Three sites of mines and one site of underground powerhouse at depths of 150m to 802m in Korea were chosen as testing sites to get informations on the initial rock stresses. Three directional borehole deformation gage was used to take a large number of strain-relief measurements, from which three dimensional stresses for each site were calculated. Attempts have been made to determine the state of absolute stresses, the influence of surface topography on the stresses and the changes of stresses in the magnitude with depth.
Trois endroits des mines et un endroit de la centrale électrique souterraine à la profondeur de 150m à 802m en corée ont été choisis comme l''endroit d''essai pour obtenir les informations sur l''effort initial de roche. La jauge de déformation de trou percé de trois orientations a été emploiée pour effecture un bon nombre de mesure de contrainte-détante, d''où les efforts de trois dimensions à chaque endroit ont été calculees. L''essai a été fait de determiner l''état des efforts absolus, l''effet de la topographie superficielle sur les efforts et la variation de l''intensité de l''effort avec la profondeur.
Die Durchführung der In-situ-Spannungsmessungen erfolgte in drei verschiedenen Bergbaugebieten und eincr unterirdischen Kraftwerke in Teufe von 150m urtd 802m in Korea. Es werden drei Bohrlöcher von unterschiedlicher Achsrichtung gebohrt, wobei die Messung nach dem Überbohrverfahren ausgeführt wird. Mit den so gewonenen Daten wird der vollständige räumliche Spannungszustand errechnet. Der absolute Spannungszustand, der Einfluβ der Gebirge auf den Spannungszustand und entsprechend variierenden Werte mit der Teufe werden näher erläutert.
The determination of the local state of stress at depth in rock masses is important in both the design of mining excavations and the understanding of the mechanism of geologic structures. If the stress is determined in the rock in an area of a prospective underground opening, it is possible to predict the stresses that will act around openings. It will then be possible to choose the ''orientation and shape of the opening so as to minimize tensile stresses and extreme concentrations of compressive stresses in its boundary surfaces. For such purposes, in-situ measurements nave been made in different geologic formations and at different depths. This paper presents current information on stress measurements in Korea. We have studied the variation of average horizontal and vertical stresses with depth in various geologic conditions and calculated the ratio of maximum to minimum principal stresses. We also discussed the relation between the principal stresses and shear stresses in rock.
The in-situ stress measurements were made by U.S.B.M. borehole deformation gage method. The overcoring procedure of making in-situ stress measurement is well known and described here briefly. As practised by the U.S.B.M., a 38mrn diameter borehole is drilled in rock mass and the gage is inserted at a proper location in hole. The gage is overcored with a 15,24 em diameter bit. As the stresses are removed from the core, the hole deformation is monitored by the gage. To calculate the absolute stress from the strain relief measurements at each site, it is necessary to know the elastic constants and Poisson''s ratio of the rocks in which the measurements are made. These constants were determined from uniaxial loading in laboratory test and biaxial loading in the in-situ test. The components of stresses were reduced by using the plane strain solution(2). By means of the least square method,.it is possible to estimate the values of the principal stresses, of their directions and of their confidence levels.
More than 70 measurements for determining the natural stresses in the undisturbed rocks have been made at six testing sites in four different localities and at depths ranging from 150m to 802m(Fig. 1) • The measurements were performed continuously in several horizontal boreholes of which length is up to 15m and strain relief deformations were taken at a distance about 2–3 times radius of the tunnel from the tunnel wall. All testing sites were more than 400m distant from areas of ~ining and could be considered to be free of stress concentration effects due to mining.
Despite of limited number of stress measurements. We have studied the effects of topographic irreqularities, the variations of vertical and average horizontal stresses with depth and the relation between principal stresses. 4.1 Influence of topography on vertical stress. The calculated average stress components are considered to represent the natural stress fields that existed prior or to excavation because the measurements were made at a distance far away from the influence zone due to the opening. The natural stress field is consisted of gravitational(current) and tectonic(current, residual) stresses(4). At the Samrangiin(relatively shallow depth) and the Kangweon, the measured Vertical stresses are greater than calculated ones by 12–18%, while at the sangdong and the 2nd Yeonhwa smaller by 5- 14%.