Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/536721
July 2015 | EquipmentWorld.com 66 work better than another type of material," For example, if you're in Florida, most of the cracks are top down cracking, developing from the top and going towards the bottom. "So if you place the geogrid at the bottom, and the cracks develop at the top, what's the point of using a geogrid?," he asks. Proper product use then comes back around to design. "It's a topic that many universities are still studying," Isola adds, saying that there's disagreement at industry confer- ences and in written papers as to whether this type of reinforcement works. "Crack propagation is not fully un- derstood," he comments. "Most of the studies say it really works, but some studies say no, it doesn't really work." Isola says the key layer in pavement isn't the asphalt layer, but rather the base, much like the foundation of any structure. If there's a solid, well-built base, then a pave- ment will last longer. That asphalt layer, he explains can always be milled or replaced, and small cracks on the face is simply minor maintenance. But doing a complete rehabilitation is going to cost much more in the long run, in materials and especially labor. "If you have problems underneath, that's going to be a serious problem," he adds. "The big savings are in the base or sub-base, and that's where geosynthetics perform very strong." Isola adds that many contractors he talks to say their goal is to give a road agencies a better and longer lasting pavement. "They say 'if we can guarantee two or three years more compared to competitors, that's a plus for us.'" If there are pavement failures after using geosynthetics, highway contractor | continued Most common uses for geogrids Geogrids are the primary workhorses of geosynthet- ics in road applications. They can cut down on the amount of aggregate needed and help in creating a solid base upon which longer lasting roads can be constructed. Brian Gee, director of marketing for Tensar, says there are four main uses for geogrids: Subgrade conditions Contractors have several options when subgrade soil can't meet proof roll or compaction requirements, Gee says. "These include excavation and replacement of the poor soils, chemical treatment of the subgrade, waiting for the soil conditions to improve (assuming the problem is excessive moisture from precipitation), or the use of geosynthetics." A geogrid, he says, can offer the least expensive and fastest solution. An example he cites is a widening project of Interstate 87 in New York with saturated and "silty" soils. "The contractor used (Tensar) TriAx Geogrid to dramatically reduce the quantity of subgrade excava- tion, reducing the cost of addressing the subgrade issues by approximately 65 percent," Gee says. Time Geogrids save time because they allow contractors to use less conventional material, thereby requiring fewer delivery trips and fewer installation hours. "One example of this was a project on Wilson Road in Fort Knox, Kentucky, where a project that was already underway, and subject to a tight construction schedule, faced a design change that would have required a significantly thicker pavement section," he says. By using a geogrid, the contractor was able to maintain grades and keep the project on schedule. Cost Outside of cost savings from fewer labor hours, geogrids allow for thinner, stabilized pavement de- signs with the same traffic capacity as conventional designs. "An example of this was a project on Carroll Can- yon Road in San Diego, California, where the city was able to reduce the thickness of the pavement section by 6 to 8 inches using our geogrid." Working around utilities "Many road projects, particularly widening projects, have to deal with utilities located in the right of way," Gee says. "Relocating utilities can be costly and time consuming, and can add financial risk to the contractor." He adds geogrids allows contractors to build thinner pavements that don't require as deep excavation as conventionally needed. Gee's example involved widening of 1st Avenue in Chula Vista, California, where the existing utilities were left intact during construction. Woven geosynthetics, such as TenCate's Mirafi RSi series used here in Washington State, are used for subgrade stabilization.