Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/307110
RoadScience 10 May 2014 Better Roads apertures, which allow for greater stability, multi-directional stabilization and ultimately maximum performance efficiency. "Biaxial geogrids are geosynthetic materials formed into a grid of integrally connected tensile elements," says Caltrans in its October 2012 publication, Aggregate Base Enhancement with Biaxial Geogrids for Flexible Pavements. "[They have] apertures of sufficient size to allow 'strike-through' and interlocking with surrounding aggregate base materials. Biaxial geogrid increases the stiffness of unbound aggregate base layers and confines the aggregate particles under repetitive loading." This begs the question of which geogrid geometry works better in creating a mechanically stabilized layer below: bi- axial (square) or triaxial (triangular) "holes." "The effectiveness of geogrid-aggregate interaction de- pends on the relative geometry of the geogrid and aggregate," Giroud says. "Square or rectangular apertures can be expected to promote a cubic arrangement of aggregate, which is a loose arrangement that would limit the benefit of interlock- ing. In contrast, triangular apertures can be expected to pro- mote a hexagonal arrangement of aggregate, which is a dense arrangement. Therefore, geogrid-reinforced aggregate bases can be expected to deform less than geotextile-reinforced ag- gregate bases." Geogrids cut base costs That use of a geogrid can save money was affirmed in a criti- cal project in a remote area last summer. In a fast-paced project last year in the north Arizona mountains east of the Grand Canyon, a geogrid helped cut a nine-month construction period to three months, while saving more than $2 mil- lion in aggregate base (AB) and trucking costs in the remote area. In the early morning hours of Feb. 20, 2013, a landslide ripped through a sec- tion of U.S. 89 along a mountain slope about 25 miles south of Page, buckling more than 150 feet of the roadway and tearing the pavement up in 6-foot-high sections. The damage forced the Arizona De- partment of Transportation (AZDOT) to immediately close a 23-mile-long stretch of U.S. 89 and begin work on a temporary detour (U.S. 89T), which follows the route of the existing Navajo N 20, a Navajo Nation road. By pav- ing US 89T, the 115-mile detour route travel time was cut in nearly half and is similar in length to the closed U.S. 89 route. The $35 million paving project to adopt N20 temporarily into the state highway system was finished only three months by FNF Construction, Tempe, an impressive feat considering the 44-mile-long tribal route was primarily a dirt road before work began in late May. "It was a high-intensity, short-dura- tion project," says Audra Merrick, Flagstaff district engineer at AZDOT. Placed between the graded sub-base and the aggregate base, approximately 583,000 square yards of Tensar TriAx 130s geogrid saved time and money for the road project. An added structural benefit of the grid was a threefold increase the design life of the road. "Six days a week, operations would start about 6 a.m. and go all day," says Steve Monroe, senior resident engineer for AZDOT's Flagstaff District. "At 6 p.m., a crew would come in and go to work; this went on 18 to 20 hours each day." The result was major savings, AZDOT's Monroe says. "The geogrid is much less expensive and easier to put down than hauling all of that additional asphalt base," Monroe says. For additional information on geosynthetics, visit betterroads.com/down-to- earth for a web extra on "Geocomposites for Drainage." The value of adding value: In urgent construction of the U.S. 89T emergency bypass in summer 2013, use of geogrid cut 2 of 6 inches of aggregate base (AB) from the project, saving $2.2 million in material costs while speeding construction. Photo courtesy of Tensar