Design and performance of geotextiles
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, says Dr P Senthilkumar
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, says Dr P Senthilkumar.
Geotextiles were one of the first textile products in human history. Excavations of ancient Egyptian sites show the use of mats made of grass and linen. Geotextiles were used in roadway construction in the days of the Pharaohs to stabilise roadways and their edges. These early geotextiles were made of natural fibres, fabrics or vegetation mixed with soil to improve road quality, particularly when roads were made on unstable soil. Only recently have geotextiles been used and evaluated for modern road construction. Geotextiles today are highly developed products that must comply with numerous standards. To produce tailor-made industrial fabrics, appropriate machinery is needed. Geotextiles have been used very successfully in road construction for over 30 years. Their primary function is to separate the sub base from the sub grade resulting in stronger road construction.
The geotextile perform this function by providing a dense mass of fibres at the interface of the two layers. 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 geogrid, geomembranes and geocomposites. The ASTM 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.
Geotextiles should fulfill certain requirements like it must permit material exchange between air and soil without which plant growth is impossible, it must be penetrable by roots etc. and it must allow rain water to penetrate the soil from outside and also excess water to drain out of the earth without erosion of the soil.
To obtain all these properties in geotextiles, the proper choice of textile fibre is of paramount importance. The different synthetic fibres used in geotextiles are nylon, polyester, polypropylene while some natural fibres like ramie, jute etc. can also be used. In this paper, the types of fibres suitable for use as geotextiles have been mentioned along with their basic characteristics, functions and applications in various areas.
Important characteristics of geotextiles are:
Physical properties include specific gravity, weight, thickness, stiffness and density. Mechanical properties include tenacity, tensile strength, bursting strength, drapability, compatibility, flexibility, tearing strength and frictional resistance. Hydraulic properties include porosity, permeability, permittivity, transitivity, turbidity/soil retention and filtration length. Degradation properties include biodegradation, hydrolytic degradation, photo degradation, chemical degradation, mechanical degradation, and other degradation occurring due to attack of rodent, termite. Endurance properties include elongation, abrasion resistance and clogging length and flow.
Functions of geotextiles
Every textile product applied under the soil is a geotextile. The products are used for reinforcement of streets, embankments, ponds, pipelines, and similar applications. Depending on the required function, they are used in open-mesh versions, such as a woven or, rarely, warp-knitted structure, or with a closed fabric surface, such as a non-woven. 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 the geotextile performs one or more of these functions simultaneously.
Separation: The introduction of a flexible, porous textile placed between dissimilar materials so that the integrity and functioning of both materials can remain intact or be improved.
The granular aggregate soil attempts to intrude into the fine soil, thereby ruining the coarse soil’s strength. When this occurs we have a situation that has been called sacrificial aggregate, which is all too often the case without the use of a proper separating geotextile.
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 improvedâ€. In transportation applications10 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.
They are used in all classes of roads and similar civil foundation as the base of construction on contaminated layer is the single most cause of premature failure. The use of separator prevents pumping effect created by dynamic load and also helps the passage of water while retaining soil particles. In these types of geotextiles, thickness and permeability are most important characteristic properties. Some of the applications areas are: Between subgrade and stone base in unpaved and paved roads and airfields, between subgrade in railroads, between landfills and stone base courses, between geomembranes and sand drainage layers, beneath sidewalks slabs, beneath curb areas, beneath parking lots, beneath sport and athletic fields and 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 considerationâ€. Infiltration, fabrics can be either woven or non-woven, to permit the passage of water while retaining soil particles. Porosity3 and permeability are the major properties of geotextiles which involves infiltration action. Application helps the replacement of graded aggregate filters by a geotextiles warping. These applications are also suitable for both horizontal and vertical drains. A common application illustrating the filtration function is the use of geotextile in a pavement edge drain.
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.
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:
In this method, the structural stability of the soil is greatly improved by the tensile strength of the geosynthetic material. This concept is similar to that of reinforcing concrete with steel. Since concrete is weak in tension, reinforcing steel is used to strengthen it. Geosynthetic materials function in a similar manner as the reinforcing steel by providing strength that helps to hold the soil in place. Reinforcement provided by geotextiles or geogrids allow embankments and roads to be built over very weak soils and allows for steeper embankments to be built.
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. Here the non-woven 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 minimising vertical flow of water into the pavement structure.
Applications of geotextiles
Civil engineering works where geotextiles are employed can be classified into road works, railway works, river canals and coastal works, drainage, sports field construction and agriculture.
Road works: The basic principles of incorporating geotextiles into a soil mass are the same as those utilised in the design of reinforced concrete by incorporating steel bars. The fabrics are used to provide tensile strength in the earth mass in locations where shear stress would be generated. Moreover, to allow rapid dewatering of the roadbed, the geotextiles need to preserve its permeability without losing its separating functions. Its filtration characteristics must not be significantly altered by the mechanical loading.
Railway works: The development of the railway networks is being greatly boosted by the present state of economy because of their profitability in view of increasing cost of energy and their reliability as a result of the punctuality of trains even in the adverse weather conditions. The woven fabrics or nonwovens are used to separate the soil from the sub-soil without impeding the ground water circulation where ground is unstable. Enveloping individual layers with fabric prevents the material wandering off sideways due to shocks and vibrations from running trains.
