On The Mechanical Coupling Between Optical Fiber And Soil For Distributed Ground Deformation Sensing

Soil at Earth's surface is the main arena for human activities and engineering constructions.Deformation and failure of soil may lead to geohazards or geotechnical problems such as landslides,land subsidence,and foundation instabilities.The spatio-temporal deformation of soil is very complicated;hence,a robust monitoring technique,which can perform long-distance and distributed sensing is urgently needed.Distributed fiber optic(FO)sensing(DFOS)overcomes the limitation of traditional point-wise sensors,which shows great potentialities in soil deformation monitoring.The deformation mechanism of soil is complicated and is easily influenced by the surrounding environment,which complicates the interaction between fiber and soil.The sensing performance of FO cables is largely dependent on their structure,modulus,and size.In addition,the quality of monitored FO data is obviously concerned with cable layout and backfilling.Therefore,from cable selection to layout,and from data collection to processing,the mechanical coupling between fiber and soil is a problem that must be answered.In this thesis,this issue is studied in detail,and the following results are obtained.(1)A pullout apparatus is designed to investigate the interaction mechanism between FO cable and soil under confining pressures(CPs)ranging from 0 to 1.6 MPa.Results reveal a critical CP that ensures a good coupling of fiber to soil.In addition,two parameters,i.e.,strain propagation coefficient and fiber-soil coupling index,are proposed to characterize the fiber-soil coupling,and a method for fast estimation of the reliability of FO data is presented.(2)A micro-anchored FO cable is designed.The influence of anchor diameter and spacing on the fiber-soil coupling is investigated through pullout tests,revealing how the fiber-soil interface shear strength and stiffness are correlated with diameter and spacing.Further,the distribution of strain along the cable is acquired using the Optical Frequency-Domain Reflectometry technique with an ultra-high spatial resolution of 10 mm.On this basis,the inclusion of anchor is equivalently correlated with CP.(3)A critical CP and a critical depth are proposed to characterize the fiber-soil interface adhesion.A cable with a low Young's modulus or a small radius corresponds to a low critical CP or depth;given the properties of soil and cable are known,the critical CP or depth is solely dependent on the maximum strain gradient.A near 4-year dataset(N = 61,092)reporting strain values measured from a 1 mm radius cable is analyzed,showing that the strain gradient normally does not exceed 2×10-3 m-1.Based on this gradient,the values of CP and depth for five typical FO strain cable are presented.(4)Based on the classical strain transfer model and Goodman's hypothesis,a stratum-backfill-cable strain transfer model is obtained.A comprehensive parametric analysis is carried out to investigate the influence of cable,backfill,and stratum properties on the strain transfer coefficient.The analysis shows that the coefficient will be large for a cable with a small radius and a low Young's modulus,a backfill with a small radius and a large shear modulus,a stratum with a low shear modulus,or a large stratum-backfill interface adhesion coefficient.These provide practical guidance for the selection and determination of FO cable and backfill material.(5)The influence of CP on the strain transfer coefficient is dependent on the parameters of both the backfill and the stratum.The strain transfer coefficient may increase monotonously,decrease monotonically,or first increase and then decrease with CP.Based on a fractional Merchant model,a revised strain transfer coefficient is also proposed.The parametric analysis shows that the revised coefficient differs from that calculated using the elastic solution.(6)The coupling between fiber and soil under shear conditions is defined from a kinematic and morphological point of view.The fiber-soil coupling is considered to be good if the deformed fiber follows the imaginary deformation line of the shear zone.Based on this,two calculation models are proposed,i.e.,"long-path model" and"short-path model".The determination of model parameters—initial length of cable,deformed length of cable,shear angle—are discussed.In addition,the effect of shear zone thickness,shear displacement magnitude,and shear angle is discussed.Finally,the proposed method is applied to analyze the results of a documented large-scale field shear test on granular material,and,hence,the method is validated.(7)The coupling between fiber and soil is analyzed for the main deformation zone in the Shengze(Suzhou)borehole.The critical CPs are calculated to be 0.82 kPa and 120.49 kPa for the 1 mm radius tight-buffed cable and armored cable,respectively;the corresponding critical depths are 0.08 m and 11.51 m,respectively.This indicates that the cable and soil bonded perfectly for the investigated deformation zone.In addition,the strain transfer coefficients are calculated to be 0.989 and 0.824 respectively for the two cables.These are used to revise the FO data monitored by the two cables,showing that the revised cable readings are consistent.Furthermore,the strain response of the main deformation zone is analyzed,indicating that the compaction of the overlying and underlying aquitards(the former in particular)contributed mainly to the subsidence between December 2012 and November 2014.Afterward,the compaction was reduced in terms of both extent and amount,and so the total subsidence reached stable.(8)Strain profiles along a borehole-embedded FO cable reveal two sliding surfaces within the Majiagou landslide(Three Gorges Reservoir region).The proposed method is employed to calculate the shear displacement along the deeper surface,and the calculation is consistent with inclinometer measurements.In addition,the landslide deformation is strongly correlated with the fluctuation of water level.This indicates that the fluctuation of water level of the Three Gorges Reservoir is a dominant factor controlling the deformation of the Majiagou landslide.