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IDSec. 1.3 Overview of GeogrIds <br />Resardinc the end use of geotexiiles on the basis oi specinc applications, a stud\ <br />conducted in 1982 gives the following distribution [15]: <br />End Use Sales 1 million yd*) <br />Priman- roads (new and overlays!33.6 <br />Secondary roads (unpaved access type)16.8 <br />Remforcement (stabilization, embankments)28.8 <br />Railroads 6.2 <br />Filtration and drainage 15.6 <br />Erosion control 12.0 <br />Other (silt fences, flexible forms)7.2 <br />120.2 <br />The future erowth of eeotexiiles is projected at 15 to 259c per year, with one recent <br />study (the most optimisric I have seen) showing sales at approximately 1.0 billion square <br />yards by 1990 (i.e.. a 34% annual growth rate). <br />1.3 OVERVIEW OF GEOGRIDS <br />1.3.1 History <br />Polymeric materials in the form of extruded nets have been used for more than 20 years. <br />These materials have very good reinforcement characteristics (e.g., modulus and <br />strength). They do, however, have a second use in the geosynthetics area: as a drainage <br />core when making a geocomposite. This area is discussed in detail in Section 1.5 and <br />Chapter 5. The relatively recent discovery' of methods to prepare high-modulus polymer <br />materials by tensile drawing [16], in a sense -cold working," raised the possibility that <br />such materials could be used in the reinforcement of a number of construction materials, <br />including soil. Today, the major function of such geogrids is in the area of reinforcement. <br />1.3.2 Manufacture <br />The basic polymer materials used in the manufacmre of high-strength geogrids are either <br />polypropylene or high-density polyethylene. Initially, they are of a heavy gauge and in <br />sheet form. Holes are then punched into the sheeting on a regular pauem, and the sheet <br />is then drawn uniaxially or biaxially. Drawing is done under controlled temperatures and <br />strain rates so as to avoid fracture but to allow free flow of the molecules into an elon ­ <br />gated and isotropic condition. The key variable in the process is the draw ratio, but other <br />variables, such as molecular weight, molecular weight distribution, and degree of <br />branching or cross-linking, are also imponant [17]. Aside from marked increases in <br />modulus and strength, the creep sensitivity of the polymers is greatly reduced during the <br />drawing process.