Some innovative developments of TDR technology for geotechnical monitoring

Up-hole time domain reflectometry (TDR) devices that interrogate passive mechanical transducers are advantageous for engineering monitoring in situ. This technology is based on transmitting an electromagnetic pulse through a coaxial cable connected to a sensing waveguide and watching for reflections of this transmission due to changes in characteristic impedance along the waveguide. Depending on the design of the wave guide and analysis method, the reflected signal can be used to monitor various engineering parameters. This paper introduces some recent developments to improve the understanding and extend the usefulness of TDR technology. TDR waveguides for engineering monitoring are grouped into three categories according to the sensing principle, namely interface type, crimp type, and dielectric type. New design concepts and data reduction procedures for each type of waveguide are introduced. These developments improve and lead to a variety of transducers, such as water level, deformation, rainfall, scouring, soil moisture, sediment concentration, and electrical conductivity. The versatility and merits of TDR are highlighted by examples of hybrid economical TDR monitoring systems composed of a single TDR device and multiple sensing waveguides.