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 in 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’s 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 a 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 are longevity, resistance to a corrosive environment, electromagnetic immunity and possibility to chain up to twelve FBG on a very long optical single measurement channel.
The GNSS monitoring system allows surveying 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 installed 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, including a robust camera taking pictures of the traffic passing on the bridge.
Displacement measurements are correlated and compared with environmental effects data from the weather station (Wind speed and direction, Temperature gradient) and live camera (estimated traffic loads) to characterize the structural behavior of the bridge over the 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 data logger gathers the data coming from all the sensors via a full optical fiber network. It process and send the data to a secured server to display data through the THM Insight© web interface. There are two main visualization 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 periods. It can show the seasonal effect, traffic effect, aging effect or any other environmental effects.
The system will allow measuring long and short term deformations, such as :
- Foundation settlement
- Bridge deck creep
- Stress relaxation
- Wind effect
- Thermal expansion
- Traffic loads.
En parallèle de l’intervention sur le pont Wadi Leban, Cementys a installé un système de surveillance similaire sur le pont Gulf situé en plein centre de Riyad.
Trente-trois capteurs à fibre optique et trois GNSS ont été nécessaires, à des positions stratégiques choisies en collaboration avec le Ministère des Transports saoudien.
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 systems 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 for 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 to warn abnormal states and give data for smart maintenance (Condition Based Maintenance).