The project involves construction of a second Gateway Bridge together with 7km of new motorway and the upgrade of another 12km of existing motorway. North of the Brisbane River, the new motorway crosses Kedron Creek, an environmentally sensitive tidal waterway supporting mangroves and aquatic life. The creek is about 3.5m deep at high tide.To provide a construction platform for the driving of piles and construction of piers to support this elevated section of motorway, there was no alternative but to fill in part of the creek. Around 25,000 cubic metres of 150mm nominal size rock fill was used for the purpose. To minimise the impact of this work, the creek banks were extended on both sides and about 40% of the width of the original creek left open to maintain water flow. Construction vehicles were able to cross the water via a barge on the narrowed creek.As the deed for the project called for reinstatement of the creek to its original shape, before any work the creek profile was surveyed from a boat, using GPS/GNSS and a depth sounder. A 3D digital terrain model was then created as a reference for subsequent creek restoration that would use machine guidance.When the structure of the motorway was completed and it came to removing the rock fill and restoring the creek, the 12 lanes of new motorway immediately above the creek blocked all satellite signals.To overcome the problem, LAJV switched over to Trimble Universal Total Station technology for the machine guidance. The technology is most commonly used on graders, where data from tilt sensors on the machine is combined with position data from an instrument or GPS to allow 3D designs to be worked with high accuracy. The technology is now finding a home on excavators, where the machine is fitted with a Trimble AS360 sensor to detect its pitch and cross slope, and Trimble AS450 tilt sensors to detect the machine’s bucket, bucket linkage, stick and boom. At Kedron Creek, the excavator doing the work has been fitted with the sensors and a Trimble MT900 prism incorporating an “active tracking” device. The device allows a Trimble SPS930 Universal Total Station to track the prism as the machine moves about and radio its location back to a Trimble SNR410 radio in the machine’s cabin. A Trimble CB430 control box in the cab takes that data and combines it with data from the various sensors to work out the exact position of the cutting edge of the bucket. On a display in the cabin, this bucket position is shown transposed over a 3D terrain model of the original creek shape that has been loaded into the CB430 from a compact flash card.The operator can view the original shape of the creek bed and banks either in plan view or elevation, together with the real-time position of both the machine and the bucket edge. The system allows the operator to reproduce the original creek shape above and below water level to an accuracy of plus or minus 20mm without using satellite signals.According to Stephen Meadows, project surveyor for the northern section of the Gateway Motorway, the Trimble system on the excavator is working extremely well because it is user friendly and works off the same Trimble SCS900 Site Controller software platform as all the machines using Trimble GPS/GNSS systems on the site.“It’s straightforward for the surveyors and for the excavator operator too,” Meadows said. “This operator has taken to it like a duck to water.”When the job is complete, there will be no need to undertake another depth-sounding survey of the creek. The authorities concerned have confirmed that the terrain-mapping recording function of the Trimble GCS900 Grade Control System on the excavator will more than satisfy their requirement for verification of the creek’s restoration to its original shape.*This article first appeared in the October issue of Contractormagazine.
