Theoretical analysis of mechanical coupling between soil and fiber optic strain sensing cable for distributed monitoring of ground settlement

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.