The North County Transit District (NCTD) of San Diego County, California, determined that three segments of railway track above coastal bluffs in Del Mar should be monitored for dangerous slope movement. This led to the development of a continuous monitoring system along 3,300 ft (1,000 m) of track. Because of the length to be monitored, the use of conventional single-point monitoring systems such as multiple tiltmeters or in-place inclinometers (IPIs) was impractical. Bluff failure could occur anywhere along the track, so the use of single-point instruments would require a large number of sensors and monitors. Instead, it was proposed to install horizontal time-domain reflectometry (TDR) coaxial cable sensors along high-concern segments of the track.
The TDR monitoring system works similarly to radar. It uses a coaxial cable as a sensor, with any slope movement deforming or shearing the cable at the location of movement. When a voltage pulse from a reflectometer encounters a deformation, or the end of the sensor, the amount of reflected energy is proportional to the extent of the deformation. The reflectometer accurately determines the location of the deformation and the relative extent of movement as noted by the magnitude of the reflection.
The monitoring system consisted of a central control and monitoring unit and three TDR monitoring stations. The central monitoring unit checked the status of each monitoring station and controlled the alert notification function of the system. The TDR monitoring stations each polled two TDR sensor cables for cable deformation or break.
At each monitoring station were a Campbell Scientific CR10X datalogger, SDMX50 TDR multiplexer, and TDR100 reflectometer. An automated telephone dialer notified railway personnel of possible bluff movement. Personnel could then contact the system by telephone and determine the location of the cable deformation so that a safety inspection of the bluff and track could be made. Each datalogger was connected to a Campbell Scientific MD9 multidrop interface. RG59 coaxial cable was used to connect these network nodes. At the central monitoring station, an additional MD9 accessed the network dataloggers through the telephone line.