Piezometers are employed to monitor construction activity below the ground water surface or where piezometric pressures may influence the stability of structures (e.g., locks, dams, levees, etc.). As such, piezometers are the most pervasive (in terms of numbers) of all geotechnical field instruments. With increasing emphasis on remediation of hazardous and toxic waste sites, use of ground water level monitoring devices is likely to increase. Traditional downhole submerged transducers for monitoring ground water elevations have not proven to be completely reliable and are time consuming and expensive to maintain. Transducers require relatively large riser pipes (usually greater than 25 mm I.D.) and must be calibrated and manually verified downhole.
This paper describes recent advancements in time domain reflectometry (TDR) instrumentation for monitoring ground water elevations and piezometric pressures. TDR takes advantage of the unique approach of pulsing a long coaxial cable and analyzing the reflected voltage signature caused by changes in impedance of the cable when submerged. While this technology has been employed for a variety of applications, recent development of a small, rugged time domain reflectometer that operates at low power has made TDR applicable for long-term field use. A reflectometer and supporting electronics are located at the surface where they are accessible and easy to maintain, and a non-electronic coaxial cable replaces the traditional downhole transducer. Installation is simple and does not require field calibration. Since only the cable is inserted in the hole, it can be employed with riser pipes as small as 12 mm inner diameter.
TDR technology for monitoring ground water is being developed and commercialized through the U.S. Army Corps of Engineers Construction Productivity Advancement Research (CPAR) program with a Cooperative Research and Development Agreement between the U.S. Army Corps of Engineers Waterways Experiment Station and HYPERLABS, Inc. The Infrastructure Technology Institute at Northwestern University is a primary participant in this cooperative research effort. Through the CPAR program a prototype time domain reflectometer has been designed and built and is currently under field evaluation. It is anticipated that this new device will significantly advance the practice of telemetric surveillance of ground water elevations.
Early civil engineering applications of time domain reflectometry (TDR) technology include measurements of ground deformation, such as displacement along rock joints (Dowding, Su and O'Connor, 1989). To deploy this monitoring system, a single solid dielectric, coaxial antenna cable is anchored to the bottom of a borehole and surrounded by an expansive cement grout that fills the hole. At the ground surface, this cable is connected to pulsing and monitoring electronics which record the reflected voltage signal along the entire length of cable. Originally, signals were collected on site with paper strip chart recorders. More recently, use of air dielectric, coaxial cables and parallel wires for measuring water level and piezometric pressure has been described by Dowding, Huang and McComb (1996). Since these cables and wires can be inserted into existing riser pipes, this approach can be employed to retrofit piezometers for telemetric surveillance.
