A novel testing technique of downhole triaxial compression tests and pressuremeter tests was proposed in 1997 for the purpose to directly measure stress and strain relationship of rock mass in the field. In 1998, a set of test equipment was developed at Central Research Institute of Electric Power Industry (CRIEPI) in Japan. This apparatus was designed to test rock specimens of 400mm in outer diameter, 86mm in inner diameter and I050mm high. Then, in 1999 after some trial tests, an attempt was made to carry out a series of field tests in rhyoritic tuffacious rock.
Evaluation of mechanical properties of rock masses is rather difficult because of their heterogeneous and/or discontinuous natures causing various engineering issues such as scale effect, quality of sampling and so forth. Thereby, large-scale loading tests, e.g. plate load tests, pressuremeter tests and rock shear tests, are commonly conducted in practical site investigation. However, several problems have been pointed out regarding these loading tests; some of them are:
inability to directly measure stress and strain relationship of rock mass;
limited load/stress paths;
inaccurate measurements of rock mass behavior;
lack of strength tests in deep ground,
separate tests in order to evaluate strength and deformation properties respectively, and so forth. Novel testing techniques of a downhole triaxial compression test and a pressuremeter test were proposed in 1997 for the purpose to directly measure stress and strain relationship of rock mass in the field (Tani. 1999). Figures 1&2 illustrate the proposed lest method and the test procedure. One of the most characteristic features of the proposed test method is the use of a hollow cylindrical specimen prepared at the bottom of a drilled hole. This can be achieved by drilling a small center hole and a concentric peripheral slit using conventional rotary drilling machinery. A solid cylindrical inner cell and a hollow cylindrical outer cell are inserted in the small center hole and the outer slit respectively. Axial as well as lateral deformation can be measured in the small center hole to evaluate average axial and circumferential strains of the specimen. Another feature is that two separate test methods can be conducted. The one is a triaxial test of an element test type, and the other is a pressuremeter test of a non-element test type.
In 1998, a set of test equipment was developed at Central Research Institute of Electric Power Industry (CRIEPI) in Japan. This apparatus was designed to tests rock specimens of 400mm in outer diameter, 86mm in inner diameter and I050mm high. The maximum confining pressure was load can be made as high 65MPa in terms of axial stress. The details of test equipments can be found in Tani et al. (2001).
In 1999 after some trial tests, an attempt was made to carry out a series or field tests.
