Study On Deformation Coupling Between Sensing Cable And Sand Under Triaxial Compression

Distributed fiber optic sensing technique is increasingly used in soil deformation monitoring due to its advantages of long monitoring distance,high sensitivity and strong robustness.The coupling between sensing optical cable and measured object is the key problem of distributed optical fiber sensing technology in geotechnical engineering monitoring.At present,there are few calibration methods and experimental studies on fiber soil interface coupling.In this paper,an optical fiber soil strain coupling calibration test device is designed and developed,which can apply confining pressure.Based on the high-resolution distributed optical fiber sensing technology,the loading and unloading cycle test and loading failure test are carried out around the coupling problem between the strain sensing optical fiber and soil,which verifies the feasibility and accuracy of distributed sensing optical fiber measuring sand compression deformation.The main work and research results are as follows(1)According to the different strain field conditions,the strain transfer model between the optical fiber and the measured matrix is established,and the effects of the embedded length of the optical fiber,the shear modulus of the coating layer and the diameter of the optical fiber on the strain transfer are analyzed and compared.(2)The cyclic loading and unloading tests of fiber sand interface coupling under low deviatoric stress were carried out.The results show that: in the loading and unloading cycle test,the coupling coefficient β of optical fiber soil deformation is mainly in the range of 0.45 ～ 0.7,and the coupling coefficient decreases with the loading and unloading cycle.And it is found that pouring sand and 3mm diameter fiber optic cable have the best coupling.In addition,with the increase of confining pressure,the coupling coefficient of fiber soil interface increases.(3)When residual strain is generated in soil,the coupling coefficient of residual strain between fiber optic fiber and sand soil is significantly smaller than that under axial stress.The coupling coefficient of residual strain under loading condition is higher than that under unloading condition.The coupling coefficient of residual strain increases with the confining pressure increasing.(4)The failure tests of fiber optic-sand interface under high axial deviatancy stress were carried out.The experimental results show that the coupling coefficient of the fiber-sand interface is close to 1,and the interface tends to be completely coupled with the increase of the axial deviator stress.This indicates that when the distributed optical fiber sensing technology is used to monitor deep strata,the interface between optical fiber and sand is in a state of high coupling,and the strain distribution data measured by optical fiber is close to the real strain distribution of formation profile.In addition,the failure of the fiber-sand interface presents a progressive failure,and the damaged interface becomes a low strain sensing section.The increase of confining pressure can well limit the development of low strain sensing and improve the shear strength of fiber optic sand interface.