COASTAL BLUFF MONITORING/ALERT SYSTEM FOR RAILWAYS
William F. Kane1, Gary R. Holzhausen2, Etienne Constable2
ABSTRACT
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
1
KANE GeoTech, Inc., Stockton, California
2
Applied Geomechanics, Inc., Santa Cruz, California
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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.
INTRODUCTION
When it is desired to monitor slope movement along a segment of a transportation
facility such as a roadway or railway, many vertical inclinometers must be installed to
ensure adequate protection. This is especially true in the area of slope monitoring where
single inclinometer values are usually spaced over a significant period of time and might
miss a significant movement or trend. Besides the singular nature in time of inclinometer
readings, they are also singular in space.
An alternate approach to a standard inclinometer installation is the use of an automated
and remote data collection and monitoring system using time domain reflectometry. The
use of automated and remote data collection and monitoring equipment for geotechnical
applications is growing rapidly. Instead of providing an isolated reading at a single point
in time, automated data collection can record data at small intervals and allow for trends
to be determined relatively quickly. Time domain reflectometry allows monitoring over
a continuous length rather than at discrete points. Coupling an automated data collection
system with an alert based on threshold values can be very valuable.
The singular nature of the inclinometer can be eliminated by installing a horizontal
sensor, capable of determining movement at any point, along the length of the
transportation facility’s exposure to the slope
The North County Transit District (NCTD) of San Diego County, California
determined that three segments of track above coastal bluffs in Del Mar, California
should be monitored for possible movement. Although the bluffs appear stable there had
been some erosional problems that had required soldier pile walls, drainage
enhancements, and shotcrete reinforcement. In addition, over fifty years ago, one
segment of the bluff had failed causing the wreck of a freight train.
SITE DESCRIPTION
Overview
The monitored areas consisted of sections of coastal bluff prone to erosion and
relatively small block failures. The bluffs were about 15 m above the beach. Railroad
tracks along the bluffs ranged from as little as 6 m to as much as 30 m from the edge of
the bluff. Soldier pile walls had been installed in sections where the distance to the edge
of the bluff was small.
General Geology
The bluffs consisted of the Eocene Torrey Sandstone and the slightly older Eocene Del
Mar Formation. At this location, the Torrey Sandstone was a relatively strong, but
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fractured and jointed sandstone, while the Del Mar Formation was soft and erodible
siltstone. Wave action and a significant amount of ground water from landscape
irrigation contributed to weakening and softening of the siltstone. The result was the
undermining of the Torrey Sandstone and local coastal bluff failure.
INSTRUMENTATION OVERVIEW
The remote data acquisition equipment included a datalogger, multiplexer,
communication devices, and a power source. In addition, software wss necessary to
program and interact with the datalogger. Only the equipment used in the case studies
is described here. The reader is encouraged to investigate other manufacturers and
approaches.
Time domain reflectometry
Time domain reflectometry (TDR) is a relatively new approach to monitoring slope
movement. Originally developed to locate breaks and faults in communication and
power lines, TDR is used to locate and monitor slope failures. This technology uses
coaxial cable as the sensor and a time domain reflectometer for measurement.
The basic principle of TDR is similar to that of radar. The reflectometer sends an
electrical pulse down a coaxial cable grouted in a trench or borehole. When the pulse
encounters a break or deformation in the cable, it is reflected. The reflection shows as
a “spike” in the cable signature. The location of the spike indicates the location of cable
damage. The size of the spike increase correlates with the magnitude of cable
deformation. Cement grout or lean concrete is necessary to insure that any earth
movement is transferred to the sensor cable. The brittle, rigid grout will crack under
even small movements and deform the cable enough to cause a reflection.
Datalogger
A datalogger is essentially a small computer CPU/voltmeter with memory. It is
programmed to do certain tasks, such as applying specified voltages over certain
durations, reading voltages, and storing values. It can also be programmed to do
calculations and store the results, such as converting the readings of a piezometer to
meters of head.
Control ports on the datalogger and excitation ports can be programmed to output
voltages at certain times to turn on peripheral equipment, such as cell phones or cable
testers. Other ports are wired to the sensors and are used to measure output voltages.
Multiplexer
A multiplexer allows many sensors to be attached to a single datalogger. The
multiplexer is wired to a single set of ports on the datalogger. A set of contacts in the
multiplexer switches between each sensor attached to it. The data is collected
sequentially by the logger. Multiplexers can even be multiplexed to each other creating
the ability to read a very large number of instruments.
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Communications
Communications with the datalogger can be by several means. “Hardwired” telephone
lines are typically the most reliable, but not always available. A telephone line only
requires a modem to transmit data and receive instructions. Cellular and satellite
telephones can be used as well as radio transceivers. These methods require a modem
in addition to the wireless telephone or radio.
Power
Power requirements vary depending on the number of instruments and the
communications device. Ideally, power is available at the site but that is not always the
case. A small system with a phone line and one or two sensors requires only a small
rechargeable gel-type battery. A large system with cellular phone and cable tester
requires a larger batteries. Typically 12 V deep cycle batteries are used. The battery is
recharged by regulated solar panels.
Software
In order to program and communicate with the datalogger, software is written that runs
under the datalogger’s operating system. This software allows the user to contact the
remote station, either automatically or manually; monitor instrument readings; and
download data. Once TDR data are downloaded, they can be plotted and analyzed using
applications running under Microsoft Windows.
MONITORING SYSTEM DESIGN
The basic layout of the system is shown in Figure 1. It 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.
