Research On Key Technologies And Applications Of Optical Fiber Monitoring For Three-dimensional Deformation Of Slope

The statistics show that landslide is a major geological hazard with susceptibility,hazard vulnerability and risk.With the frequent occurrence of severe weather such as extreme heavy rainfall and the increase of human engineering activities,the occurrence of landslide disasters has been further aggravated,directly threatening people's lives and property safety.In the process of instability and destruction of slopes,there will be some common characteristics,for example surface cracking,vertical deformation and internal lateral deformation.Therefore,deformation monitoring of landslide disasters is an important basis for effective early warming,evaluation and prevention.The optical fiber sensing technology that has emerged in the 1980 s takes light as the carrier and optical fiber as the transmission medium.It has many obvious advantages of real-time detection,anti-electromagnetic interference,large-scale network monitoring and long-distance signal sensing and transmission over the traditional electromagnetic sensing and monitoring technologies,which will bring new opportunities and ideas for prevention and mitigation of landslide disasters.Therefore,this paper presents a method of slope three-dimensional deformation monitoring based on optical time domain reflection(OTDR)and weak fiber Bragg grating(w FBG).Firstly,a series of optical fiber displacement sensors with better performance for monitoring surface cracking,vertical deformation and internal lateral deformation of the slope are developed,and an early warning mechanism of the optical fiber sensing system for landslides disasters is also constructed.Then through theoretical analysis,calibration tests,numberical simulation and model experiments,the new principles,new methods and new technoloigies of the optical fiber sensors for landslide deformation monitoring are systematically proposed.At last,the monitoring application is combined with the field practical engineerings to provide advanced technical means and platforms for landslide monitoring.The main work and achievements of this paper are sumarrised as follows:(1)Based on the principle of optical fiber bending loss,two kinds of optical fiber displacement sensors with high-precision and large range for slope surface deformation monitoring are developed.The sensor structure design and packaging,the modulation mechanism of the optical loss and displacement as well as the reliability and stability of the sensor are studied theoretically and experimentally.It is proved that the sensor can be effectively used for real-time monitoring of slope surface crack opening process.(2)An optical fiber winding modulation structure based on a cylindrical spiral spring is developed,and its optical loss sensitivity mechanism is analyzed.The geometric structure design,loss sensing characteristics and mechanical properties of the optical fiber winding structure are studied through theoretical calculation and tests,which have well verified the sensing ability of the spring-shaped optical fiber winding structure.The packaging structure and quasi-distributed multiplexing method of the spring-shaped optical fiber displacement sensor for the soil vertical deformation monitoring are developed.The sensor calibration and soil settlement model test are carried out,which have proved that the developed spring-shaped optical fiber sensor can effectively monitor the soil vertical deformation in a real time.(3)Based on OTDR technology and the macro-bending loss characteristics of optical fibers,a fiber loop bending modulator mechanism is proposed,and the basic structure and the loss sensitive mechanism of this fiber loop are studied.Through theoretical derivation and calibration test,the relationship between the output optical loss and the sensing displacement can be determined.A series of displacement sensors based on the bending structure of the fiber loop are developed by ourselves.The shear tests with single or double sliding planes of rock stratum are carried out.The detection abilities of the sensors for shear sliding failures of different rock stratum are analyzed and proved.In order to verify whether the sensor is effective in the soil slope,a model test of soil slope sliding failure is carried out,and the results prove that the sensor can effevtively determine the whole deformation process of the soil slope.Finally,the working mechanism and applicable characteristics of these sensors are also discussed.(4)Based on the w FBG sensing technology,a w FBG strain tube for monitoring the deep deformation of a slope is designed with a PPR tube as the carrier.The requirements of large-scale optical fiber sensing for FBG arraies and the factors affecting the strain transfer rate of FBGs are analyzed.Based on w FBG sensor array,the conjugate beam method and compoiste Simpson method are proposed to transform the strain distribution into the displacement response.Through numerical simulations and model tests,the conjugate beam method,compoiste Simpson method and common difference equation method are compared together.The calculation results of various algorithms for different types under deformation conditions are analyzed.It is shown that the conjugate beam method and composite Simpson method combined with the w FBG strain tube are more suitable for displacement monitoring,with higher calculation accuracy and adaptability.(5)The developed fiber loop displacement sensor is applied to monitor a shallow slope with a supporting system during construction;the developed distributed fiber loop displacement sensors and a w FBG strain tube are used together for the deep displacement monitoring of an excavated slope;the developed spring-shaped optical fiber displacement sensor is used to monitor the load deformation of a fill slope.The monitoring results are compared and analyzed,which further verify the long-term stability and effectiveness of these newly developed optical fiber displacement sensors in this paper.