Man & machine: lessons in grading
Making great grades in a single pass
Grading a roadbed. What could be simpler? Place and rough grade your 25, 75 and 150 millimetre sands and gravels using a dozer; then finish it off with a motor grader. But not so fast, say the experts, there’s more to grading than that. To find out just how much more, we talked to a project owner, project managers and equipment operators about some of the unique challenges they recently faced on a major grading project in British Columbia.
GET IT RIGHT THE FIRST TIME
Drivers had two bits of good news on the day last December when a brand new 37 kilometre, four lane stretch of highway opened up in Greater Vancouver: the new alignment would not only speed them to home and office more quickly, the road surface would provide a gloriously smooth ride. The last thing on commuters’ minds? How much planning and work was focused below surface—on up to 450 millimetres of sand and gravel—to ensure that faster, smoother ride home.
“In the old days we used to kind of do it off the back of a cigarette pack,” chuckles Geoff Freer, project director for the South Fraser Perimeter Road (SFPR). “Nowadays it’s just amazing what they’re doing in terms of planning and achieving more efficiency.”
Freer says one of the first challenges confronting planning engineers and builders at B.C.’s Ministry of Transportation and infrastructure was giant swaths of soft soils and peat beneath the site of the SFPR. To offset the loss, due to settlement, of approximately one million cubic metres of these softer materials, three million cubic metres of preload sand were dredged from the Fraser River and pumped to storage points along the SFPR corridor.
This decision accounts for the project’s success and, in particular for the success of the grading operations, says Freer. But even this wasn’t enough, he says. That preload—making up a sizeable portion of the alignment’s eventual sub-grade—would also require a surcharge of sand for additional settlement. Only when it was removed using rock trucks could the sand sub-base be graded to the tolerance required for the gravels and asphalt that would follow.
The litmus test for the SFPR, as it is on any grading job, was the amount of re-work required. “I saw very little re-work. The contractor and subcontractors were just excellent,” says Freer. He attributes this in part to the use of GPS machine controls, ensuring this initial grading phase occurred in a single pass. The GPS controls would also prove helpful laying down the three-inch minus crushed rock that followed the sub-base grading.
Hauled down the Fraser River by barge and loaded on to tandem dump trucks, both the sub-base and base gravels were spread using a Cat D6K dozer. No small task given the tight time schedule the operator was working to, says FTG Constructors’ section manager Sean Frost. “The operator was placing a 2,200 to 2,500-ton barge in five to six hours and putting it on grade.” Getting 250 to 450 millimetres of sub base gravel down that quickly “exceeded our expectations,” says Frost.
GPS WON’T DO IT ALL
But the best was yet to come. Despite a thickness half that of the sub base of between 300 and 450 millimetres, and having to travel farther to place it, the same amount of 150 millimetres base gravel was placed just as quickly as the sub-base. The resulting grade was “fantastic,” says Frost. “People made comments after he was done like ‘Boy you must have placed this with a grader.’ Nope. We just had Robbie and the GPS dozer pushing it down.”
“Robbie” is dozer operator Rob Gordon. “I tried to stay a little on the high side so that the grader, after compaction, would be just working with an inch maximum. On an average day I put down between twenty three and twenty six hundred tons.” Gordon gives a lot of credit to his two rovers helping him to adjust the GPS to meet the layout requirements. Where he demurs a little is on the usefulness of GPS itself.
“They sell it as a unit that monkeys can run. But GPS is not fool proof. If you don’t know how to manage materials GPS doesn’t really help you.”
A case in point is the grader operator’s use of the track marks left by Gordon’s dozer after it left the site. The grader operator can get to grade, says Gordon, by simply cutting out the one or two inch track marks that the dozer leaves on the ground. Not that he’s proposing this as an alternative to GPS; Gordon is simply stressing the importance of matching GPS smarts to driver smarts. GPS won’t always save you from trees or power lines, he says, but a sharp eye on the ground just might.
Gordon’s boss, FTG project director Ramon Fiuza, agrees. “The critical part of the operation is the operator. You can have higher productivity using automated systems on difficult tie-ins or interchanges, but not with an unskilled operator.” That became especially apparent when several different grader operators tried placing 25 millimetre gravel using a GPS equipped motor grader. Their difficulty was getting the grader’s larger blade to work with the GPS surface model.
“The blade does some strange things,” says Frost. “When it gets to the edge it might jump up or down…And sometimes that was a challenge having the right guy in the seat who knew when to turn it off.”
