ABSTRACT: The rock stress is measured by means of borehole deformation gauge in several test sites in the western region of Tokyo. The accuracy of rock stress measurement with this gauge is discussed from the technical point of view, The state of rock stress determined by this gauge is compared with those determined by other principles of rock stress measurements, and discussed from the view point of plate tectonics, It is indicated that results of the present measurement agree with those of several other measurements in a reasonable range of deviation from the view points of measurement engineering and plate tectonics.
ZUSAMMENFASSUNG: In unterschiedlichen Orte im Westen von Tokyo, wird die Gebirgsspannung mittels eines Verformungsmessgeräte des Bohrloch gemessen. Es behandelt die Messgenauigkeit des Gebirgsspannungen aus dem messtechnischen Gesichtpunkt. Der Zustand der Gebirgsspannungen in dieser Region die ist bestimmt mittels der Verformungsmessgeräte wird mit die bestimmt mittels anderes Messprinzip verglicht und aus dem Gesichtpunkt der Plattentektonik bewertet. Es wird gezeigt dass die Abweichung zwischen die Ergebnisse dieser Messungen und die einiger anderen Messungen ist vernachlässigt in Hinsicht auf den Messtechnik wie auch Plattentektonik.
RESUME: Les contraintes dans les roches s''ont mesurés à une cellule extensométrique dans les trous de forage à l''ouest de Tokio. La précision au mesure des contraintes Par la cellule extensométrique sont en discussion au point de vue technique. L''etat de contrainte dans les roches mesuré par la cellule extensométrique se compare aux résultats dérivés par les principes différentes au mesure, Nous met tons le champ de contrainte en discussion en vue de la tectonique de plaque, Il s''indigue que les résultants aux présentes mesures sont en accord avec les résultats aux methodes différentes à la portée acceptable de déviation aux points de vue de l''art de la mesure et de la tectonique de plaque.
INTRODUCTION One of the most reliable and popular principles of rock stress measurement would be the stress relief principle by overcoring among several principles proposed for rock stress measurement, Several techniques or gauges have been developed and used in practice as applications of the stress relief principle, For examples, it is reported that the complete state of stress has been determined in a single borehole by means of CSIR triaxial strain cell (Leeman 1968) as well as the doorstopper type strain cell (Hiramatsu, et al 1979). However, they have an inherent difficulty to be applied in the borehole in which the wall is wet and/or some water inflows occur, because the strain cell must be glued to the rock wall of borehole. As another technique of the stress relief principle, the borehole deformation gauge (Obert 1962, Suzuki et al 1970) can be applied in the wet or submerged borehole, although this technique needs three boreholes at least, for determination of the complete state of stress in the rock mass. The accuracy of rock stress measurement has been discussed by several authors for each technique, and the coefficient of variance more than 20% has been sometimes reported (Gray et al 1975, Van Heerden 1976). A few authors have measured the rock stress at the same test site by two different techniques, and reported the deviation of about 20% of the major principal stress measured by doorstopper strain cell from that measured by borehole deformation gauge (Van Heerden et al 1967, De La Cruz et al 1972). As well known; all of techniques of rock stress measurement by overcoring becomes more difficult to be conducted as deeper becomes the borehole. This is a reason that the hydrofracturing technique is frequently used for rock stress measurement in the deep borehole (Haimson 1978). However, it is very difficult to determine exactly the direction or azimuth of generated crack. In order to reaveal the accuracy of hydrofracturing technique, the ''rock stress is measured by means of hydrofracturing in the test site where the rock stress has been determined by the borehole deformation gauge in short boreholes drilled from an underground roadway (Haimson et al 1974). The discrepancies between the results by these two techniques have been given as 9% and 30% for the major and minor principal stresses in the horizontal plane, respectively. Since 1978, the authors have conducted the rock stress measurement in several test sites in Kanto-Tokai region, Japan. In this series of rock stress measurements which have been conducted as a part of the national project for prediction of large scale earthquake, the substantial interest concerns to the state of stress in the horizontal plane in the rock mass. As more data of rock stress measurements have been accumulated and compared with each other, as more necessities for evaluating the accuracy of measurement have been revealed. In this paper, the authors describe the procedure and result of rock stress measurements conducted by means of borehole deformation gauge.