Mechanical coupling between borehole backfill and fiber-optic strain-sensing cable
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摘要: 钻孔回填料与直埋式应变传感光缆之间的耦合性,是决定分布式光纤感测（DFOS）技术能否利用一个钻孔精细化监测钻孔地层剖面变形分布的关键。利用自主研制的可控围压光缆-土体相互作用特性试验装置,探究了0~1.6 MPa围压下传感光缆与松填砂土以及击实砂-黏混合土之间的耦合性。试验与理论分析结果表明：在拉拔状态下,光缆-土体界面呈现渐进性破坏特征;在低围压下,光缆轴向应变随拉拔位移的增大而增大,且不断向光缆尾部传递;在高围压下,应变的扩展与传递被限制在很小的范围内;理想弹-塑性拉拔模型可较好地描述光缆-土体界面的渐进性破坏特性。提出采用光缆-土体耦合系数ζc-s定量描述光缆与土体之间的耦合性,将10000 με下的ζc-s值作为评价长期监测条件下两者耦合性的指标,并根据0.5,0.75,0.9三个ζc-s值将两者的耦合性分为强、较强、较弱以及弱4类。一定围压下,光缆与土体具有强耦合性;对于松填砂土与击实砂-黏混合土,该临界围压值分别为0.55 MPa与0.17 MPa。以苏州盛泽地面沉降为例,分析了钻孔回填料与传感光缆之间的耦合性,结果表明：距地表约16 m深度以内两者具有较强耦合性;而约16 m深度以下两者具有强耦合性,传感光缆的应变数据可准确反映地层的变形。这一研究成果为DFOS技术应用于钻孔剖面地面沉降监测提供了坚实的理论依据。Abstract: The mechanical coupling between borehole backfill and fiber-optic strain-sensing cable is the key factor affecting distributed fiber-optic sensing (DFOS)-based land subsidence monitoring. A new pullout apparatus is designed to investigate the interaction mechanism between cable and soil under confining pressures (CPs) ranging from 0 to 1.6 MPa. The test results and analytical analyses show that the cable-soil interface fails progressively under pullout conditions. Under low CPs, the axial strain increases and propagates toward the cable toe under increasing pullout displacements. In contrast, the propagation of strain is restrained around the cable head under high CPs. The ideal elasto-plastic pullout model can reasonably describe the progressive failure behavior of the cable-soil interface. A new coefficient is proposed to characterize the cable-soil mechanical coupling for long-term monitoring purposes (the maximum axial strain of 10000με), together with a classification of the mechanical coupling based on this coefficient. The case of the Shengze land subsidence in Suzhou of China is presented to illustrate how these findings can be applied to the field. The analyses demonstrate the strong coupling of the cable to the borehole backfill below a depth of 16 m. It may provide a sound basis for monitoring land subsidence using the DFOS technique.
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