Abstract
Abstract. The observation and estimation of the deep crustal stress state is a key and difficult problem for in situ stress measurement. Using a borehole wall strain gauge based on the overcoring stress-relieving method is one of the main methods of in situ stress measurement. In this paper, a strain-sensing array based on fiber Bragg grating (FBG) is designed by using the main structure of the classical hollow inclusion cell, and its layout scheme on the hollow inclusion is studied. According to the layout scheme, the in situ stress inversion algorithm of hole wall strain to stress is deduced. Following this, the triaxial loading and unloading experiment platform is built, and the calibration experiment for the FBG strain sensor is designed. Finally, Abaqus finite element software is used to simulate the in situ stress measurement process of the overcoring stress relief. The FBG strain values of each measurement direction before and after the overcoring process are extracted, and the stress inversion equation is used to carry out the stress inversion. The comparison of the inversion results proved that the FBG strain sensor group is feasible and reliable. The quasi-distributed FBG sensor module designed in this paper can invert the three-dimensional in situ stress by measuring the hole wall strain, which places a theoretical and experimental foundation for the development and application of an FBG hole wall strain gauge. It makes up for the deficiency of the existing hole wall strain gauge based on a resistance strain gauge, provides direct and accurate observations for hole wall strain measurement, and has important practical value for the development of in situ stress measurement technology.
Highlights
The undisturbed stress in rock mass is called geo-stress or in situ stress (Amadei and Stephansson, 1997)
The fiber Bragg grating (FBG) sensor and resistance strain gauge are used to measure the longitudinal strain of the cylindrical specimen under uniaxial compression; the strain measured by the resistance strain gauge is regarded as the actual strain of the sample, which is proportional to the wavelength shift ratio kλ of the FBG
Through data processing methods and error analyses, we have proven that the FBG strain sensor array is feasible and reliable for in situ stress measurement based on the overcoring method
Summary
The undisturbed stress in rock mass is called geo-stress or in situ stress (Amadei and Stephansson, 1997). This gauge can measure the stress tensor in a single hole to obtain the three-dimensional stress state of rock mass and form a set of standardized measurement procedures, making it one of the most applicable and reliable in situ stress measurement methods (Cai et al, 2000; Liu et al, 2011). There has been less research and fewer applications of FBG strain detection technology in the field of in situ stress measurement: there are only a few borehole Earth strain meters, geo-stress sensors, and volcano monitoring sensors based on optical fiber sensing technology, and there are even fewer mature and sta-. The arrangement of the sensor array is studied, and the in situ stress inversion algorithm of FBG strain gauge is deduced, which lays an important theoretical and experimental foundation for the development and application of an FBG hole wall strain gauge
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