The D6K by comparison was a little more agile with a much narrower blade. But FTG also had Rob Gordon, brand new to GPS himself, but equipped with a ton of experience. “We basically threw the manual at him and said strap the equipment onto the machine and ‘Go play.’”
“Once he figured out that GPS it was just phenomenal, he just made that thing fly. The second that blade did something that he didn’t anticipate he knew there was a little problem and he’d just override it.”
The hands of surgeon…
“For one day of paving, we needed about five days of grading.” Trouble was, says Imperial Paving project manager Robin Smith, the main grading contractor couldn’t keep up during the fine grading phrase. Worse still, GPS once again was failing to meet expectations.
“The paver wasn’t completely satisfied with it because every time they’d start up with it there were bumps and a ripple in the gravel. It met [the main contractor’s] criteria but didn’t meet mine.”
To rectify matters Smith called on veteran grader operator Gary Haac. Though Haac imagines he will someday gravitate towards the use of GPS, that day has not come yet. Instead, mounted on a relatively older Cat 140H motor grader in the summer of 2013, Haac set to work fine grading on 176 Street up to Highway 17. The result: his bosses couldn’t have been more pleased with his finish.
“I guess I had a better feel for the machine and an eye for the holes than GPS. That and the ride of the machine; you could just feel them.”
A nice sharp cutting edge and well-balanced machine are the two key aspects of a good motor grader, says Haac. Something else that is “so, so important,” on a project like this, adds Smith, is watering. Sometimes during fracture, aggregate will develop an electric charge, preventing amalgamati
on of the rock and fines. “It’s almost got an umbrella around it that nothing will adhere.”
Watering the aggregate and letting it sit over a weekend will release that charge. But over water the aggregate and you create another problem: segregation. “What was happening is too many people were watering it and the fines would work their way down into the bottom and the rock would stay on the top. You don’t want to pave just on rock, you need the fines to pull it together.”
As a result, Haac’s blade would begin fine grading soft soil, only to unexpectedly drop when it encountered segregated. “It creates a horrible problem because then I can’t hold my blade.” The only solution at that point, says Haac, is to cut out the segregated area and refill it with non-segregated gravel with a higher fines content. On a tightly scheduled project like the SFPR this costs time and money.
“Sometimes it would cause me a couple of days to get rid of the segregation. I had to grade it out, pick it up, bring new stuff in and repack it all.”
MAINTAINING YOUR EDGE…
The object of any grading job is to ensure your grade and sub-grade hold firm for the long-term. Nearly as important as the life of the grade is the life of your grader, particularly the cutting edge of your blade. As Robin Smith explains, the blade on a motor grader may wear more on the inside of the edge than on the outside, creating a bow effect. “What we do then is we’ll run it on the base surface asphalt to square it up again.”
But the overall health of the moldboard, “that’s all on me,” says Gary Haac. Understanding what impact the position of your moldboard has on your cutting blade is the first step towards keeping the blade’s cutting edge sharp (an upright blade will maintain a sharp edge; curling or rolling the blade forward tends to blunt the edge). But while an upright blade will preserve the cutting edge, says Haac, it “doesn’t cut as nice a grade as when it’s curled over.”
The compromise is to occasionally tilt the moldboard back to even out wear across the bottom surface of the blade. An experienced operator like Haac will continuously change angles as a way to preserve the life of a blade while simultaneously preserving the finished grade. “When I’m finished with the cutting edge it’s worn the same thickness from one end to the other, because one side doesn’t wear faster than the other.”
The other challenge Haac faces is trying to see over the blade when it’s in an upright position, “so you can’t tell how much you’re cutting.” When he’s bulking, Haac has the blade upright so that he can move the maximum amount of material quickly. Fine grading requires more finesse, rolling the blade forward for greater visibility while applying equal pressure along the entire length of the moldboard for a more precise finish.
At the end of the day it all comes down to having the right touch or feel for the machine, sand and gravel underneath it. Smith had it when he operated a motor grader years ago. Haac has it today. “The good grader operator will have a better sense of the machine than the mediocre operator,” says Smith. He believes for all the uncertainties around the use of GPS more good operators will eventually have a better feel for it, too. He, Haac and Gordon understand it’s the wave of the future.
David Godkin is a B.C.-based freelance writer and regular contributor to On-Site. Send comments to firstname.lastname@example.org