1974, Bieniawski 1973), but it seems that most of them are too complicated and inconvenient to use. The rock mass conditions should be expressed by simple methods. The borehole jack tests have been employed at many sites. This method is easier and less costly to conduct than other in situ techniques and a lot of data relating to rock mass conditions can be obtained without serious errors..
At the beginning of this paper, the outlines of borehole jacks used in this investigation are summarized. Next, the correlations between the jointed rock mass properties and borehole jack test data are presented.
At the end, this paper proposes an engineering classification of rock mass based on the borehole jack test data.
The borehole jack is a unidirectional loading tester used in a borehole to measure the deformability of rock or soil quantitatively. It consists of a steel loading plate, several small oil jacks, a linear motion potentiometer and a pressure transducer. The loading plate drives to borehole wall by jacks, and diametral displacement is measured by potentiometer. The looks of borehole jack device (KKT) are given in Figure 1. These geometries aresummarized in Table 1. Among these, C type KKT is mainly used in this investigation. The features of KKT are shown as follows.
The higher pressures possibly allow the test to be carried from soft rocks to hard ones.
The loading being unidirectional, the test can be oriented and focused on the geological targets, if visual observation (borehole TV) is made before and after jacking.
The loading method of KKT is as follows.
The simple loading in Figure 2 is used to assess the rock mass conditions.
The repeated loading shown in Figure 2 is conducted to determine the rock mass deformability.
The rate of loading is from 5 to 20 kg.f/cm2 /min. The typical curves of pressure vs. displacement for the KKT tests using simple loading are shown in Figure 3. The deformation modulus are obtained by the Goodman's equation as follows (Goodman 1968).
(Figure in full paper)
(Table in full paper)
In general, rock mass classifications have been conducted on the basis of core data. But the softer and weaker components of the rock mass tend to be lost at boring and the fabric of the rock block fracture system in situ is not sampled. Therefore, the informations from the boring core data are too crude and primitive for designing rock structures.
The classification index called rock quality designation (RQD) is based on analysis of recovered core from a borehole.
