8素子ゲージ法による地殻応力測定法の精度とその改良

抄録

In situ stress measurements have been carried out by an “8-element-gauge” method which is one of the stress releasing methods. Several problems remain unsolved in the techniques of measurement and analysis. Their causes are exactly investigated and the accuracy of the stress value estimated is discussed in the present article.
Data employed for estimating the stress value involve systematic errors. Among them, errors in strain sensitivity matrices and those in rotational angles which determine the angles between each strain gauge and boreholes are predominant.
In order to reduce the systematic errors, the strain sensitivity matrix is recalculated and the rotational angle is corrected. Results obtained are compared with ones previously determined. The following conclusions are obtained as for the eight sites where the Research Group for Crustal Stress in Western Japan measured in situ stresses.
(1) Vertical stresses at a few sites previously determined are significantly underestimated, whereas the present ones can be explained by the gravity loading and topographic effect.
(2) Average horizontal stresses as well as maximum horizontal shear stresses vary by their corrections and are propotional to the depth from the ground surface, except for those at Ohtani mine.
The accuracy of the stress values obtained by the “8-element-gauge” method is several MPa for the measurements using two boreholes and a few MPa for ones using three boreholes, if the conditions of the measurements are the same as those at eight sites described above.