<p><p><figure id='attachment_2596' style='max-width:947px' class='caption alignnone'><img class="wp-image-2596 size-full" style="border: 2px solid #696969;" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Diagram showing the construction sequence of geosynthetic encased columns using the displacement method." width="947" height="496" /><figcaption class='caption-text'> Construction of Geotextile Encased Columns with Displacement Method (Raithel, M. and Henne, J., 2000)</figcaption></figure><h2>Basic Function</h2>Geotextile Encased Columns (GECs) stabilize the soil using a geotextile tube filled will with sand or gravel.<br><h2>Advantages:</h2><ul> <li>Can be used in very soft soils where conventional stone columns are not effective or efficient</li> <li>Provide excellent vertical drainage, which may lead to more rapid construction due to dissipation of excess pore water pressure</li></ul><h2>General Description:</h2>GECs were developed to overcome the problem of bulging of sand or stone columns installed in very soft soils, under vertical loading. The seamless geotextile casing around the column provides additional lateral confinement for the column needed in a very soft soil to carry vertical loads. GECs have been primarily used for embankment foundations with very soft clays (undrained shear strength less than 15 kPa) in Germany, Sweden, and The Netherlands since the 1990s.<br><h2>Geologic Applicability:</h2><ul> <li>Geotextile encased columns are used to improve clays, silts and sands.</li> <li>GECs can be used in very soft soils with undrained shear strengths less than 15 kN/m2 where stone columns are not effective or efficient.</li></ul><h2>Construction Methods:</h2>GECs can be installed in two ways: replacement or displacement. For the replacement method, an open steel pipe is driven into the ground and the inside soil is<br>removed by an auger. For the displacement method, a steel pipe with two closed base flaps is vibrated into the ground and the soil around the pipe is displaced. Then, for both installation methods, the geotextile casing is lowered into the pipe and filled with sand or gravel. After the pipe is withdrawn under vibration out of the ground, a geotextile encased column with sand and or gravel at a medium density is completed. Compaction of the sand or gravel fill is achieved by gravity drop and further compaction occurs when the pipe is vibrated out of the ground at the end of installation.<br><h2>Additional Information:</h2>GECs have been used to increase bearing capacity, shear strength, and rate of consolidation, and to reduce settlement. Design of GECs is typically performed by determining the layout (triangular or rectangular), spacing, depth of columns, and the hoop stress in the geotextile to meet design requirements. The typical diameter of columns is 0.8 meters (32 inches) and the spacing ranges from 1.7 to 2.4 meters (5.5 to 8 feet) (i.e., 10 to 25% area replacement ratio). The design principle is similar to sand or stone columns in soft clays, i.e., treating a column and its surrounding soil in a unit cell as a composite foundation. Due to the encasement, the GECs are stiffer than conventional sand or stone columns installed in very soft soils.<br><h2>SHRP2 Applications:</h2><ul> <li>Embankment and roadway construction over unstable soils</li> <li>Roadway and embankment widening</li></ul><h2>Example Successful Applications:</h2><ul> <li>Railway Spur Roadbed, Oakland, CA</li> <li>Embankments, Vijfwal Houten, the Netherlands</li></ul><h2>Complementary Technologies:</h2>Load transfer platforms<br><h2>Alternate Technologies:</h2>Preloading, stone columns, jet grouting, piles, and deep mixing methods<br><h2>Potential Disadvantages:</h2><ul> <li>A proprietary technology</li> <li>Has not been widely used in the U.S. to date</li> <li>Seamless geotextiles, which require specialty manufacturing, are used for GECs.</li></ul><h2>Key References for this Fact Sheet:</h2>Alexiew, D., Brokemper, D., and Lothspeich, S. (2005). “Geotextile Encased Columns (GEC): Load capacity, geotextile selection and pre-design graphs.” Contemporary Issues in Foundation Engineering, GSP No. 131, ASCE, Reston/VA: 1-14.</p><p>Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J. G., and Berg, R. R. (2006). “Ground Improvement Methods”- Volume I. Federal Highway Administration Publication No. NHI-06-020.</p><p>Raithel, M. and Kirchner A. (2008). “Calculation techniques and dimensioning of encased columns – Design and state of the art.” Proceedings of the 4th Asian Regional Conference on Geosynthetics, Shanghai: 718-723.</p></p>