Geotextiles in Transportation Applications

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Text of Geotextiles in Transportation Applications

  • Paper presented at the Second Gulf Conference On Roads, Abu Dhabi, March 2004,

  • Authors:

    Khalid Ahmed Meccai Eyad Al Hasan Khalid Meccai obtained his Masters degree in Technology with specialization in Geotextiles in 1984. With over 18 years experience in geotextiles, he is currently heading the Marketing and Technical Services department at Alyaf.

    Eyad Al Hasan obtained his Bachelor's degree in Civil Engineering from U.K. in 1982. He is currently working with Alyaf as Sales Manager.

    Summary:

    Geotextiles play a significant part in modern pavement design and maintenance techniques. The growth in their use worldwide for transportation applications in particular, has been nothing short of phenomenal. The focus of this paper is on better understanding of this relatively new tool available to the transportation engineer. The paper provides an overview of the current geotextile technologies and highlights the functions geotextiles perform in enhancing the performance and extending the service life of paved roads. Three key application areas of geotextiles, construction of pavements, in asphalt concrete overlays and for drainage systems along with impetus on the current design methodologies available in geotextile design and selection are addressed.

    Keywords: Geotextile, pavement, drainage, reflective cracking, subgrade, functional properties.

    1. Introduction Geotextiles have proven to be among the most versatile and cost-effective ground modification materials. Their use has expanded rapidly into nearly all areas of civil, geotechnical, environmental, coastal, and hydraulic engineering. They form the major component of the field of geosynthetics, the others being geogrids, geomembranes and geocomposites. The ASTM (1994)[1] defines geotextiles as permeable textile materials used in contact with soil, rock, earth or any other geotechnical related material as an integral part of civil engineering project, structure, or system. Based on their structure and the manufacturing technique, geotextiles may be broadly classified into woven and nonwoven. Woven geotextiles are manufactured by the interlacement of warp and weft yarns, which may be of spun, multifilament, fibrillated or of slit film. Nonwoven geotextiles are manufactured through a process of mechanical interlocking or thermal bonding of fibers/filaments. Mechanical interlocking of the fibers/filaments is achieved through a process called needle punching. Needle-punched nonwoven geotextiles are best suited for a wide variety of civil engineering applications and are the most widely used type of geotextile in the world. Interlocking of the fibers/filaments could also be achieved through thermal bonding. Heat-bonded geotextiles should be used with caution, as they are not suitable for filtration applications or road stabilization applications over soft soils [2].

    2. Geotextile Functions: The mode of operation of a geotextile in any application is defined by six discrete functions: separation, filtration, drainage, reinforcement, sealing and protection. Depending on the application

    Second Gulf Conference On Roads, Abu Dhabi, March 2004,

  • the geotextile performs one or more of these functions simultaneously. The protection function is not discussed here as it is not related to transportation applications.

    2.1. Separation:

    Separation is defined as, The introduction of a flexible porous textile placed between dissimilar materials so that the integrity and the functioning of both the materials can remain intact or be improved (Koerner, 1993) [3]. In transportation applications separation refers to the geotextiles role in preventing the intermixing of two adjacent soils. For example, by separating fine subgrade soil from the aggregates of the base course, the geotextile preserves the drainage and the strength characteristics of the aggregate material. The effect of separation is illustrated in figure 1.

    Figure 1, Concept of separation Function

    2.2. Filtration:

    It is defined as the equilibrium geotextile-to-soil system that allows for adequate liquid flow with limited soil loss across the plane of the geotextile over a service lifetime compatible with the application under consideration (Koerner, 1993) [3]. To perform this function the geotextile needs to satisfy two conflicting requirements: the filters pore size must be small enough to retain fine soil particles while the geotextile should permit relatively unimpeded flow of water into the drainage media. A common application illustrating the filtration function is the use of a geotextile in a pavement edge drain, as shown in figure 2.

