Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/307110
Better Roads May 2014 7 and as a fluid barrier. A subset of geotextiles is paving fabrics used between pavement lifts. • Geogrids are formed by a network of tensile elements with openings of sufficient size to allow interlock with the surrounding fill materials. They provide reinforce- ment or stabilization, with geogrids oriented such that their principal strength is in one direction (uniaxial), both directions (biaxial geogrids) or in three directions (triaxial). • Geocomposites are a combination of two or more geo- synthetic materials, such as geotextiles with a core, and are used to enhance drainage, for example as prefabri- cated longitudinal edge drains or a layer feeding those drains. • Geomembranes are a single, solid sheet of polymeric material, used in construction as an impermeable barrier. Use of a geosynthetic for separation prevents intermixing between two adjacent dissimilar materials, so the integ- rity of materials on both sides of the geosynthetic remains intact, according to Caltrans. This keeps weak subgrade soils from pumping through overlying fill or prevents contami- nation of select fill by intrusion into the subgrade. "This may allow stable construction over soils that may otherwise require expensive ground improvement technologies," Caltrans says. Geosynthetics can add value to the construction sequence by saving material costs associated with working surfaces, ex- pediting construction schedules and providing a permanent working surface that yields a smooth riding surface, Tensar says. Their added cost, though, must be balanced by savings in material and labor, along with long-term benefits in increased pavement durability. Finding the right combination is the challenge of the civil engineer. Geotextiles separate layers Whether woven or nonwoven, the "planar" geotextile (vs. 2D or 3D geogrid) must allow migration of water while keeping dissimilar materials apart. They perform according to their apparent opening size (AOS), which must be smaller than the smallest size particle to be retained, and still allow for the flow of water through the geotextile material. Geotextiles (woven and nonwoven) are used as separa- tors and filters to prevent soils from migrating into drainage aggregate, granular bases or pipes, while maintaining water flow through the system, according to the Iowa DOT. "Geotextiles are also used below riprap and other armor materials to prevent erosion of the soils from the stream bank," Iowa DOT says. "The geotextile should be selected to prevent the migration of fines based on the AOS criteria. In addition, geogrids may also be used as a separator to prevent the migration of granular materials (aggregate) into fine- grained, soft subgrade soils. However, this application will not prevent the migration of fines from the subgrade soil into the aggregate." Even as separators, nonwoven needle-punched geotextiles and geocomposites provide drainage by allowing water to drain from or through low-permeability soils. Paving fabrics, warm-mix asphalt Geosynthetics are used within asphalt pavement structures as well as below them. "Geosynthetic pavement interlayers [also called paving fabrics] provide numerous benefits to a pave- ment system," say Bradley J. Putman and William P. Bolger, Clemson University, in their 2014 Transportation Research Board paper, Laboratory Investigation of the Effect of Compaction Tem- perature on Geosynthetic Pavement Interlayers Made with Warm Mix Asphalt Overlays. When used properly, these interlayers can prevent water from infiltrating a roadway base and also assist in absorbing pavement stresses, which then extends the life of a pavement. "Geosynthetic pavement interlayers lead to an increase in the performance of a pavement overlay by retarding fatigue and reflective cracking," Putman and Bolger say. Typically, the most common geosynthetic interlayer system consists of the existing base pavement, tack coat, nonwoven geotextile paving fabric and an asphalt overlay. One of the most important factors affecting the performance Here's a typical cross section of a paving fabric interlayer system. RoadScience Photo courtesy of Putnam and Bolger