Mechanical coupling between borehole backfill and fiber-optic strain-sensing cable

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.