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Geosynthetic Engineering: Hydraulic Barriers

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Geosynthetic Engineering: Hydraulic Barriers Geosynthetic Engineering: Hydraulic Barriers Course No: G02-001 Credit: 2 PDH Yun Zhou, PhD, PE Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980 P: (877) 322-5800 F: (877) 322-4774 [email protected] 10.0 GEOMEMBRANES AND OTHER GEOSYNTHETIC BARRIERS 10.1 BACKGROUND Barriers are used in earthwork construction to control movement of water, other liquids and sometimes vapors. Barriers are used to waterproof structures, to prevent moisture changes beneath roadways, to contain water and wastes, and to support other applications in transportation works. The function of these barriers is to either prevent damage to highway pavements and structures or to contain water or waste materials. Barriers must be engineered to perform their intended function for the particular application and project being designed. Traditional barriers, or liners, are field-constructed of soil or aggregate-based materials. Thick compacted clay layers, cast-in-place concrete, and asphalt concrete are used to construct liners. Another conventional liner material is geomembranes, which have been used in transportation applications for more than forty years. The U. S. Bureau of Reclamation has been using geomembranes in water conveyance canals since the 1950s (Staff, 1984). Other types of geosynthetic barriers have also been used in transportation applications. These include thin-film geotextile composites, geosynthetic clay liners, and field-impregnated geotextiles. While soil or aggregate-based liners are well-suited to some applications, geomembrane and other geosynthetic barriers are more appropriate for other projects. Suitability may be defined during design and with due consideration to material availability, long-term performance, and cost. For example, rigid concrete and asphalt liners or semi-stiff compacted clay liners are not well-suited to sites where barriers are subject to foundation settlements; conversely a geomembrane which has adequate flexibility would be suitable. 10.2 GEOSYNTHETIC BARRIER MATERIALS Geosynthetic barrier materials can be classified as geomembranes, thin-film geotextile composites, geosynthetic clay liners, or field-impregnated geotextiles. Materials within each classification are reviewed herein, starting with a general material definition. The components, manufacturing processes, resultant product characteristics, and typical dimensions are presented. Available test standards used to quantify property values are listed in Appendix E. Geomembranes and Other Geosymhetic Barriers 367 10.2-1 Geomembranes The term geomembrane is defined as a very low-permeability synthetic membrane liner or barrier used with any geotechnical engineering-related material to control fluid migration in a man-made project, system, or structure (ASTM D 4439). However, within this document the term geomembrane will be used to specifically describe materials which are manufactured of continuous polymeric sheets. Commonly available geomembranes are manufactured of the polymers listed in Table 10-1. TABLE 10-1 COMMON TYPES OF GEOMEMBRANES Available POLYMER TYPE Abbreviation without with scorn scorn reinforcement reinforcement Chlorinated Polyethylene CPE II II Chlorosulfonated Polyethylene CSPE II II Ethylene Interpolymer Alloy EIA II High-Density Polyethylene HDPE II Polypropylene PP II II Polyvinyl Chloride PVC II II Linear Low-Density Polyethylene LLDPE II The HDPE and LLDPE geomembranes are supplied in roll form. Widths of approximately 4.6 to 10.5 m are available. Roll lengths of 200 to 300 m are typical, although custom roll lengths are available. These materials are generally available in sheet thicknesses of 1.0, 1.5, 2.0, and 2.5 mm. These PE geomembranes are usually of singular (i.e., not composite) manufacture. However, thick coextruded PE composites are available with light-colored heat reflective surfaces, electrical conductive surfaces (for leakage testing), or a LLDPE layer sandwiched between an upper and lower layer of HDPE. Geomembranes manufactured of CPE, CSPE, PVC, and PP are supplied in large panels, accordion-folded onto pallets. The panels are fabricated by factory-seaming of rolls, typically 1.4 to 2.5 m in width. Panels as large as 1,800 m2 are available. These geomembranes may be of singular manufacture or composites with a fabric scrim incorporated to modify the mechanical properties (Table 10-2). Geomembranes are relatively impermeable materials - i.e., all materials are permeable, but the permeability of geomembranes (on the order of 10-14 m/s) is significantly lower than that of 368 April 1998 compacted clays. Hence, geomembranes are sometimes referred to as being impermeable, relative to soil. Theoretically, multiple