Fiber-Optic Sensors at the Circle Line

April 24, 2011 | By More

Singapore MRT map showing Circle Line (yellow) stations under construction

Following the Nicoll Highway collapse in 2004, the Singapore Land Transport Authority (LTA), who is responsible for all road and public transportation in Singapore, engaged the predecessor of Alphatech to do a large scale test bed of their sensor system based on fiber optic technology developed at NTU (read more about Fiber Optic Sensors).

The company had already validated much of their technology in several commercial projects around Singapore (you will read more about them later). But this time, in order to prevent a further incident like that at Nicoll Highway, the LTA is taking no chances. A ‘belt-and-braces’ approach was taken with the fiber-optic system.

Telok Blangah excavation challenges

Contract 856 includes of excavation and construction of Telok Blangah, Labrador Park, Pasir Panjang and West Coast MRT stations. The challenge at Telok Blangah is the proximity to an existing overhead expressway, the Pasir Panjang Semi-Expressway (PPSE). Any movement to the ground below due to the excavation can be catastrophic to the overhead expressway.

I-beam struts are the most important structures in the excavation of any hole measuring 85m wide. They keep the walls from collapsing onto itself as the excavation increases in depth – up to 20m. Should there be any problem with the retaining walls (of the hole), these struts will become deformed, as was seen at Nicoll Highway excavation days before the eventual collapse.

PPSE shown in the background (left), and i-beam struts keeping the hole open (right)

The risks at Telok Blangah were enormous as the overhead PPSE was only 10.5m from the excavation. A conventional vibrating-wire sensor system was already part of the tender and being put in place by an existing vendor.  LTA wanted to give fiber-optics a chance.

Installing inclinometers to a depth of 30m

First, the retaining wall need to be continuously monitored along the side of the overhead semi-expressway. A second-generation in-place inclinometer system was designed for this purpose using patented fiber-optic technology to continuously monitor along the wall of the excavation up to 30m in depth.

As the struts are the most important indicators of impeding problems, different strategies were deployed using multiple sensors. Along the length, fiber-optic strain gauges were placed to measure longitudinal stresses transmitted from the retaining wall. Fiber-optic deformation sensors are also to be placed at regular intervals to monitor torsional stresses should the I-beam struts warp. Additionally, fiber-optic profilometers were also installed to measure their curvature should the walls be causing it to compress.

Deformation sensors (left), Profilometers (center), and Strain Gauges (right)

Altogether, 470 fiber-optic sensors were deployed at the three sites – Telok Blangah, Pasir Panjang and West Coast MRT station sites – deploying 79 inclinometers, 212 strain gauges, 64 temperature sensors, 114 profilometers and 21 deformation sensors. The system included a web-based reporting and SMS alert system for real-time response should safety limits be reached.

At the conclusion of the test bed, LTA commented that fiber-optic sensors provided comparable results to conventional vibrating-wire sensors but were less susceptible to interference and so fluctuated less. This implies fiber-optic sensors are less likely to cause spurious alarms and therefore more valuable for real-time monitoring.

Actual data readings comparing vibrating-wire with fiber-optic sensors

This large scale test bed was concluded on June 2008.  Throughout its duration, the project was independently monitored by Singapore’s Defence Science and Technology Authority (DSTA) who published a final Project Report (PC09-59-67) on Feb 2009.

For more information about this and other projects, contact us directly.

References:
History of the Nicoll Highway accident and the subsequent investigation
.
Detailed presentation summary of the findings of what caused the accident.

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Category: Projects, Sensor Systems

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About the Author ()

EngTong, pioneer and innovator. Graduated from Imperial College London with an MBA from Cranfield School of Management. Lived in Scotland, England, California, Beijing and led teams in Italy, France, Japan, Taiwan and Malaysia to do the impossible. Now based in Singapore and believes the future is to blend the sophistication of western management practices with the strength of Asian Values. Trained as a Chartered Engineer. Member of IET, Associate of City and Guilds and a certified SixSigma Champion.

Comments (4)

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  1. RW says:

    Dear Sirs,

    This article is quite interesting, and I wonder what kind of optical fiber is used? Glass, single mode? Plastics?
    We are very much interested in this new technology.

    Sincerely,

    • Error: Unable to create directory uploads/2017/06. Is its parent directory writable by the server? ET says:

      Thank you for your interest and reading. We use single mode glass fiber, same one as used in telecoms.

  2. Joseph Chiang says:

    Where can I find a final Project Report (PC09-59-67)of said projects

    • Error: Unable to create directory uploads/2017/06. Is its parent directory writable by the server? ET says:

      Joseph,
      This is a DSTA report. You should contact them.