    Figure 2 Filtration and Transmissivity Functions

    Second Gulf Conference On Roads, Abu Dhabi, March 2004,

  • 2.3. Drainage (Transmissivity): This refers to the ability of thick nonwoven geotextile whose three-dimensional structure provides an avenue for flow of water through the plane of the geotextile. Figure 2 also illustrates the Transmissivity function of geotextile. Here the geotextile promotes a lateral flow thereby dissipating the kinetic energy of the capillary rise of ground water.

    2.4. Reinforcement:

    This is the synergistic improvement in the total system strength created by the introduction of a geotextile into a soil and developed primarily through the following three mechanisms: One, lateral restraint through interfacial friction between geotextile and soil/aggregate. Two, forcing the potential bearing surface failure plane to develop at alternate higher shear strength surface. And three, membrane type of support of the wheel loads.

    2.5. Sealing Function:

    A nonwoven geotextile performs this function when impregnated with asphalt or other polymeric mixes rendering it relatively impermeable to both cross-plane and in-plane flow. The classic application of a geotextile as a liquid barrier is paved road rehabilitation, as shown in Figure 3. Here the nonwoven geotextile is placed on the existing pavement surface following the application of an asphalt tack coat. The geotextile absorbs asphalt to become a waterproofing membrane minimizing vertical flow of water into the pavement structure.

    Figure 3 Sealing Function

    3. Design Properties and Tests: Standardized testing of geotextile properties has evolved over a very short time reflecting the increasing rate with which these materials are used. Sufficient number of standardized tests both ASTM [1] and EN [4] are available with which to assess the suitability of the geotextile to the specific application. The design engineer incorporating geotextiles needs to understand these test methods and specify only those properties that govern the functional needs of the end application. By taking these steps, the engineer not only protects the clients interest in getting the right product for the application but also invites the largest number of geotextile manufacturers

    Second Gulf Conference On Roads, Abu Dhabi, March 2004,

  • possible; thereby assuring that the cost of the construction materials is kept to a minimum. In this paper only the functional properties that govern typical geotextile applications are addressed.

    3.1. Puncture Strength: ASTM D 4833.

    This test is intended to measure the puncture resistance of geotextiles and geomembranes and simulates the puncture strength of the geotextiles to static loads of aggregates. In this test the geotextile is secured in a ring clamp. A steel rod with a conical tip is then forced through the material and the resistance to puncture is measured in Newton.

    3.2. Burst Strength: ASTM D 3786.

    This test simulates the strength of the geotextile to a continuous hydraulic/mechanical load. In this test the geotextile sample is secured over an inflatable membrane. As the membrane is inflated, the geotextile deforms to a hemispherical shape. The force causing rupture of the geotextile is recorded in units of pounds per square inch or kilo Pascal.

    3.3. Dynamic Puncture: EN 918.

    This test is intended to measure the strength of the geotextile to falling objects and simulates the placement of aggregates over the geotextile during the installation stage. In this method the geotextile is fixed in a ring clamp and a steel cone of 1000 grams is dropped from a height of 500 centimeters over the geotextile. The diameter of the hole created by the impact is measured and expressed in millimeters. Note, the smaller the hole size the tougher the geotextile is to dynamic loading.

    3.4. Grab Tensile Strength and Elongation: ASTM D 4632.

    This index test measures the tensile strength and elongation along the plane of the geotextile by loading it continually. In this test the specimen sample of dimensions 100 by 200 millimeters is secured in clamps of a tensile testing instrument and loaded with a constraint strain rate of 300 millimeters per second. The value of the breaking load is expressed in Newton and the elongation at break in percent.

    3.5. Permeability:

    This test is intended to measure the rate at which liquids can pass through the geotextile. The test method ASTM D 4491 measures the permittivity, which is related to permeability, by the following equation. = k / t - (1) Where: = geotextile permittivity (sec-1) k = geotextile permeability (cm/sec) t = geotextile thickness (cm)

    Second Gulf Conference On Roads, Abu Dhabi, March 2004,

  • Alternatively the test method BS 6906 Part 3 measures the permeability as flow rate and the value expressed in liters per square meters per second.

    3.6. Apparent Opening Size (AOS): ASTM D 4751 The apparent opening size reflects the approximate largest opening dimension available through which the soil may pass. In this test method, the geotextile is secur