Study On Influence Of Backfill On Coupling Between Fiber And Sandy Soil In Borehole-based Full-section Monitoring Of Land Subsidence

Land subsidence can cause a series of secondary disasters,which can seriously lead to the destabilisation of buildings,endanger people's lives and property and limit urban development.With the development of distributed fiber optic sensing(DFOS),full-section monitoring of land subsidence provides a new technical means for land subsidence monitoring.The coupling between soil,borehole backfill and fiber optic cable is vital factor affecting the validity of groud settlement data monitored by DFOS.The choice of borehole backfill material and method directly affects the coupling.The backfill is usually a sandy soil when the borehole is located in loose Quaternary soil.Based the pullout tests and numerical modelling in kinds of sandy soil sample,the influence of grain size,gradation and cable diameter on coupling of cable-sand is analyzed systematically.Combined with the groud settlement data montiered by automatic land subsidence monitoring system in Hengshui,borehole backfill quality and data validity are analysised quantitatively.The conclusions of the paper are summarized as follows:(1)The effect of backfilling with different slurry concentrations and backfilling methods is investigated through simulated backfilling tests.Adopt the way of interval backfill and the backfill volume should not exceed 1.0 kg,the local stratification influence can be ignored,and meet the sampling resolution of BOTDR.The mass ratio of water to bentonite should be not less than 1?0.15 to avoid the slurry concentration being too high or too low which may cause difficulty in settling or particle sorting of the backfill.To avoid the slurry concentration being too high or too low which may cause difficulty in settling or particle sorting of the backfill,the ratio of water to bentonite should be not less than 1?0.15.(2)Based on the interfacial shear stress at cable-sand interface,the evaluation of shear damage is proposed.Effects of grain size,gradation,compactness and water content on the interface shear strength was analysed based on Coulomb's law.The indexes of cable-sand coupling coefficient and strain propagation depth are proposed to quantitatively describe the coupling.(3)The pullout tests are carried out in sand soil sample with different grain sizes and gradations.The test results show that the coupling of cable-sand is negatively correlated with the grain size in a single grain size sandy soil.In coarse and fine mixed sand,the coupling is positively correlated with the fine particle content when the fine particle content is between 10%and 30%,and negatively correlated above 30%.Soil moisture in sandy soil reduces coupling(4)Using the discrete element numerical simulation software Mat DEM,a two-dimensional discrete element pull-out test model is established and simulated pull-out tests are carried out on cables with diameters of 1 mm,2 mm,4 mm and 8 mm.The coupling is positively correlated with the cable diameter.(5)The automatic land subsidence monitoring system based on weak-reflection fiber gratings is proposed and established in Hengshui,China.Combined with the study on the coupling of sandy soil and fibre optic cable,when the borehole depth is below26 m,z_c-s>0.75,cable has a strong coupling with borehole backfill,and the soil deformation data is accurate.The research results of this paper provide a new theoretical basis and practical guidance for the study of borehole backfill in fiber monitoring of land subsidence.