LOOPING TDR CABLES FOR INCREASED SENSITIVITY TO EXTENSION

Changes in coaxial cable geometry that result from shearing, crimping, extending or kinking cause voltage reflections at the point of faulting. While coaxial cables are often employed as sensors to measure the magnitude of shear deformations with time domain reflectometry (TDR), they are much less responsive in tension. This paper describes a unique approach to increasing the sensitivity of coaxial cables in tension by prescribing loops along the cable length. A series of laboratory tests are performed on several flexible cables with braided shields, with and without polymeric jackets. These cables have a minimum radius of curvature of no more than 3 cm. When cables are bent such that the loop radius is decreased below the minimum radius of curvature, the cable kinks at the loop. As the radius of curvature progressively decreases, tests have shown that the voltage reflection directly increases. Since the loops are compressed when the cable is extended from one end, looped cables may be used to measure and monitor tensile displacement. Through a controlled set of experiments, correlations are developed among the tensile load and displacement, loop radius and voltage reflection as the cable is extended. Rigid cables with solid shields require a much larger radius of curvature to prevent kinking, and thus are not considered feasible for field use and are not pursued in this study. Potential applications of looped TDR cables include measuring soil settlement, recording movements in reinforced earth walls, and monitoring movements beneath bridge pier footings that result from bridge scour.

C.E. Pierce
Assistant Professor, Department of Civil and Environmental Engineering
University of South Carolina
Columbia, South Carolina

C.M. McAlister
Graduate Research Assistant, Department of Civil and Environmental Engineering
University of South Carolina
Columbia, South Carolina