In this paper, the principle, structure, performance and calibration of the Type YG-73 gauge are described. The verification of the reliability of the gauge in laboratory and in the field and some results of rock stress measurement in situ are given.
L''article presente la jauge du type YG-73 en indiquant ses principes, structure, performance ainsi que la methode de la calibration et en fournissant les resultats de la Verification de sa fiabilite au laboratoire et sur Ie terrain et ceux des mesures sur les contraintes de roche in situ.
In dieser Arbeit werden die Prinzip, die Struktur, die Eigenschaft und Einstellungs-methode des Spannungsmessers YG-73 Typ empfohlen. Es werden die Prüfung der Zuverlasstgkeit des Spannungsmessers im labor und auf dem feld und die Messungsergebnisse in situ gegeben.
The measurement of rock stress is of great significance in the design of mining, dam, power station, earthquake prediction and geotectonic analysis. Since 1962, the YG-73 type piezomagnetic stress gauge (shortly called YG-73 cell) and its prestressing apparatus have been developed as a modification of Hast''s cell (Hast 1958). Using the stress gauge, we have carried out a good many experiments which show that the YG-73 cell has a good linearity, repeatability, stability and sensitivity enough for measurement and gives a reliable result of measurement. At present, it has been widely used in China.
The stress is recorded on the principle of magnetostriction. If a load is applied to the cell parallel to its axis, the magnetic permeability of the nickle alloy is changed, and with it the impedance of the coil is changed too. If a stable alternating current is ~assing through the coil, there is a potential drop across it.
There are three cells in a probe at angle of 60'' between directions of cells. The cell is mounted in a brass housing. By moving the wedge through prestressing apparatus, the cell can be fixed in the bore hole and prestressed to a suitable value. The structure of the cell is designed in consideration of stability of contact state between the cell and borehole wall. During overcoring, even light relatively slip between the cell and the wall is not allowed. For this reason, the contact surface between the parts of the cell must be ground and fitted well, especially the two sides of the cell contacted with the wall of borehole should be parallel strictly to each other. The angle between the two surfaces of the wedge should be smaller. The sensitivity and stability of the cell depends on the material of alloy, heat-treatment, cell structure and machining precision.
In the past, we used electric bridge for the measurement. At'' present, we have specially developed a digital piezomagnetic stress indicator that consists of a stable signal source and a digital voltage meter. The signal source supplies a stable alternating current of 1000Hz for the coil of the cell.
A rock hole of 36mm in diameter is drilled at the point where the stress is to be determined. A stress gauge is placed at the required position in the hole and pre-stressed to a suitable value, which is recorded, and the overcore is made. During overcoring, the readings are recorded. When the readings are no longer changed with overcore, the rock core is taken out of the hole and put into a calibrator for calibration (see section 4). The difference between the original readings and the last readings are used to calculate principle stresses. For two dimensional stress state, from three readings in different directions of diameter in single hole, the principle stresses can be calculated (Pan 1981). If at least six readings are obtained in three holes perpendicular to each other, the three dimensional stress state can be determined. In order to obtain more reliable results of measurements, it is necessary that many measurements are carried out at a same measuring site and the least square method is used for data processing (Panek 1965, Wang 1977, Wang 1979).
For convert ion of readings into recorded stress (or reduced displacement) so that the principle stresses can be calculated, it is necessary for cell to be calibrated, and the calibration curve should be drown (Fig. 4). In the past we used a special rock prism for calibration (Fig. 5). At present, we have developed a new calibration method-the confining pressure calibration which can eliminate the two weaknesses mentioned above. A confining calibrator is used in this kind of calibration. The calibrator consists of a pump and a confining pressure container. Confining pressure is applied to the core to a expected value by pumping oil into the container. Then the pressure is reduced and t he readings are recorded.