River canals and coastal works: Geotextiles protect river banks from erosion due to currents or lapping. When used in conjunction with natural or artificial embankments, they act as a filter. For erosion prevention, geotextile used can be either woven or nonwoven. The woven fabrics are recommended in soils of larger particle size as they usually have larger pore size. Nonwovens are used where soils such as clay silt are formed. Where hydrostatic uplift is expected, these fabrics must be of sufficiently high permeability.
Drainage: In civil engineering, the need for drainage has long been recognised and has created the need for filters to prevent in-situ soil from being washed into the drainage system. Such wash in soil causes clogging of the drains and potential surface instability of land adjacent to the drains. The use of geotextiles to filter the soil and a more or less single size granular material to transport water is increasingly seen as a technically and commercially viable alternative to the conventional systems. Geotextiles perform the filter mechanism for drainages in earth dams, in roads and highways, in reservoirs, behind retaining walls, deep drainage trenches and agriculture.
Sports field construction: Geotextiles are widely used in the construction of Caselon playing fields and Astro turf. Caselon playing fields are synthetic grass surfaces constructed of light resistance polypropylene material with porous or nonporous carboxylated latex backing pile as high as 2.0 to 2.5 cm. Astro Turf is a synthetic turf sport surface made of nylon 6,6 pile fibre knitted into a backing of polyester yarn which provides high strength and dimensional stability. The nylon ribbon used for this is of 55 Tex. It is claimed that the surface can be used for 10hr/day for about 10 years or more. Modern Astro Turf contains polypropylene as the base material.
Agriculture: It is used for mud control. For the improvement of muddy paths and trails those used by cattle or light traffic, nonwoven fabrics are used and are folded by overlapping to include the pipe or a mass of grit.
Giroud-Han Design
The Giroud-Han (G-H) method replaces the widely used method published by Giroud and Noiray and has been included in the updated “Geosynthetic Design and Construction Guidelines†manual by the Federal Highway Administration (FHWA, 2008). Development of the G-H method was a long and complex effort in the late 1990s and early 2000s. The length and complexity were justified by the authors’ desire to openly provide all details and calculations pertaining to the development of the method. However, the need for a summary of the method has been expressed. This article presents a summary of the method’s features.
Even though the G-H design method has been adopted by consultants and geosynthetic manufacturers, a number of issues have arisen, which are clarified in this article. In particular, this article clearly indicates the equations that are generic and can be used with any geosynthetic with appropriate calibration and the equations that were calibrated for specific geosynthetics. This distinction between generic and calibrated equations is crucial because it was not clear to some readers of the original publications of the G-H method. Also, the calibration steps were not easy to follow due to the length and complexity of the original papers. In this article, they are presented in a concise manner.
Unpaved roads typically consist of an aggregate layer (often called “base course†or simply “baseâ€) resting on the subgrade. When a geosynthetic is used in an unpaved road, it is generally placed at the base/subgrade interface. The use of geosynthetics in unpaved roads is a mechanical stabilisation technique that is different from chemical stabilisation. In mechanical stabilisation, the base is improved via the inclusion of a geosynthetic layer (or layers) and the aggregate remains unbound.
Chemical stabilisation involves inclusion of chemicals (e.g., lime, cement, binders) to bind aggregate materials or the subgrade soils.
It is important to distinguish between aggregate that is bound (as a result of chemical stabilisation) and aggregate that is unbound. In this paper, only unpaved roads constructed with unbound aggregate are considered. These roads can be either unreinforced or reinforced using geosynthetics. The term reinforced is equivalent to mechanically stabilise throughout this article.
The use of the terms reinforced and reinforcement in the context of unpaved roads does not imply that the geosynthetic simply adds force (i.e., simply adds its strength) to the unpaved road structure. As shown in the original publication Giroud and Han a geosynthetic improves an unpaved road through complex mechanisms that mostly do not involve the strength of the geosynthetic per sec. Therefore, in the context of unpaved roads, reinforced and reinforcement should be regarded only as convenient terms established by tradition Development of generic equation of G-H method: The G-H method can be used for the design of both unreinforced and geosynthetic reinforced unpaved roads constructed with unbound aggregate. In the development of the G-H method, the stresses at the interface between the base and subgrade are estimated using a stress distribution angle. The effect of base stiffness on the stress distribution angle is quantified using an approximate relationship between the stress distribution angle and the base to subgrade modulus ratio based on the classical Burmister’s two-layer elastic solution.
In the field, the stress distribution angle decreases progressively because of the progressive deterioration of the base due to cyclic loading resulting from trafficking. Laboratory tests by Gabr on unreinforced bases and on bases reinforced with biaxial geogrids, have led to a linear relationship involving the stress distribution angle and log N, where N is the number of load applications (i.e., the number of axle passes in the field).
The G-H method takes into account the progressive decrease of the stress distribution angle with a term k log N, where k is a dimensionless parameter that depends on the radius of tire contact area (which is assumed to be circular), the base thickness, and the geosynthetic. Indeed, the inclusion of the geosynthetic at the base/subgrade interface reduces the deterioration rate of the base; as a result, the rate of decrease of the stress distribution angle is reduced.
