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 waveguide and analysis method, the reflected signal can be used to "sense" various engineering parameters. When embedded in a geotechnical structure, TDR can be seen as a "geonerve" system. New design concepts and data reduction procedures for each type of sensing waveguide are introduced in this article. The versatility and merits of TDR are highlighted by a TDR slope monitoring system example composed of a single TDR device and multiple sensing waveguides.
Time domain reflectometry (TDR) is a principle of measurement based on a cable radar (formally called time domain reflectometer) and sensing waveguides. The cable radar transmits an electromagnetic pulse through a coaxial cable connected to a sensing waveguide and watches for reflections of this transmission due to changes in characteristic impedance along the waveguide. It was originally developed for detection of cable faults and later applied to dielectric spectroscopy in physical chemistry. In the past two decades, TDR technology has been adapted to geotechnical applications. A basic overview of TDR technique can be found in a book by O'Connor and Dowding (1999). This article introduces some further developments that improve the understanding and extend the usefulness of TDR technology in geotechnical engineering. Depending on the design of the waveguide and analysis method, the reflected signal can be used to "sense" various engineering parameters. When embedded in a geotechnical structure, TDR can be seen as a "geo-nerve" system. In particular, an example of an integrated TDR monitoring system composed of a single TDR device and multiple sensing waveguides are given in the context of slope monitoring.