layers of geomembranes and drains can be utilized to construct impermeable structures (Giroud, 1984). Leakage, and not permeability, is the primary concern when designing geomembrane containment structures. Leakage can occur through poor field seams, poor factory seams, pinholes from manufacture, and puncture holes from handling, placement, or in-service loads. Leakage of geomembrane liner systems is minimized by attention to design, specification, testing, quality control (QC), quality assurance (QA) of manufacture, and QC and QA of construction. Geomembrane materials are better-defined than other geosynthetic barriers, due to their widespread use in environmental applications and their historical use in other applications. Manufacturing QC/QA standards, index property test methods, performance test methods, design requirements, design detailing, and construction QC/QA are well-established (Daniel and Koerner, 1993). 10.2-2 Thin-Film Geotextile Composites The moisture barrier commonly used in roadway reconstruction is a thin-film geotextile composite. These composites are used to prevent or minimize moisture changes in pavement subgrades, as discussed in Section 10.3. Two styles of composites, as illustrated in Figure 10-1, are available. One commercially available composite consists of a very lightweight PP nonwoven geotextile sandwiched between two layers ofPP film (Figure 10-la). This product has a mass per unit area of 140 g/m2and is available in roll widths of 2.3 m and roll lengths of 91 m. Another commercially available composite consists of a polyethylene (PE) film sandwiched between two layers of nonwoven PP geotextiles (Figure 10-1 b). This product has a mass per unit area of 300 g/m2 and is available in roll widths of 3.65 m and roll lengths of 91 m. Similar products are available with PVC, CSPE, and PP geomembranes as the core. PP Film PP Geotextile ] PE Film (0) PP Geotextlle ~~~------------ --- ----- PP Film:;; Figure 10-1 Thin-film geotextile composites: (a) PP film I PP geotextile I PP film; and (b) PP geotextile - PE film - PP geotextile. Geomembranes and Other Geosynthelic Barriers 369 10.2-3 Geosynthetic Clay Liners Geosynthetic clay liners (GeLs) are another type of composite barrier materials. A dry bentonite clay soil is supported between two geotextiles or on a geomembrane carrier, as illustrated in Figure 10-2. Geotextiles used above and below the dry clay mayor may not be connected with threads or fibers, to increase the in-plane shear strength of a hydrated GeL. Approximately 5 kg/m2 of dry sodium bentonite is used in the manufacture of GeLs. The bentonite is at a moisture content of 6 to 20% in its dry condition. This dry bentonite hydrates and swells upon wetting, creating a very-low permeability barrier. The fully hydrated bentonite typically will have a permeability in the range of 1 to 5 X 10-11 m/sec. GeLs are supplied in roll form. Widths of approximately 4.1 to 5.3 m are available. Rolliengths of 30 to 60 m are typical, although custom roll lengths may be used for large projects. I ~Y»'J;>'J):;Y»'J»'J>;'J,),'J>;Y»'J»'J) -5mm Cloy + Adhesive· L fwwwmwwwmmww> (0) (c) 1.>.'>-::.'»'''.'7:>:::»»'J»'J)>'J)''''7:.'>-::.;"'7:.'>:::)/'>";>:::») -5mm . Clay r- Cloy + AdhesIve --'.5mm I 1 ,. '" "V'V'V'" ,./' , ~ >»,Y»'Y>;'/»,Y»'Y»'Y)).'/>,Y)~j)-;,/>;,:// ~ E§!~~~~~§3 (b) (d) Figure 10-2 Geosynthetic clay liners: (a) geotextile / bentonite clay / geotextile; (b) stitched bonded geotextile GeL; (c) needle punched geotextile GeL; and (d) bentonite clay / PE geomembrane (after Koerner, 1994). 10.2-4 Field-Impregnated Geotextiles Impregnated geotextiles are also used as moisture and liquid barriers. The coating treatment is applied in the field, after the geotextile is deployed and anchored. A nonwoven geotextile is used with a variety of coatings, including asphalt, rubber-bitumen, emulsified asphalt, or polymeric formulations. The coating may be proprietary. The geotextile's type and mass per unit area will be a function of the coating treatment, although use of lightweight nonwoven geotextiles, in the range of 200 to 400 g/m2, is common. Heavier-weight, nonwoven geotextiles may be used to provide gas venting, if gas potential exists on a site. The barrier is formed as sprayed-on liquid solidifies into a seam-free membrane. Although sprayed-on membranes are seam-free, bubbles and pinholes may form during installation and can cause performance problems. Proper preparation of the geotextile (i. e., clean and dry) to be 370 April 1998 sprayed is important. These types of barriers have been used in canals, small reservoirs, and ponds for water control. Water storage applications have used air-blown asphalt coatings. (Matrecon, 1988) Engineers also use field-impregnated geotextiles to provide moisture control in friable roadway soils. Pavement
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