WHEN the surface of the former BHP site in Newcastle was capped to contain contamination in the soil, laser guidance of final grading was the only way the surface could be finished to the specifications required.
Hunter Valley civil contractor Road & Rail Civil won the contract to reshape and cap the Newcastle site. The original surface was far from flat, and waste from BHP’s blast furnace and steel pickling operations had been dumped randomly over the entire area, to depths of about one metre.
Road & Rail’s job was to pull the contaminated material into the hollows to form a flat deck over the site, cap the material with a 400 mm compacted layer of clean clay sourced from the site and to place 500 mm of compacted clayey gravel over that.
To move 45,000m3 of contaminated material the contractor used up to three hydraulic excavators in the 22-30t range (fitted with batter buckets to scrape away every trace of contamination in areas not being capped), together with three articulated dump trucks, a D6 dozer, a D9 dozer, a Cat 140H grader, pad foot and smooth drum rollers and a water cart.
The purpose of the cap is to prevent rainwater seeping through and taking contaminants into the water table below, so it was important for rainwater to run off the site and not pool on the surface. To meet this requirement on a site designed with a finished surface slope of just 1 per cent, Road & Rail’s grader needed something better than a few pegs for level guidance.
The contractor had not used laser guidance on a grader previously, but opted to contact the local Coates Shorco branch – the Trimble ’s machine guidance reseller in Eastern and Central Australia – to supply and install a Trimble GCS600 Grade Control System operating with a Spectra Precision Laser GL742 dual grade laser transmitter and a single Trimble LR410 laser receiver mounted on the grader blade.
Dual receivers on the blade would have allowed the grader’s blade to be automatically controlled travelling in any direction across the site. However Road & Rail chose the single receiver because the site suited itself to the grader travelling either across the slope with the blade angle set at 1 per cent or up and down the slope with a horizontal blade.
Road & Rail made the most of having the GL742 laser transmitter on site and, before getting to the grading work, used the LR410 receiver on a stick to manually guide the dozers working the contaminated material before the capping was placed.
Initial grading of both the cap and the top layer was carried out with the GCS600 in indicate mode, but the system was switched to automatic grade control for the final trim. Site supervisor Rob Tickle says that on the whole job they only used two pegs. Each morning they set up the transmitter over one peg and, in a one person operation using two-way radio, to remotely align the transmitter to a second peg.
Delete catch drain
Approximately 1.5 ha had been completed when a good rainstorm gave the finished surface its first real test. “Our clients were concerned about the potential for localised concentrations of water and scouring,” Tickle said, “but the deepest puddle on the deck was just 20 mm deep after all the rain. We’ve got the surface so flat the water flows cross the site and down the batter in a sheet.
“When they saw that, they changed the design,” says Tickle. “We were meant to build a 600m long catch drain, along the top of the batter, and then take the water down the batter every 50m with rip-rap channels. But the engineers were so stoked with what we’d done that they chose to delete the catch drain. It’s all because of the laser”.
After their work on the BHP site, Road & Rail decided to upgrade their GCS600 Grade Control System by adding a second LR410 laser receiver to the grader blade. Now the company is looking at a further upgrade to a GCS900 3D grade control system with a choice of either dual GPS antennas or robotic total station or GPS with laser augmentation, depending on the job requirement. The full upgradeability path of Trimble’s GCS system means that Road & Rail will have a common grade control system platform across their entire fleet and they could move a system from one machine to another.
Source: Construction Contractor