Central Control and Monitoring Unit
A simplified schematic of the central control and monitoring unit is shown in Figure
2. An uninterruptible power supply with surge protection supplied power to the unit. It
was recharged by a 120 V AC power source. The power was converted to 12 V DC to
operate a Campbell Scientific CR23X datalogger and three telephone autodialers, one for
each monitoring station.
The purpose of the CR23X was to monitor output voltages from each of the TDR
monitoring stations. Zero voltage on any monitoring station channel meant that there
was no alert condition. If a voltage was present, then an alert condition existed and the
appropriate telephone dialer was activated to make an alert call.
The 120 V AC was also converted to 48 V DC and supplied to each monitoring station
through individual wires enclosed in conduit below and alongside the tracks.
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Figure 1. Central control and monitoring unit schematic.
TDR Monitoring Stations
At each monitoring station the voltage was stepped down to 12 V DC to power a
Campbell Scientific CR10X datalogger, SMX50 TDR multiplexer, and TDR100
reflectometer (Figure 2). The datalogger polled two horizontal TDR cables every four
minutes and compared the reflected waveform to a baseline signature. If the reflected
data indicated a deformed or broken cable, an alert signal, consisting of 5 V DC was
applied to a wire monitored by the CR23X at the central control and monitoring unit.
The datalogger was capable of distinguishing between a low level alert (deformed cable
indicating small movement), and a high level alert (sheared cable and possible large
magnitude slope failure).
TDR sensor cables were routed under the tracks and installed parallel to the tracks in
a shallow trench. The cables were enclosed in a cement/sand grout to ensure deformation
should a slope movement occur. A total of approximately 1000 m of sensor cable was
installed.
Telecommunications
Telecommunications were made using a multidrop network (Figure 3). Each
datalogger was connected to a Campbell Multidrop MD-9 network interface. RG59
coaxial cable was used to connect these network nodes. At the central monitoring station
an additional MD9 was attached to a null modem/power supply connected to a telephone
modem. The telephone modem was used for accessing the network dataloggers through
the telephone line.
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CAMPBELL
SCIENTIFIC
INC.
DC/DC
DC/DC
COM210
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AD- 2000
MULTIDROP INTERFACE
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CAMPBELL
SCIENTIFIC
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CAMPBELL SCIENTIFIC INC.
POWER
SUPPLY
Figure 2. Monitoring system schematic.
In addition to the modem, the three autodialers also used the telephone line. When
activated, the dialers called up to eight telephone numbers and delivered an alert message
applicable to the type of action required.
Construction
Coaxial sensor cables were installed using a tractor-mounted trencher. Trenches were
cut 0.7 m deep parallel to the tracks about 4 m from the track centerline between the
track and the bluff edge. A 50 mm layer of lean concrete was placed on the bottom of
the trench and the sensor cable was laid on top. An additional 0.3 m of concrete was
placed on top of the cable. After the concrete set, the remainder of the trench was
backfilled with compacted fill.
The Central Monitoring Unit and the Remote Monitoring Stations were housed in
stainless steel enclosures with a powder coating colored to blend with the surrounding
soil. The enclosures were mounted on poured concrete slabs with conduit cast into the
slab. Pre-existing conduit buried alongside the track was utilized for communication,
power, and alerting cables.
All instrumentation and power supplies were placed in weatherproof enclosures within
the main enclosure.
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STATION
RED
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CAMPBELL
SCIENTIFIC
INC.
RED
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CAMPBELL SCIENTIFIC INC.
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TDR 100
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Figure 3. TDR monitoring station schematic.
IMPLEMENTATION
Programming
The remote monitoring station dataloggers were programmed to pulse the TDR sensors
every ten minutes with the TDR100 reflectometer. The program then compared the
sensor reflectance to a predetermined value. Cable deformation or failure triggered an
alert condition. This condition caused a datalogger control port to output 5 V DC to one
of the alerting cables. An alert condition was also set if the station power supply fell
below a minimum level.
The Central Monitoring Unit datalogger was programmed to read the alerting cables
and detect a non-zero voltage on any of the cables. When a voltage was detected, the
datalogger activated one of the telephone autodialers depending on the nature and
location of the alert.
Alerts
One aspect of the alerting feature was the ability to call up to eight different phone
numbers to notify personnel of a possible failure condition. Each of the three dialers was
dedicated to one fo the remote monitoring stations. For example, a sensor deformation
on Remote Monitoring Station 2 would trigger calls to eight different individuals to alert
them of a possible slope movement in the vicinity of Station 2.
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Data
Data consisted of digitized TDR signatures that were downloaded automatically by a
computer at the office of the owner’s consultant. Data were plotted periodically to
determine if there were any cable deformations not located by the datalogger alerting
feature.
CONCLUSION
To the knowledge of the authors, this project represents the largest single application
to date of TDR slope monitoring with movement alerting capability. The use of multiple
telephone autodialers to notify key personnel was one of several innovative features of
the project. In addition, the areal extent monitored and the distances that cables had to
be run presented many challenges. TDR-based monitoring has excellent potential for
safeguarding railroads and highways wherever they are threatened by dangerous slope
movements..
ACKNOWLEDGMENTS
The authors would like to acknowledge the following for their assistance and expertise
in helping to make this project successful: Paul McKee, J. C. Baldwin Construction,
Steve Hoyle, North County Transit District, and Rafael Martinez, KANE GeoTech, Inc.
8