Equipment Type

Bridge Breakdown On I-79

Road construction projects come about for many different reasons. A heavily traveled thoroughfare may need to be widened to accommodate more lanes of traffic. The asphalt surface on a county road may be severely cracked due to wear and age. And then there are those times when the earth could possibly give way and cause a highway's bridge to collapse, creating the likelihood of a really bad day ...

January 15, 2007

Road construction projects come about for many different reasons. A heavily traveled thoroughfare may need to be widened to accommodate more lanes of traffic. The asphalt surface on a county road may be severely cracked due to wear and age. And then there are those times when the earth could possibly give way and cause a highway's bridge to collapse, creating the likelihood of a really bad day for unsuspecting motorists.

While the latter scenario may seem a little on the unusual side, it is a real circumstance the Pennsylvania Department of Transportation (PennDOT) was preparing for in August 2006. Looking to avoid such an occurrence, PennDOT contracted Clearwater Construction, Inc. to demolish and rebuild a small stretch of Interstate 79 — including a 108-foot-long bridged section — near Waynesburg, Pa.

The potential bridge failure did not come about because of a natural occurrence, but rather is one side effect of longwall mining, a coal extraction method that has been gaining popularity in southwestern Pennsylvania for the past 25 years. The process works by first establishing a horizontal section, or panel, of coal to be mined. A single panel can be up to three miles long and 1,500 feet wide. A longwall shearer moves across the width of the panel and breaks away the coal, which in turn is transported out of the mine by a conveyor belt.

Unlike conventional room and pillar mining, where coal columns are left behind to support the earth's surface above, longwall mining completely removes the coal from the area being mined. Hydraulic shields support the mine roof, but only until the shearer has removed the coal from a particular area. As mining progresses, the shields move and the mine roof is allowed to collapse, causing overlying rock and other material to fill into the void left behind. Although the mining occurs hundreds of feet underground, there is usually at least some surface subsidence.

Despite inevitable surface disruptions, the longwall technique makes sense from an economic standpoint in that it allows for much faster coal mining with far fewer workers. The process gained official acceptance when the state passed a 1994 law that allows coal companies to perform longwall mining under certain homes, structures and property, provided that the companies compensate owners for any losses sustained from the resulting subsidence. With thousands of properties in the region already affected, it was only a matter of time before roadways would feel the impact as well.

"The longwall mining could cause a drop of a couple feet in certain areas along the interstate," said Gary Gorski, operations manager for Clearwater Construction. "A road at surface level will usually drop pretty evenly, so you can pave over the affected spots and it should be fine. But when there's a bridge involved, a 2-foot drop in the earth could cause a support beam to crack and the outcome could be catastrophic. PennDOT saw the potential was there for something bad to happen. And since the county road that ran underneath wasn't vital, they decided to close the road, take down the bridge and bring this stretch of the interstate to ground level."

Based in Mercer, Pa., Clearwater Construction is a civil construction contractor that specializes in heavy concrete and bridge work in western Pennsylvania, with an occasional venture to the eastern end of the state. The company's experience with bridge demolition made it an ideal candidate to complete the work on I-79. Even more than the type of work to be performed, Clearwater was a good choice due to the tight timeline of the project.

"Fast track work is right up our alley," said Gorski. "The scheduled mining in the area was coming up quickly. We had a nine-day window to get traffic to one lane, demolish most of the bridge and fill underneath it, pave a new road to interstate specifications and then get traffic completely off of the bridge and onto the new surface."

The idea of combining demolition, earthwork and roadbuilding while racing against the clock may have had some contractors picking up the phone to find subcontractors to assist with the various phases of the job. But Clearwater didn't view the situation as a daunting task that required outside help, just another challenge that would involve some improvising.

Because of the variety of projects that it takes on, Clearwater keeps its equipment fleet relatively light. While the company will bring its own excavator and some other machines to most jobs, other requirements at a given site could be anything from portable light towers to compaction rollers to demolition tools. Clearwater routinely works through various local dealerships to rent equipment that properly addresses its needs for specific assignments.

Groff Tractor & Equipment, Inc. is one dealer that Clearwater has worked with frequently over the past few years. For the I-79 job, Clearwater was particularly interested in what Groff Tractor could offer in the way of hydraulic breakers that could demolish the bridge.

"We rented an Atlas Copco breaker from Groff last spring to take down a big concrete arch in Philadelphia," said Gorski. "Our superintendent on the project was very impressed with the equipment. Ever since, we've been renting Atlas Copco breakers whenever possible."

Leaving nothing to chance with the tight deadline looming, Clearwater rented three Atlas Copco breakers, each mounted on Case excavators. The company estimated that the three units could take down the bridge in four days, including some nighttime work, leaving an additional five days for the remaining work to be completed. "Timing was a big issue, especially because the bridge was in very good condition," said Gorski.

The breakers used by Clearwater on I-79 included two HB 3000 breakers and an HB 2200 unit. Both models deliver a maximum impact rate of more than 500 blows per minute. The HB 3000 provides more than 4,500 foot-pounds of impact energy while the HB 2200 delivers more than 3,500 foot-pounds of force.

"The bridge spans on the project were each 108 feet long," said Gorski. "There were 10 pre-stressed concrete I-beams to break, 66 inches high and 28 inches wide. We used the larger units on the I-beams while the HB 2200 was taking off the diaphragms and backwalls. We also used all three breakers on the existing road surface. The performance of the equipment was outstanding and we finished much faster than we thought we would."

In fact, Clearwater wrapped up the demolition phase of the project in just two days. "I've been doing this stuff for 30 years and had never seen production like this," said Gorski. "The superintendent estimated four days. I was thinking four days. And we wound up cutting it in half. In hindsight, the breakers were so effective that we could have even gone with smaller units."

While the performance of the breakers and ensuing results were impressive, Gorski and Clearwater's crew found helpful the convenience provided by the automatic lubrication system mounted directly onto each breaker. "I personally wasn't aware of the system," said Gorski. "I told the operators beforehand to be sure to grease the hammer every two or four hours. It was nice to find out we didn't have to. Basically it takes some downtime and possible operator error out of the picture."

Clearwater found another feature of the breakers to be a welcome benefit when it came time to recycle the concrete from the bridge, which was being incorporated into the aggregate base for the new road surface. Atlas Copco's AutoControl, which limits the breaker's output energy to 50 percent until the tool steel contacts a solid surface, helped amidst the unstable piles of material. By using only half of the available power, Clearwater's operators were able to break down the concrete in a more efficient manner.

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