EXPERIMENTAL STUDY ON MECHANICAL COUPLING BETWEEN ANCHORED STRAIN SENSING OPTICAL CABLE AND SOIL DEFORMATION UNDER LOW CONFINING PRESSURES

The mechanical coupling between strain sensing optical cable and soil deformation is vital to determine the accuracy of distributed fiber-optic strain data. An anchored strain sensing optical cable is designed to enhance the cable-soil mechanical coupling for the use in geotechnical model test. Using a newly devised pullout apparatus, optical cable pullout tests are carried out to explore the influence and mechanism of anchor on the cable-soil coupling under low confining pressures. Test results show that the axial strain increases under increasing pullout displacements, but the strain propagation is limited to a length of 0.35 m. The anchored strain sensing optical cable requires more pullout forces than the unanchored cable under the same pullout displacement. The anchor averages the measured strain within a range of 0.05~0.075 m near the anchor. An increase ratio is proposed to characterize the effect of anchor on enhancing the cable-soil coupling. The ratio is calculated to be 97.41%in this test, which indicates that the anchor can effectively enhance the mechanical coupling between cable and soil. A mechanical model is proposed to simulate the pullout process of anchored strain sensing optical cable in loose sand, which accurately describes the relationship between pullout force and anchor force and surface friction of cable. This study provides a basis for the application of distributed fiber-optic monitoring technology in geotechnical model tests.