Geotechnical Monitoring of a Heritage-Listed Bridge Underpass in Brisbane
RPS, a leading global professional services firm with over 200 surveyors across Australia, was recently commissioned by the Brisbane City Council in Queensland, Australia, to monitor the stability of the heritage-listed Story Bridge. For this task, RPS utilised an automatic geotechnical monitoring system from Trimble® during a recent construction project.
Pedestrian bridge (circled) under construction, left. The monitoring site (arrow) is to the right.
Opened in 1940, the steel cantilever Story Bridge spans the Brisbane River, connecting the northern and southern suburbs. In 2023, a new underpass was added on the southern side to improve access for pedestrians and cyclists to the Kangaroo Point Green Bridge.
The Project Brief
Led by Nick Morey, surveyor and practice leader for RPS in Brisbane, the RPS team was tasked with ensuring the stability of the Story Bridge during construction of the new underpass. Their primary goal was to monitor potential shifts in the bridge’s columns caused by the excavation work.
The Solution
The RPS team, in collaboration with Trimble Authorised Dealer UPG, deployed seven Trimble LaserTilt90 devices on key columns around the site, focusing on areas most vulnerable to the excavation. These devices were programmed to take hourly measurements, with alerts set to trigger if any reading exceeded a pre-determined threshold. The sensors transmitted data via an on-site router to Trimble 4D Control (T4D) software, where it was collected, analysed, logged, and used to manage the alert system.
The Challenge
Amid the usual chaos of a construction site, an unexpected issue arose when a newly constructed concrete wall required temporary struts to be placed on the same columns as the monitoring units. In some instances, these wall supports obstructed the distance measurements to the original measurement points.
“Thankfully the affected baseline distances were able to be reset within T4D after traditional measurements confirmed that no column shifts had occurred,” Morey said.
The disruption could have been avoided if the brackets had been placed slightly higher or lower, or if site staff had been made aware of the lasers in use through proper signage to prevent obstruction. This situation underscores the critical importance of effective communication on construction sites, especially in remote monitoring projects, where each site presents unique variables that need to be carefully managed.
The Results
No shifts in the columns were detected during construction. While T4D issued several alerts throughout the project, on-site investigations revealed that all alerts were caused by construction interferences, not by actual movement of the columns.
Manual measurements taken at the start of the project and after the site disruptions revealed no discrepancies, with an average residual of approximately -1 mm. These tight residuals confirm that the construction activities did not have a measurable impact on the position of the structural columns. Despite the challenges faced during the project, the sensor monitoring system proved highly effective, thanks to the multiple redundancies built into the monitoring network.
“The amount of data that we collected is much more useful for them than the small amount of data that’s collected using traditional techniques, Morey said,” More data is useful in identifying the time and the source of a movement if there is an instance where movement is detected.
He concluded, “We are certainly looking forward to the next project that we can deploy this monitoring equipment on and welcome future collaboration with UPG.”
