In a unique application, a Texas highway contractor is using surface miners to excavate highway right-of-way in Fort Worth.
There—as blasting is not allowed or not practical—Mario Sinacola & Sons Excavating is using Wirtgen 2200 SM and 2500 SM surface miners to remove layer after layer of hard limestone bedrock from the right-of-way quickly and efficiently, at far less expense in time and money than breaking and hauling or ripping with dozers and crushing.
Surface miners offer advantages over conventional surface-mining techniques under everyday mining conditions in open-cast mines. The mobile surface miners cut, size and load material in one pass, achieving great savings in machinery and personnel costs while allowing continuous refinement of pit drainage and haul patterns at the same time. But surface miners have not typically been used for road construction.
The 28-mile Chisholm Trail Parkway (S.H. 121)—formerly called the Southwest Parkway—is being constructed by the North Texas Tollway Authority (NTTA), working with its partners, which include the Texas Department of Transportation, Johnson and Tarrant counties and the cities of Cleburne, Burleson and Fort Worth. The parkway runs from I-30 south of downtown Fort Worth to S.H. 67 near Cleburne.
The NTTA is not a private company or corporation, but a political subdivision of the state of Texas that is authorized to acquire, construct, maintain, repair and operate turnpike projects in the North Texas region. The NTTA raises capital for construction projects through the issuance of turnpike revenue bonds. Tolls are collected to repay those bonds and to operate and maintain the roadways.
Crushing’s defeat
“We’ve had to deal with a lot of rock,” said Channing Santiago, project administrator for Mario Sinacola & Sons Excavating Inc., Frisco, Texas. “There is very little overburden—no more than 5 ft that you can dig with a backhoe—and once you get past that there is solid rock, which is why we are utilizing the surface miners.”
“We are working within the city limits of Fort Worth, and blasting is off the table,” said Mike Manley, equipment division manager for Mario Sinacola. “Inside the city limits, and in the surrounding area, blasting has become a thing of the past.”
The terrain is primarily rolling prairie with a good deal of hills. “We’ve had to cut through much hilly terrain in addition to road cuts for intersections,” Santiago said. “In the middle of the city you can’t blast, and there are gas lines and other underground utilities that you have to be careful with.”
“It’s a large-scale mass excavation,” said Chris Logan, project manager for Sinacola. “Everything is so much larger in scale, with a lot more work and a lot more rock. The surface miners are crucial in certain areas where we would have to utilize multiple hammers and excavators. They allow us to make a usable product in situ, and the gradations are such that we can use it right on-site.”
The benefit is a remarkable improvement in productivity, while eliminating the problems of blasting in a congested, populated area.
“It’s a definite improvement over ripping and crushing,” Santiago said. “Your machine is milling the rock, processing it and getting it to a size that can easily be loaded into a truck. You don’t have to use your ‘hoe hammer’ to break it up in advance of additional processing. It’s much more productive than other means of construction.”
In the past, without the surface miners, the work would have been very slow going, Santiago said.
“The work would have been done with hydraulic hammers. We would have had to hammer away month after month. It would have been very costly. The surface miners are the best equipment to use in this application.”
The limestone being removed is a tough material, measured between 6,000 and 8,000 psi at the low end and up to 16,000 psi at the high end, with the majority being between 6,000 and 10,000 psi.
Mario Sinacola found that for the 2500 SM, the optimum cut depth is 18 in., which moves the machine at a relatively quick pace; for the 2200 SM, a 10-in. cut is optimum. The 2500 SM, with its 8-ft-2-in.-wide cut, has been used in the main corridor; the 2200 SM, with its 7-ft-2-in.-wide cut, is used as needed for tighter areas such as ramps and intersections.
One of the machines is using a Trimble robotic system for fine-tuning the grade. “We use the same system on surface miners in Europe,” said Robert Bauer, director of Wirtgen America Inc.’s mining division in North America. “The integrated GPS system provides interface with the surface miner’s internal computer, providing a very fine degree of control.”
Filling in
Depending on the application, Sinacola will either load cut material into a haul truck or leave it at the rear of the surface miner for removal by a front-end loader. “We’ve done both processes,” Santiago said.
The cut stone ranges from a 9-in. top size, with 98% minus 6 in., with varying gradations down to fines.
“In our applications, we use it as fill in the project,” Santiago said. “It’s easily crushed to smaller gradations that meet our needs by running over it with our dozers and compactors.”
This eliminates the need for a crushing plant and is another advantage of using a surface miner in road construction.
The teeth or picks or cutting tools used are basically the same as used in mining applications.
“We have the latest technology—hard-rock pick on the cutting drum, with the newly developed pick holder system—which allows for a pretensioned clip ring on the pick tip itself,” Bauer said. “You don’t need pliers or a hammer to get them out; instead, a pick extractor speeds replacement.”
The pick holder system is useful in applications where hard, highly abrasive material must be mined and high hourly—and annual—production rates must be achieved. The tool holders are rigidly welded onto the drum body in order to resist the tremendous peak loads when cutting irregular rock formations. The system has 42-mm pick bores and an increased outer diameter as wear cladding.
By completion of the excavation, approximately 2.8 million cu yd of rock will have been excavated by the surface miners for this project.
“They have asked us to additionally process the bottom material, grinding it to another 8 in. in advance of liming the material,” Santiago said. This becomes a stabilized, granular sub-base.
The ultimate section of the highway will be 8 in. of lime-stabilized subgrade, 4-in. bituminous base course and a 13-in. continuously reinforced concrete pavement, three lanes wide, with 10-ft inside and 8-ft outside concrete shoulders. R&B
