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Grouting Tips: An Outlook on Post-grouted Drilled Shafts

Benjamin S. Rivers, PE
Geotechnical Engineer
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FHWA Announces Excellence in Highway Design awards Up front News and Views on 2005 competition. (December 2004)

Army of Steel Pipes Support Tough Pennsylvania Highway (2/14/05) By Jonathan Barnes
Courtesy of ENR Magazine

Mile by mile and pin by pin, contractors are stabilizing and widening a Pennsylvania highway once studded with crosses marking the sites of traffic fatalities.

The notoriously dangerous two-lane, 7-mile stretch of Lewistown Narrows highway is now undergoing a $105-million retrofit and widening to four lanes. It is the state's most complex highway project ever, says Gary Hoffman, PennDOT deputy secretary for highway administration.

Walsh Construction, Chicago, won the four-year contract to reconstruct the length of Route 22/322 in 2004. Hemmed in on the north side by boulder-strewn slopes and on the south side by the Juniata River and the Pennsylvania Canal, crews also must allow two lanes to remain open for 20,000 vehicles a day to pass.

The road will be reconfigured so that the eastbound, riverside lanes are separated from and lower than the westbound lanes. Concrete barriers will separate the sets of lanes. "We're putting in fill at the base of the slopes that will act as a counterweight [against the slopes of the mountain], and the westbound lanes will be on that fill," Hoffman says.

Pilings drilled into the bedrock pin the roadway in place. The technique is rare in Pennsylvania and has never been used to such a great extent in the nation, says Neil Fannin, a PennDOT geotechnical engineer. "The amount of pilings we're using there is exceptional."

The 7-in.-dia steel pipe pilings range in length from 15 to 45 ft. The pilings are drilled through a top layer of soil, then through a clay layer and embedded 6 ft into the bedrock. The "micropiles" are filled with grout, which will bond the pipes to the rock. Each pipe has a 0.5-in. wall thickness and are spaced 1 to 2 ft apart for three miles, says Jonathan Raab, geotechnical engineer with Harrisburg-based GTS Technologies Inc., development and design consultant. "Without a doubt, the Narrows is the largest project with these types of stability issues that has ever been done in the state."

The Federal Highway Administration did not have a strong precedent for the design of the project, Raab notes. "This method of micro-pile stabilization has been used elsewhere, but on a much smaller scale," he says.

With the scale and the variety of soils, "the application here is unique because of the geology of the site and the difficulty of what we're trying to do," FHWA geotechnical engineer Silas Nichols says.

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