Light Rail Tunnels
Kuala Lumpur, Malaysia: Real-time monitoring of two light rail transit (LRT) tunnels is providing safety for commuters while major road construction takes place just a few meters overhead.
Kuala Lumpur City Hall is constructing an underpass to carry traffic on Tun Razak Road beneath Ampang Road. The foundation of the underpass will be just 4.5 meters above the crowns of two LRT tunnels. Both the roads and the transit tunnels must continue operation during construction, which is expected to take about 21 months.
The project involves installation of sheet pile walls and steel decking for a temporary road surface, ground improvement including dewatering, jet grouting, and ground anchors, and excavation to 20 meters below grade. Since these activities could affect the LRT tunnels, a comprehensive suite of instrumentation has been installed in and around the tunnels.
The objectives of the instrumentation are to monitor movements of the tunnels and the ground around them, to provide early warning of potentially damaging movements, and to permit safe operation of the LRT system as construction proceeds.
The primary monitoring system, which is operated continuously in near-real time, consists of Bassett convergence systems and EL beam sensors. The Bassett systems are installed at four locations in each tunnel to monitor cross-sectional deformations. The beam sensors are installed along the crown of each tunnel, to monitor vertical movements of the tunnel. Computers at the site office retrieve data from the data loggers every five minutes and MultiMon software processes the readings and updates graphic displays.
A typical display shows the outline of the tunnel with readings superimposed at the location of each sensor. Clicking on a reading pops up a 5-day trend plot. A replay function allows readings to be reviewed off site.
If a reading exceeds an alarm threshold, MultiMon pops up a color coded warning, showing the severity of the condition. The same displays and alarms are duplicated at the LRT control center, which is off site.
If the condition warrants, key personnel are paged automatically with a message that contains sensor the IDs, data values, and alarm type that triggered the alarm. Alarm thresholds and actions to be taken are outlined in a contingency plan developed by the contractor, LRT engineers, underpass designers, and City Hall.
Secondary instrumentation consists of an automatic total station and 130 prisms mounted on the tunnel lining. Surveys are obtained on a regular schedule to supplement the primary system and also when an alarm is triggered. However, the secondary system can be used only between 1 and 4 o'clock in the morning, when trains are not running.
Construction is underway
The monitoring of the tunnels commenced one month before construction started so that the baseline behavior of the tunnels could be recorded. At the time this story was written, the instrumentation system had been operational for two months and construction was in progress. The critical period - during excavation to the foundation level for the underpass - is still ahead. It is expected to last for six months. Monitoring will continue until the completion of the project.
Thanks to Mr Munning Jamaludin of CE Instruments for providing this story. As Slope Indicator's distributor for Malaysia, Mr Jamaludin and his company supplied and supervised installation of the Bassett Systems, EL beam sensors, data loggers, and MultiMon.