In situ stress measurement is usually done based on borehole methods such as hydraulic fracturing and borehole breakout. Borehole shape changes under the action of in situ stress and similarly we can calculate in situ stress based on borehole shape analysis. In this study, borehole shape is analysed under far field stress state based on theories of elastic mechanics. We theoretically prove a circular hole under far field stress state will be elliptical in shape, and derive the equations between the elliptical parameters and the stress. Formulas for calculating ellipse shape and azimuth by using deformation data of 3 points on borehole are deduced. A new method for in situ stress measurement based on borehole shape analysis is proposed and a new contact type borehole shape measuring device is developed. Through laboratory test, this method is proved to be feasible in measuring the magnitude and direction of in-situ stress.
The in-situ stress is the natural stress which is not disturbed by engineering in the formation, also known as the initial stress of rock mass or the stress of original rock. It is the fundamental force that causes the deformation and destruction of underground or open-pit excavation projects, such as mining, hydropower, civil construction, railway, highway and military. It is a necessary prerequisite to determine the mechanical properties of the engineering rock mass and to analyze the stability of the surrounding rock. In recent 30 years, with the continuous development of in-situ stress measurement, various measuring methods and measuring instruments are developing constantly. Amadei & Stephansson (1997) divide the commonly used in-situ stress measurement methods into borehole based methods and core based methods. Among them, the most applications in rock engineering are based on drilling method, and the typical methods are stress relief method and hydraulic fracturing method. The hydraulic fracturing method is a direct in-situ measurement method which is directly obtained by hydraulic fracturing. It is especially suitable for measuring deep in-situ stress, but there are some limitations in determining the direction of the principal stress and the requirements of the testing environment. The stress relief method is an indirect measurement method which can get the in-situ stress by measuring the displacement of the borehole wall caused by in-situ stress. The method can accurately get the magnitude and direction of the in-situ stress. But the testing process is more tedious, each step needs to be completed by the drilling rig, especially the line problem when the sensor is laid, which seriously limits the application of this method in the deep borehole. Therefore, it is an important direction for the development of the in-situ stress measuring technology to break through the deterministic problem of the principle stress direction, the limitation of the measurement depth and the complexity of the testing process, realizing the rapid and accurate in-situ stress measurement in the deep borehole with the complex environment.
