Abstract: The piezomagnetic overcoring method is an in-situ stress measurement technique that developed since 1950 s. In order to implement three dimensional stresses measurement in a single borehole, the total stresses sensor has been developed. The structure and calculating theory of piezomagnetic total stresses sensor are introduced. The comparing study of in-situ stress measured has been carried among the piezomagnetic overcoring method using several inclined boreholes which are oriented in different directions, piezomagnetic method in single borehole and hydraulic fracturing at Jinping Ⅱ Hydropower Station. The measurement result indicate: in-situ stress at shallow depth of tunnel is controlled by deadweight, topography and region formation, formed typical stresses distribution of V-shaped river valley; the maximum stress is about 11 MPa and the direction is about NNW. The in-situ stress at deep depth of tunnel is higher, the maximum stress is about 40 MPa, and the direction is about vertical. The direction of maximum stress changes from level state to vertical state along with increasing of level burying; and it indicates that the deadweight stress is dominant effect. After comparison of in-situ stresses measured, it is shown that they are consistent fundamentally. We consider the piezomagnetic overcoring three dimensional stresses measurement method in a single borehole can provide reasonable in-situ stress information as same with the piezomagnetic overcoring method and hydraulic fracturing using several inclined boreholes which are oriented in different directions.

Key words: over-coring, in-situ stress, hydraulic fracturing