By Brian O’Sullivan and Rodney Garrett
SPECIAL TO CEG
Where would Americans be without their highways? Their love of the automobile would be consigned to the garage; there would be no ’road movies;’ a good proportion of their rock & roll would have lost its subject matter and McDonald’s would be at a bit of a loss as to where to put the next diner.
Luckily, the good ol’ US of A highway is alive and well. In fact, highway development has consumed so much aggregate in recent years that to ensure a ready supply at reasonable cost, specialist road contractors are increasingly vertically integrating into the quarrying business.
Such is the case at Bourbon Limestone, a quarry operation in the Kentucky town of Paris. The company is a joint venture between The Walker Company and Hinkle Contracting Corporation, both highway and heavy construction contractors serving the eastern half of Kentucky.
At first, quarrying at Bourbon Limestone was not difficult, the 65-ft. (20 m) deep formation of Lexington limestone lay directly beneath the overburden. However, half a century of quarrying this stone has all but exhausted these reserves – and the rock directly below the quarry floor is not economically feasible for aggregate production.
There is another layer of limestone in the quarry that is viable for extraction. Unfortunately though, it is 270 ft. (82 m) below the current quarry floor, leaving the company with the choice of either closing the quarry once the surface layer rock reserves are depleted, or going under-ground to mine these lower deposits of Camp Nelson limestone.
As Camp Nelson limestone has good blasting and crushing properties, making it suitable for producing Superpave aggregates, and most Kentucky road paving projects call for Superpave, Bourbon Limestone opted to go subterranean. The first step was to consult with specialists about the mine’s design and construction and then construction of an access road leading to the mine’s portal began.
Access to the mine entrance required a 1,240-ft. (378 m) long road with a downward slope of 14 degrees. This road starts at the existing quarry floor and descends to the portal, 310 ft. (94 m) below.
A total of 1.1 million cu. yds. (841,000 cu m) of rock was excavated to build the access road. At the start, an excavator and two 100-ton (90 t) capacity, rigid-frame trucks were used to transport the rock. Unfortunately, while rigid-frame haulers are often good performers in such applications, in this instance, on such an extreme gradient, the rigid-frame trucks proved unsuited to the task.
In the course of hauling the first 500,000 cu. yds. (382,000 cu m) of excavated rock, both trucks’ transmissions experienced problems. One had to be replaced and the other rebuilt.
Tim Hatton, quarry manager, said fully loaded rigid-frame trucks are not designed for constantly negotiating such extreme gradients. In addition, it was difficult to maintain a high standard of surface on this steep grade, meaning that the terrain of the mine access road was rough during its construction. To make matters worse, the road would become extremely muddy during rainy periods. The end result was one of poor production performance from the rigid-frame haulers.
The issues encountered with the rigid haulers prompted the company to try new Volvo articulated haulers to carry out the remaining 600,000 cu. yds. (458,000 cu m) of materials yet to be excavated. It was decided that these haulers, once the access road was completed and the excavation of the mine begun, could also be used as the muck-out hauling system.
“We reasoned that Volvo haulers could be the solution to our problems as we already had some good experiences using them on construction projects,” explained Tom Hinkle, vice president of Hinkle Contracting. Hinkle uses two A35 model Volvos at another quarry for transporting blasted rock down a steep haul road with a 20 percent gradient. Each hauler has clocked up 15,000 operating hours on this mountain quarry, and each with a minimal downtime. The Volvo haulers have operated so cost-effectively in these examples of extreme downhill applications, as well as on general construction projects, that both Hinkle and Walker felt that they would be a good bet for uphill hauling too.
Their reasoning proved correct –– “performance at the Bourbon Limestone quarry has been excellent,” said Walker Co.’s Arthur Walker III. “We have used four new Volvo A35D trucks here with practically no downtime during their 8,000 hour operating times. Since the start, we only had to replace a couple of differentials,” Walker continued.
Production efficiency at Bourbon has also been good. At cycle distances of up to 4,000 ft. (1,219 m) –– 2,480 ft. (756 m) of which is going up and down the steep grade) –– the haulers’ cycle times were between eight and nine minutes.
This includes loading and dumping. Typically, a loaded truck negotiated the 1,240-ft. (378 m) long access road
in three minutes.
The company has already mined and hauled away enough rock to install a new 48 by 1,380 ft. (1.2 by 421 m) long conveyor for transporting the primary-crushed rock from the mine to a new tower-mounted triple-deck screen. Even though the Volvo haulers have been released from their muck-out hauling duties, they are still not going to be retired from this project. They will now be used for the next year to haul the blasted rock from the mine headings to a jaw crusher installed near the portal.
Ultimately, larger-capacity rigid-frame trucks will take over the underground task of hauling the blasted rock from the headings to the jaw crusher. Currently this is not possible as the first advancement of headings have only an 28 ft. (8.5 m) head clearance.
A big rigid frame hauler needs more headroom for the body to fully tip. With this low roof, only the likes of an articulated hauler can dump and still have a superior payload capacity when compared to highway end-dump trucks. It will not be until a 12-ft. (3.6 m) cut is made to lower the mine’s floor that the larger-capacity rigid-frame trucks will replace the articulated trucks.
The story doesn’t end there; Art Walker said his company will continue to use them on construction and excavation projects. “Trucks this versatile will never sit idle for long,” he said.
(Story courtesy of “Volvo Spirit.”)