The new bridge requires a new piled steel retaining wall to be built further East of the existing wall prior to the removal of the existing wall and the construction of the bridge tunnel. The proposed piled wall is within a close proximity to the existing retaining wall, and it is anticipated that piling activities onsite will have some impact on the existing retaining wall.

MONITORING OBJECTIVE

The structural health monitoring is to provide 24/7 observation of the existing piled steel retaining wall, identifying any changes in the structures behaviour and position, which would suggest a loss in the structure’s stability and present risk of structural failure ahead of the planned demolition of the wall. The monitoring sensors are then to be moved to the proposed wall to monitor its behaviour and stability during the remaining works onsite and completion of the bridge.

INSTALLATION CONSIDERATION

SENSORS: The sensors needed to be moved from the existing retaining wall to the proposed retaining wall once construction was complete. To extend the range of observation of the tilt sensors and prevent the sensors simply measuring variation in the surface of the steel piles in localised areas due to temperature variation, the tilt sensors were to be installed on 1m tilt-beams.

POWER: The site's power is provided by an onsite generator. The generator is turned off overnight and during weekends. The monitoring system required 24/7 power to provide continuous monitoring data.

MONITORING SYSTEM

• 16no. Triaxial Tiltmeters

• 12no. Triaxial SHM Accelerometers

• 1no. Communications Gateway

OUR COMPONENTS

• 28no. Magnetic Mount Kits

• 12no. 1m Magnetic Tilt-beams

• 1no. Back-Up Power System

• 1no. GNSS Wall Bracket

The tiltmeters and SHM accelerometers were installed along the length of the piled steel retaining wall to provide comparable data on the position of the wall and the frequency response of the wall during and after the piling works. The sensors were installed using our magnet mounts so that the system could be transferred to the proposed piled steel retaining wall once completed, to monitoring its behaviour during the remaining construction activities onsite. Our back-up power system ensured the sensors and gateway stayed online 24/7, taking over powering the gateway when the generator was switched off, and recharging when the generator power returned.

PROJECT NOTES

The monitoring system installation was completed in 6 hours including the assembly of the tiltmeters to the tiltbeams and SHM accelerometer to the magnet mounts, which was completed onsite due to the short lead time for the system installation. All sensors were online and communicating with the Gateway within 20 minutes of the Gateway being powered.

The monitoring installation will be in situ for approximately 12 months in total with the sensors being repositioned on to the proposed piled steel retaining wall once completed.

CONCLUSION

Utilising our magnetic mount kits enabled the sensors to be quickly and easily relocated to the new retaining wall without the onerous task of undoing the fixings from the old wall and re-drilling and fixing to the new wall.

As a coincidental benefit, the sensors were regularly knocked and hit by site workers and the compacting machinery as the channel they were operating in was less than 1m. Had the sensors been drilled in to the old retaining wall, the sensors would likely have been damaged. As they were magnet mounted the sensors were simply knocked off when hit hard and were able to be repositioned undamaged.

SUPPORT

We attended the site to support the installation and revisited some weeks later to resolve an issue with the back-up power system. The vibrations from the compacting works hard worked loose a wiring connection within a sealed unit which led to the battery inconsistently recharging. The system was completely checked and all connections tested to, as far as possible, prevent a repeat occurance.