ABSTRACT

This paper presents the study in situ on stress field of rock mass in a large dam site. By means of two different measure instruments and the method of stress relief, a lot of measure in situ has been done, including stress measurement of three dimensions at ten points and two deep-hole stress measurements in the perpendicular boreholes which are 59.4m and 53m deep seperately, and the basic accordant results corresponding two methods have been got. In the process of the deep-borehole stress measurement we got the stress value in critical state in which cylindrical core would become core-disking in the high stress area, which was first got in the world. The maximum principal stress was up to 650 kg/cm². The stress value obtained by test and the thickness of rock disks all basically accord to the analysis by mechanical mechanism of core-disking. The plane linear elastic finite element analysis has been completed. Results of inverse calculation all accord to the measurement results in four different positions. The whole outline of the stress field of dam site was delineated clearly. The investigating stress field of rock mass is one of crucial works to evaluate stability of rock engineering. The stress measurement of rock mass in situ and the finite element analysis is introduced in this paper.

METHODS FOR MEASURING ROCK STRESSES AND RESULTS

1. To clear up rock stresses around the dam, two different methods for measuring stresses in- situ were used:

deep-hole stress-relief method with borehole deformeter as transducers, and stress-relief method with piezomagnetic stress units as transducers. Borehole diameter deformation method uses type 36-2 borehole deformer as a transducer of whose sensitive elements are steel rings with good elasticity; and four strain gages, making up a doublebridge, are stuck on the ring. When deformation of borehole occurs, YJ-5 resistance strain instrument can record the message output from the double-bridge through rigid feelers; and then the change of diameter along feeler direction can be obtained (Fig 1.). The four steel rings fixed in a deformer can record at the same time deformations of a borehole along four orintations inclined at 45º each other. According to the deformations recorded, values of stresses and orientations in the plane normal to the axis of the hole would be calculated. Busing three intersected-boreholes method, values and orientations of cubic stresses can be obtained. Undergoing a great number of tests for reliability and stability and engineering 36-2 borehole deformer has been proved stable and accurate enough to sense stress change up to 0.5 kg/cm , and has a relative error less than 10 % when measuring principal stress. In piezomagnetic method, YJ-73 piezomagnetic stress gage is used as a transducer which is designed according to piezomagnetic effect.

2. Measuring poits were mainly arranged inside rock mass where the axis of dam and undergrbund building would be located. Cubic stress measurements with three intersected-boreholes method were performed in syenite in prospecting excavation 2 (pro. Exc. 2), and in basalt in pro. exc. 4.

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