Theoretical analysis of mechanical coupling between soil and fiber optic strain sensing cable for distributed monitoring of ground settlement
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摘要: 地层、钻孔回填材料和传感光缆之间的耦合性是决定地面沉降钻孔全剖面分布式光纤监测精度的关键因素。提出了地层-钻孔回填材料-传感光缆耦合性的明确定义和地层变形光纤监测数据有效性的评价方法。地层、回填材料和传感光缆达到完全耦合应满足两个条件:地层-回填材料界面与回填材料-传感光缆界面剪应力均不能超过其抗剪强度;地层应变能有效地传递至纤芯。提出了回填材料与传感光缆处于黏结状态的临界围压与临界深度;传感光缆弹性模量、半径或最大应变梯度越小,则临界围压或深度也越小;当回填材料、传感光缆及其界面参数确定时,临界围压或深度仅与最大应变梯度有关。基于经典应变传递模型与Goodman假设建立了地层-钻孔回填材料-传感光缆应变传递模型,并研究了传感光缆、回填材料以及地层性质对应变传递特性的影响,结果表明:传感光缆半径和弹性模量、钻孔半径、地层剪切模量越小,或回填材料剪切模量、地层-回填材料界面黏结系数越大,则应变传递性能越好;围压对应变传递特性的影响则取决于回填材料与地层参数之间的相对关系。最后,采用苏州盛泽地面沉降光纤监测数据验证了该评价方法的有效性。研究结果为钻孔地层剖面的全剖面、精细化分布式光纤监测提供了理论依据。Abstract: The mechanical coupling between soil and fiber optic cable is vital to the validity of ground settlement data monitored using distributed fiber optic sensing (DFOS). Here a perfect stratum-backfill-cable coupling is clearly defined—the interface shear stresses do not exceed the strengths, and the strain transfers efficiently from the strata to the fiber core. The critical confining pressure and the critical depth are proposed to characterize the backfill-cable interface adhesion. The cable with a low Young's modulus or a small radius corresponds to a low critical confining pressure or depth. Given the backfill and cable properties are known, the critical confining pressure or depth is solely dependent on the maximum strain gradient. Based on the classical strain transfer model and the Goodman's hypothesis, a theoretical model is established to quantify the stratum-backfill-cable strain transfer efficiency. A comprehensive parametric analysis is carried out to investigate the influences of cable, backfill and strata properties on the strain transfer coefficient. Finally, the proposed method is validated using the field monitoring data collected from a DFOS-instrumented borehole in Shengze (Suzhou, China). This study may provide a sound basis for monitoring the ground settlement using the DFOS technique.
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