Design And Experimental Research Of The Fiber-optic Ground Settlement Sensor Based On Optical Time Domain Reflectometry

Ground subsidence may be caused by groundwater exploitation,underground mineral resources exploitation and poor foundation,which may lead to ground deformation,affect the normal use of ground buildings and even cause damage.With the acceleration of urbanization in various countries,the scope and harm degree of land subsidence are expanding day by day,which has become an environmental geological problem that can not be ignored in the sustainable development of today's society.In order to effectively prevent and control land subsidence,the state has established a land subsidence monitoring network in key subsidence areas to monitor the law of land subsidence development and change.The existing land subsidence monitoring methods are either point measurement,or can not implement large-scale and long-distance monitoring.Some of them are easy to be disturbed by electromagnetic field,easy to rust and corrosion,and poor durability.Optical time domain reflection(OTDR)technology plays an increasingly important role in engineering monitoring with the advantages of real-time dynamic monitoring,corrosion resistance and electromagnetic interference resistance.Based on this,this paper analyzes the current situation of land subsidence monitoring,summarizes and analyzes the optical stress sensor based on OTDR technology,designs a kind of optical fiber spring sensor combined with spring and single-mode optical fiber,and a settlement displacement sensor based on optical fiber spring,and carries out theoretical analysis and experimental verification.The main contents and achievements of this paper are as follows:(1)G652D optical fiber is calibrated with different wavelength and pulse width,and the combination of wavelength and pulse width suitable for follow-up test and research is optimized.The structural characteristics of existing fiber bending loss sensor are analyzed and summarized,and a kind of optical fiber bending loss modulation structure fiber spring which pastes optical fiber on the outermost side of spring wire is proposed.(2)Combined with theoretical analysis and numerical simulation,the influence of geometric characteristics and axial deformation of optical fiber spring on its range,loss and sensitivity is analyzed;indoor calibration test and mechanical property test are carried out with two kinds of substrates(nylon carbon fiber and spring steel)fiber spring,the sensing performance of optical fiber spring is explored,and the structure optimization is carried out.(3)A settlement displacement sensor based on optical fiber spring is proposed.The measurement principle and displacement loss relationship are analyzed.The displacement loss relationship of the sensor is calibrated through indoor calibration test,and the performance of the sensor is discussed.The quasi distributed multiplexing technology of the sensor is analyzed.(4)Indoor soil settlement model test is carried out to verify the sensor's single and quasi distributed settlement monitoring,and the reliability and applicability of the sensor are proved from the perspective of test application.