Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/632397
AGGREGATES MANAGER / February 2016 37 While using a pit crusher makes sense on massive freeway projects and where stringent aggregate specifica- tions for structures like bridges are well justified, there are better, more economic options for gravel roads, particularly remote ones. Pit mining vs. linear crusher Sweet Grass County, Mont., depends on good gravel roads for much of its timber, oil, mining, and agricultural transporta- tion. But maintaining these gravel roads was becoming too costly and difficult with traditional pit mining. "Previously, we used pits or, when the haul got too far, we'd push shale rock on the road and run it over with vibratory rollers, trying to break it down to make a drivable surface," says Cory Conner, Sweet Grass County public works director. Instead of burying near-surface subgrade problems under excess gravel from a pit, a growing number of counties and road contractors are discovering that linear crushers can repair a gravel road for significantly less money, while correcting the underlying subgrade problems. Unlike mobile rock crushers, which are stationary when operating, linear crushers move along the road being repaired, crushing oversize rock along it in a crushing chamber. The oversized rock, existing gravel, crushed rock, and natural fines (soils) are all windrowed together and processed through the crusher in one pass, which leaves the reduced material in place as a stable crushed layer. When used in on-road repair, it can help achieve "aggregate lock," a natural binding of soil and gravel when wet, that can prolong the life of the road surface beyond that of clean- er gravel from a pit that may lack soil mixed in as a binding agent. Unsatisfied with the traditional ap- proach to maintaining gravel roads and concerned about shortening the lifespan of the county's vibratory rollers, Conner favored the linear crusher approach. After research and watching demonstra- tions of two on-road crushing systems, he chose a front loader-based design over a tow-behind design. "The tow-behind design would've required us to buy a dedicated tractor for about $400,000, while the Vanway lin- ear crusher works with the front loaders we already have," Conner says. Vanway International, a linear crusher manufacturer, makes machines capable of crushing any rock, including construction grade rock (110-210 MPa compression strength) such as trap- rock like basalt and gabbro, as well as quartzite, granite, and dense limestone. "The tow-behind also did not blend fines or crushed rock, so we would've had to do that manually," Conner says, noting that the front loader can uniform- ly crush rock to the size he needs, and blend it with fines. "We can just blade it, roll it out, and call it a finished product." According to Conner, properly blend- ing fines and crushed rock with the linear crusher is critical for Sweet Grass County to achieve the good gravel road surface sought. "Without blended crushed rock and fines, you'll get rock pockets, rocks kick- ing out, washboard, or potholes," Conner says. "Half your road will be a muddy mess, half will be nothing but rock be- cause the fines will turn to mud or slime when really wet. When you lay crushed rock and fines in the right blend, you get a good, lasting driving surface that won't easily dust up, kick off rocks, or allow water penetration and damage to the road surface." After purchasing the linear crusher, Conner says, "We can take existing roads that are wearing out and rehab them without having to open a pit and do a whole DEQ pit process, then the whole reclaim process. We'll save a lot of mon- ey using existing material on the road shoulder and surface. "Compared to our typical gravel road cost of about $20,000 per lane mile hauling from a pit, we expect to rehab the same road for about $5,000 to $7,500 per lane mile," he adds. "We should be able to do about two to three times the road repair we'd done previously with the same crew, while extending the lifespan of our roads and vibratory rollers." Travis Clark, operations manager for Roadtech, a contract road construction firm based in St. Maries, Idaho, also relies on a linear crusher for cost-effec- tive gravel road repair and construction. "Compared to burying a road's subgrade problems with gravel from a pit crusher, we can often repair the wear surface and correct subgrade problems for up to 66 percent savings," Clark says. "All that material that's been pushed off the edge of the road for years — from ditches, berms, subgrade, oversize — becomes our lift material," Clark adds. "Our linear crusher usually runs at a cost comparable to a pit crusher, but doesn't Using a linear road crusher, some road contractors are finding that gravel roads can be repaired at a fraction of the cost of traditional methods.