<p><p><figure id='attachment_2589' style='max-width:831px' class='caption aligncenter'><img class="wp-image-2589" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Photograph of aggregate being spread out over a geotextile separation layer in a pavement system." width="831" height="569" /><figcaption class='caption-text'> (Photo courtesy of Minnesota Department of Transportation)</figcaption></figure></p><p><figure id='attachment_1463' style='max-width:829px' class='caption aligncenter'><img class="wp-image-1463 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Photograph showing geosynthetics and aggregate being spread out over weak subsoils." width="829" height="552" /><figcaption class='caption-text'> (Photo courtesy of National Highway Institute)</figcaption></figure><h2>Basic Function:</h2>Geosynthetics as separation in pavement systems are used to prevent base aggregates from penetrating into soft subgrade.and the soft subgrade from pumping or migrating up into the base<br><h2>Advantages:<em> </em></h2><ul> <li>Relatively fast and easy placement;</li> <li>Improvement of pavement performance</li> <li>Increases the reliability and service life of pavements,</li> <li>Prevents aggregate penetration of subgrade,</li> <li>Reduces the depth of excavation needed for the unsuitable subgrade material removal,</li> <li>Provides the secondary functions such as reinforcement, filtration, and drainage in pavement systems, and</li> <li>Geosynthetics are not significantly affected by moisture.</li></ul><h2>General Description:</h2>Geosynthetics, especially geotextiles, are used in pavement systems at interface between subgrade and base material to prevent the penetration of base aggregates into subgrade and intrution of subgrade fine into base aggregates.<br><h2>Geologic Applicability:</h2>Class 2 moderate strength geotextile separators are necessary for a subgrade soil with laboratory soaked CBR greater than 3 and less than 8.<br><h2>Construction Methods:</h2>For separation design, the use of a geosynthetic does not reduce the base coarse thickness and is required to adequately carry the design traffic loads for the design life of the pavement. Placement of the geosynthetic between the subgrade and base layers is relatively fast and easy and the benefits are immediate. The geotextile must have sufficient strength to survive installation and to provide separation. Generally, moderate to heavy-weight geotextiles are used to check the possible damage during installation, placement, and compaction of the aggregates base coarse. The geotextile is placed on the prepared subgrade maintaining the overlap requirements. The subbase or base coarse materials shall be placed by end dumping on the geotextile from edge of the geotextile. The specified density shall be obtained by compacting using the smooth drum roller but not by using the vibratory compaction.<br><h2>Additional Information: <strong> </strong></h2>The geotextile must function as a filter between the subgrade and the base coarse to provide drainage of the base coarse. Research is needed to demonstrate the life-cycle benefits of geosynthetics and to verify the results of physical model tests on long-term performance. Quality control during the installation of the geosynthetic layer is crucial to the performance.<br><h2>SHRP2 Applications:</h2>Stabilization of Pavement Working Platforms<br><h2>Example Successful Applications:</h2><ul> <li>Performance of geotextile separators five years after installation – Washington</li> <li>Long-term field performance of a secondary road incorporating geosynthetics at subgrade – base interface - Virginia</li></ul><h2>Complementary Technologies:</h2>Geogrid/geotextile geocomposities and Geocomposite with horizontal drainage layers.<br><h2>Alternate Technologies:</h2>Granular aggregate/base layer; combinations of cement-treated granular material, asphalt chip seals, and dense-graded asphalt concrete; over-excavation and replacement<br><h2>Potential Disadvantages:</h2><ul> <li>Current AASHTO M-E design procedure does not include a methodology for incorporation of separators in pavement structures.</li> <li>Lack of demonstration of life cycle cost benefits</li> <li>Limited test data for the long-term performance.</li> <li>Not cost beneficial in areas where the base and subbase materials are inexpensive and of good materials</li> <li>Geotextile is difficult to place under windy conditions and susceptible to ripping and puncture.</li></ul><h2>Key References for this technology:</h2>AASHTO. (2006). “<em>Standard specifications for geotextiles - M 288</em>.” Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 26th Edition, American Association of State Transportation and Highway Officials, Washington, D.C.</p><p>Christopher, B.R., Holtz, R.D, and Berg, R.R. (2008). “<em>Geosynthetic Design &amp; Construction Guidelines Reference Manual</em>”, U.S. Department of Transportation, National Highway Institute, Federal Highway Administration, Washington DC, FHWA-NHI-07-092</p></p>