One of the major overcoring techniques widely used in Japan has adopted a new procedure of determining orthotropic anisotropy by a pressure cell test of a retrieved over-core. A procedure of stress analysis considering the anisotropy of rock has also been devised. This new method of rock stress measurement has been applied to a soft sedimentary Neogene rock in Japan. This paper describes the outline of the development and the result of the application.
Overcoring is one of the most reliable method for rock stress measurement, and many versions of it have been developed so far. In Japan, there are a few overcoring methods and they have been used in many projects in Japan (Kanagawa et al. 1986) (Obara et al. 1997). This paper deals with one of them. The method is categorized in the borehole deformation group. The deformation in the hole during overcoring is measured by 3 dimensional deformation sensors. Another feature of the method is that the elastic constants is determined from a pressure cell test using the retrieved over-core. In this way, the method is almost immune to the variability of elastic constants from place to place. This method gives stable curves of deformation during overcoring and pressure cell test. Owing to its high reliability, the method has been commercially used in the design of almost all the underground power stations in Japan. So far, the method has only considered the case that the rock is isotropy.
But as rock mechanists and rock engineers well know rock is anisotropic from a slight to a grave level depending on the nature of the rock. Amadei (1983) has pointed out the importance of taking into account the anisotropy in overcoring rock stress measurement. However, it may be said that there have been few overcoring techniques which take into account the anisotropy and are widely used in practical engineering. One of the major reasons for that is that determining anisotropy relevantly to rock stress measurement has been not easy. In this paper, by adopting a method of anisotropy determination (Shin, 2003) which uses a retrieved over-core after overcoring, a procedure of stress analysis in the case of anisotropy is presented. An example of its application to a Neogene sedimentary soft rock in Japan is also presented.
Japan Islands and the peripheral area are located at a boundary region where the Pacific plate, the Philippine Sea plate, the Eurasian plate and the North American plate collide together (Figure 1). The Pacific plate and the Philippine Sea plate are subducting beneath
(Figure in full paper)
the Eurasian Plate. The biggest island, called Honshu, is divided into two segments, the southwestern and the northeastern, by the Fossa Magna. In-situ stress measurements by means of 3 dimensional deformation gauges a-type (3DDG(a)) have been conducted in the Pacific side of the northeastern region of Japan. The depth of the measurements are about 45 m and the topography of the ground surface is just about flat.