Wadi Leban Bridge Project

Wadi Leban Bridge Project

At the end of 2018, Cementys installed with success for the first time a monitoring system combining GNSS/Insar technologies and optical fiber sensors on Wadi Leban Bridge in Riyadh, Kingdom of Saudi Arabia.

Wadi Leban Bridge is a cable-stayed bridge located at the south-west of Riyadh. It connects Riyadh to its south suburbs hence a lot of traffic is crossing the bridge. Thereby, monitoring such a very busy bridge through seasonal and traffic effects is a crucial issue.
During last year intervention, Cementys did an upgrade of the still running monitoring system that was installed in 2014 by adding more optical fiber sensors and a brand new GNSS/Insar measurement combination at strategic location provided by the Ministry of Transport and the King Saud University.

The optical fiber sensors are based on the Fiber Bragg Grating (FBG) technology. A grating similar to a « barcode » is photo-inscribed in the fiber core. A wide spectrum light is injected into the fiber and when passing through the sensor, the gratings will reflect a certain wavelength proportional to the step of the grating. Three types of sensors were installed (displacement, strain or tilt sensors). The advantages of FBG technology is longevity, resistance to corrosive environment, electromagnetic immunity and possibility to chain up to twelve FBG on a very long optical single measurement channel.

GNSS monitoring system allows to survey 3D displacements (Easting, Northing and Up) of the bridge components. Three receivers were installed to survey pylons and mid-span deck.

They provide regular and continuous measurements of :
• The Inclination and potential settlement of the pylons.
• Relative and absolute displacements of the pylons in lateral & longitudinal directions.
• Deck deflection
• Accelerations, Vibrations, Modal frequencies can be also measured by adapting the system to high frequency measurements (10Hz)

In total, there are twenty-eight optical sensors insytalled on the bridge : two displacement sensors (one at each joint), four tiltmeters (two on each pier measuring tilt on bridge axes and transverse axes) and twenty-two strain sensors measuring tension in cable stays. There are five GNSS/Insar measuring points and a weather station measuring temperature, wind speed, wind direction and relative humidity, inclusing a robust camera taking picture of the traffic passing on the bridge.

Displacements measurements are correlated and compared with environmental effects data from weather station (Wind speed and direction, Temperature gradient) and live camera (estimated traffic loads) in order to charaterize the structural behaviour of the bridge over service life.

The system is fully automatic using a power system essentially based on solar panels and batteries. In addition to the GNSS monitoring system, Radar Corner Reflectors were also installed in collocation with GNSS antennas to experience monitoring the bridge by radar interferometry (InSAR).

A datalogger gathers the data coming from all the sensors via a full optical fiber network. It process and send the data to a secured server in order to display data through THM Insight© web interface. There are two main visualisation modes :

The Real Time Display mode allows the user to see the last measurement taken and see the current state of the bridge.

The Historical Data mode allows the user to see the evolution of data through long time periods. It can show the seasonal effect, traffic effect, ageing effect or any other environmental effects.

The system will allow to measure long and short term deformations, such as :

  • Foundation settlement
  • Bridge deck creep
  • Stress relaxation
  • Wind effect
  • Thermal expansion
  • Traffic loads.

In parallel, Cementys did a similar upgrade with Gulf Bridge located in the center of Riyadh. In total thirty-one optical sensors and three GNSS system were installed at strategic locations decided in collaboration with the Ministry of Transport and the King Saud University.

Structural Health Monitoring is a useful tool in bridge maintenance and management. Monitoring the bridge deformation is essential to detect and predict changes in the material and/or geometric properties and performance.

The system THM Insight© collects continuous displacements measurement, environmental parameters and operating conditions for the purpose of warning abnormal states and giving data for smart maintenance (Condition Based Maintenance).

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