As the stress distribution angle decreases, the maximum vertical stress at the base/subgrade interface increases. Bearing capacity failure of the subgrade occurs when the stress distribution angle decreases to a point where the stress at the interface exceeds the mobilised bearing capacity of the subgrade. The mobilised bearing capacity of the subgrade depends on the undrained shear strength of the subgrade, the surface deformation or rut depth, the tire contact area, and the base thickness.
The presence of a properly selected geosynthetic at the base/subgrade interface results in a stabilisation effect, which decreases subgrade deformation and allows for a higher bearing capacity factor than if there was no geosynthetic. Giroud and Noiray suggested bearing capacity factors of 3.14 and 5.14 in the case of unreinforced and geotextile-reinforced unpaved roads, respectively. These bearing capacity factors have been adopted in the G-H method. In the case of a geogrid-reinforced base, the lateral restraint due to geogrid-aggregate interlock results in an inward shear stress on the subgrade, which increases the bearing capacity factor from 5.14 to 5.71.
The process includes four steps:
It is possible, however, to conceive a one-step calibration/validation process where the parameter k in Equation 2 would be calibrated using field tests that would simultaneously provide validation, which would lead directly to an equation similar to Equation 5.
Applicability and limitations
Giroud and Han stated that the G-H design method is applicable and limited to the following conditions:
Therefore, the validity of the method is currently limited to rut depths ranging between 40 and 100 mm. More calibration work, based on more field data, would be required to extend the validity of the method to a broader range of rut depths. These rut depths, essentially due to the deformation of the subgrade, are measured at the surface of the aggregate base. These are different from surface ruts, which may form during the construction process due to surficial disturbances of the base materials and not because of subgrade deformation. These surface ruts should be filled, rather than graded, prior to proof rolling to maintain the required base or sub base thickness above the geosynthetic.
Transmissivity
Transmissivity is evaluated by the amounts of water to be passed through the geotextile specimen flow under the confined normal stress and the specific hydraulic gradient in accordance with ASTM D 4716. The principal transmissivity mechanism of smart geotextiles in this study is analysed by equation. If water flows along the surface of geotextiles horizontally and the amounts of water-in should be equal to those of water-out, flow rate of water, q, for drainage system could be written by equation from Darcy’s law.
Design methods for geotextiles include design by cost and availability, design by specification and design by function.Design by cost and availability: Geotextile design by cost and availability is simple. One takes the funds available divided by the area to be covered and calculates a maximum allowable geotextile unit price. The geotextile with the best available properties is then selected within this unit price limit. One’s intuition plays a critical role in the ultimate selection process. The method is obviously weak technically but is still sometimes practiced. Design by specification: Geotextile design by specification is common and is used almost exclusively when dealing with public agencies. In this method, several application categories are listed together with critical fabric properties.
Allowable versus ultimate geotextile properties
A particular value of a property cannot be used directly from the laboratory test and must be suitably modified for in-situ conditions. This could be done directly in the test procedure (i.e., by conducting a completely simulated performance test). But in many cases it is simply not possible. Such situations as full-size test specimens, long-term creep testing, use of site-specific liquids, representation of in-situ pore water stresses, complete stress state modeling, etc., are generally not feasible for laboratory situations. To account for the difference between the laboratory-measured test value and the desired performance value, two approaches can be taken: Use an extremely high global factor of safety at the end of a problem, and use partial factors of safety on the laboratory-generated test value to make it into a site-specific allowable value.
Designing for separation: For a separation function to occur, the geotextile must be placed on the soil subgrade and then have stone placed, spread, and compacted on top of it. A number of scenarios can be developed showing what geotextile properties are required for a given situation.
Burst resistance: Consider a geotextile on a soil subgrade with stone of average particle diameter (da) placed above it. If the stone is uniformly sized, there will be voids within it that will be available for the geotextile to enter into.
This entry is caused by the simultaneous action of the traffic loads being transmitted to the stone, through the geotextile, and into the underlying soil. The stressed soil then tries to push the geotextile up into the voids within the stone.
Conclusion
Designers need to take careful consideration of geotextile properties with respect to the selection and specification of geotextiles. Many designers refer to layers within the construction that require specific properties merely as geotextiles. The variability in performance with commercially available geotextiles is vast. They can vary in thickness from a few microns to tens of mm, can be manufactured from a diverse range of raw material (for example polyethylene, polypropylene, polyesters) and be any blend of the foregoing with various mixtures of virgin or recycled material. Geotextiles can be woven, nonwoven, needle punched or thermally bonded all with different pore sizes and permeability. All these aspects give rise to a huge variance with regard to physical properties and performance of geotextiles, together with UV resistance, durability and robustness during installation. All too often designers specify a geotextile based on a popular brand name alone without due consideration of the required material properties. Guidance will provide recommendations on the specification of geotextiles to ensure that they provide adequate filtration to prevent migration of fine soil particles, together with appropriate permeability so they do not limit flow of water in the system.
References
pp. 26–54.
