Paper No. ______03-3774_____ TITLE: HYDRAULIC COMPATIBILITY OF GEOTEXTILE DRAINS WITH FLY ASH IN PAVEMENT STRUCTURES Author(s): M. Emin Kutay, and Ahmet H. Aydilek Department of Civil and Environmental Engineering 1173 Glenn Martin Hall University of Maryland, College Park, MD 20742 [email protected] Session Title: Recent Improvements in Performance of Pavement Drainage Systems Sponsoring Committees: A2KO6 on Subsurface Drainage and A2KO7 on Geosynthetics Paper accepted for presentation at the Transportation Research Board 82nd Annual Meeting January 12-16, 2003 Washington, D.C. Abstract: The legislations have been promulgated in many states that remove barriers to large-scale beneficial re-use of waste materials. As a result, there is a renewed emphasis on incorporating suitable waste products into highway construction. Fly ash is one of these materials, and is increasingly being used as fill materials for highway embankments, and as grout mixes for road bases. In most cases, a geotextile is in contact with the fly ash or fly ash-treated soil, and drainage is one of the duties expected from the geotextile. Hydraulic compatibility of geotextile drains with fly ash was evaluated through laboratory gradient ratio tests. The results indicate that fly ash is compatible with a variety of woven and nonwoven geotextiles. Clogging of the geotextile or excessive piping of fly ash was not observed, even under relatively high hydraulic gradients. The gradient ratio test had some limitations when used with fly ash. Long-term tests should be performed for evaluating the performance of the fly ash-geotextile systems, and changes in hydraulic conductivity should be analyzed along with the measured gradient ratios.Kutay and Aydilek 1 Key words: fly ash, drainage, geotextile, geocomposite drain, gradient ratio test. INTRODUCTION Geotextiles are increasingly being used in transportation applications due their ease of construction and economy over traditional methods. Geotextiles are often used in embankment construction; their two most important roles being the reinforcement of the foundation and separation of the embankment fill from the foundation soil. In addition to these roles, geotextiles provide lateral drainage of percolating water and prevent the build-up of excess pore water pressure. Pavement subsurface and highway edge drainage systems are other application areas in which geocomposite (GC) drains are commonly employed. GC drains are composed of a geonet sandwiched between two geotextile layers, and have been cost effective alternatives over traditional drainage systems for the last two decades (Allen and Fleckenstein 1991, Christopher et al. 2000). The current design of GC drains and geotextile drains is primarily dependent on their flow rate capacities. However, the hydraulic compatibility of a geotextile with the contact soil is an important issue and should be considered in design procedures. This compatibility is usually analyzed through laboratory soil filtration tests. The first main requirement for ensuring this hydraulic compatibility is that the drain should not be clogged throughout the life of the structure. The second requirement is that the soil piped through the geotextile should be minimal, so that the internal stability and modulus of the soil are not adversely affected. Allen and Fleckenstein (1991) reported excessive clogging of GC drains in two different projects. The drain was completely clogged due to accumulation of soil fines at the surface and inside the geotextile (blinding and clogging phenomenon, respectively), resulting in excess pore water pressure build-up under the pavement. Similar problems, excessive clogging of geotextile component of GC drains with fine-grained soils, were also reported by Highlands et al. (1991). These failures indicate that the hydraulic compatibility of contact soil with the geotextile component of a drain is an important issue, requiring consideration during pavement drainage system design. The problem of fine particle clogging becomes more cumbersome when industrial by-products are in contact with geotextiles in pavement drainage systems. The nature of these geomaterials is different than regular soils, often consisting of significant amounts of fines. The existing geotextile selection criteria may not be directly applicable to these materials and, in most cases, their filtration or drainage performance with geotextiles should be investigated by conducting laboratory tests. Fly ash is one of these industrial by-products, and has increasingly being used in transportation applications as fill materials or grout mixes for highway embankments, as well as grout mixes for road bases (Akram and Gabr 1997, Edil et al. 2002). Beneficial reuse of fly ash in pavement bases has gained wide acceptance due to its abundance. For instance, 3.5 million tons of fly ash was used in pavement construction in the U. S. in 1996 (American Coal Association 1996). In spite of ongoing efforts to use fly ash in highway construction, limited information is available about its hydraulic compatibility with geotextiles (GabrKutay and Aydilek 2 and Akram 1996). Clogging of a geotextile drain by fly ash particles may cause significant reduction in permeability, thus reducing the flow capacity of the drain. Even though, the fly ash is mixed with other aggregates (e.g. sand), reduction of the permeability will be caused as a result of movement of fly ash particles through the drain. Therefore, the fly ash should be hydraulically compatible with the adjacent geotextile. In order to respond to this need, a series of laboratory gradient ratio tests were conducted to investigate the clogging behavior of various geotextiles with fly ash. The retention performance of these geotextiles was also investigated by analyzing the results obtained from the gradient ratio tests. MATERIALS AND METHODS Geotextiles One woven and three nonwoven geotextiles were employed in the testing program. The nonwoven geotextiles were selected from the ones most often used in filter applications and had a wide range of porosity, apparent opening size (AOS or O95) and permittivity (ψ). The woven geotextile was a high-strength one (ultimate wide-width tensile strength=175 kN/m), and has commonly being used in reinforcement applications. In these applications filtration is usually