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Allegheny River Bridge Project will take on the appearance of a natural hillside

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By Shannon Rengers
Sunday, Aug. 16, 2009
 

The Pennsylvania Turnpike is in the process of receiving a "rocking" makeover.

As part of the Allegheny River Bridge Project -- the building of a new multi-lane span to carry the Turnpike over the river between Plum and Harmar, Walsh Construction of Chicago is excavating four bridges and seven retaining walls. The work is needed because, in addition to the new bridge, the Turnpike is being widened from four to six lanes.

But all of that digging away of hillsides creates sheer rock walls overlooking the road -- and the threat of future rockslides like those that have plagued the Route 28 Expressway at Harmar.

Part of Walsh's job is to make sure those rockslide don't happen.

Enter "Shotcrete" -- appropriately named because the concrete mixture literally is shot out of a high-pressure hose.

In addition to making the hillsides more stable, the mixture can be sculpted and contoured to resemble natural rock strata similar to what would be seen by simply cutting away a hillside.

At highway speeds, motorists might not know the difference. But Turnpike maintenance crews, who won't be clearing rockslides, will notice.

"The Shotcrete will hold up for a very long time," said project manager Craig White for McTish Kunkel. "I do not want to say 85 years, but as long as the road is there, the walls should be, too."

Walsh Construction contracted McTish Kunkel as the engineering consultant firm.

Faking it

White explained how to make a real hillside into a safer, fake hillside.

The Shotcrete application begins with the excavation of existing ground. The excavation widens the road and creates a cross section of the hillside.

Next, soil nails, which are long pieces of rebar that stitch the hillside together, are drilled into place. Once the hillside is locked, the construction team shoots a primary layer of Shotcrete to seal and stabilize the hill. This is dubbed the "anchor wall."

The anchor wall consists of 6 to 8 inches of the cement and is the structural portion that protects the rock underneath. Shotcrete keeps the underlying rocks solid so that the weather cannot touch them.

The outer layer is sprayed on with color and mostly aesthetic. It is 4 to 5 inches thick and gives the impression of a natural hillside. The outer layer is first carved to replicate real rock, then it is chemically stained and shaded to add realistic features of hillside formations.

Walsh also subcontracted the carving and staining of the Shotcrete to companies Cemrock and Bella Vernici.

Once Shotcrete is applied to stabilize the rock and the outer layer is sprayed, carving begins.

Cemrock, an artificial rockwork company based in Tucson, was contracted for the carving. The company uses sticks, rakes, shovels, and brooms to carve out designs imitating rock formations to create an artificial environment. Typically, designs and walls similar to these are seen in zoos or amusement parks to hide unpleasant structures. Now the turnpike is following that lead.

Cemrock was subcontracted by Walsh because the construction called for widening of the road.

"When Walsh needed to make the roadway as wide as possible and needed to create a vertical wall instead of a gentle slope our system was used," says Brian Olson, president of Cemrock. "Engineers come to learn they can employ Cemrock to make structures look indigenous to the geology that may already exist."

Naturally local

Cemrock is used all over the country to replicate natural geology. In Pennsylvania the firm chose to simulate natural coal seams because coal naturally occurs in this area. California, for example, would see the use of a sand replica or sandstone because it is seen in that region.

The carving was done with hand tools and took roughly nine months to complete. The four carving artists on the Allegheny River Bridge Project use rakes, shovels, and landscaping devices modified to recreate this particular region's geology.

Lead artist, Richard Tribel, hid 5 keystones, the logo for the Turnpike, on the surface of the retaining walls which he carved.

When carving was complete, architectural concrete company Bella Vernici stepped in. The company, based in Orlando, is responsible for the staining of the rock. Bella Vernici's role, is adding shadow and character through the use of artistic fading and shading. The stain that is used on the carved retaining walls is water based, which reacts chemically with the concrete.

Art Director Rodney Ray describes the stain Bella Vernici uses as "environmentally friendly. It does not fade, and never requires reapplication. The stain penetrates the concrete and changes the pH so that the color is permanent."

Bella Vernici's team makes a custom color combination on site to accurately match the hillside. The application process includes plastic sprayers and mobile backpack sprayers used by artists on lifts. Bella Vernici began staining the new Oakmont wall in November of 2008 and will continue through August of 2010. The firm works in 2-week increments and will finish the project with the piers of the new Allegheny River Bridge. Every "stone" on the piers will be stained by hand.

Once complete, the finished product is a hillside that is structurally sound with a pleasing surface, giving motorists the impression of a real hillside as they cruise the Turnpike.

Additional Information:

Big contract

Walsh Construction, of Chicago Illinois, recently singed a three-year, $190 million dollar contract with the Pennsylvania Turnpike Commission to excavate four bridges and seven retaining walls as part of the Allegheny River Bridge Project. This contract is the largest single contract that the Turnpike Commission has ever signed.

The Allegheny River Bridge and nearby retaining wall replacement includes improvement of retaining walls along the Turnpike near UPARC, Oakmont Country Club and ramps at the Allegheny Valley interchange. The project began in May of 2007.

 

 
 


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