Coastal Bluff Monitoring/Alert System for Railways

The desire to monitor a section of coastal bluff for slope movements along the North County Transportation District/Amtrak tracks in Del Mar, San Diego County, California led to the development of a continuous monitoring system along approximately 1000 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 deemed impracticable. Since bluff failure could occur anywhere along the track, the use of single-point instruments would require a large number of instruments, with the possibility that a movement could still occur between points and be missed by the 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 similar to radar. It uses a coaxial cable as a sensor grouted in a trench between the bluff edge and the tracks. Any slope movement will deform or shear the cable at the location of movement. A reflectometer sends a voltage pulse along the sensor. When the pulse encounters a deformation, or the end of the sensor, some or all of the energy is reflected. The amount of reflected energy is proportional to the extent of the deformation with all energy reflected from the end of the sensor. The reflectometer accurately determines the location of the deformation and the relative extent of movement as noted by the magnitude of the reflection. The Del Mar Bluffs TDR monitoring system used three on-site dataloggers and reflectometers to monitor three different sections of the bluff. Each reflectometer was connected to two sensors about 175 m long through a multiplexer. The sensors were pulsed every several minutes to determine if sensor deformation had occurred. In the event of a deformation, a signal was sent to a central monitoring unit where 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.