Kercher Engineering, Inc.

Although geosynthetics have been available for use in a variety of public works applications for many years, they have not been widely used. A major reason for this under-utilization is that public works personnel have not been informed about the benefits of using geosynthetics. The purpose of this article is to provide an overview of what geosynthetics are and why they should be used, as well as, to illustrate practical applications.

Geosynthetics is a generic term that is used to describe a variety of synthetic materials that are manufactured for use with soil, rock or other materials used in public works projects. Although geotextiles are probably the most well-known of the geosynthetics, there are six basic types:

Geotextiles are a permeable synthetic material made of textile materials. They are usually made from polymers such as polyester or polypropylene. The fibers are then made into either woven (Figure 1) or non-woven fabrics (Figure 2). In general, woven fabrics have higher strength, are less likely to stretch, but may have reduced water flow characteristics when compared to non-woven fabrics.

Although both woven and non-woven fabrics can be used for a variety of applications, one type of fabric will usually perform better in a specific situation. For example, if a geotexile is being used for reinforcement, a low modulus/high stretch non-woven geotexile is probably not the best selection. However, if the geotexile were being used for separation on a rough terrain where it is important that the material stretch and conform to the uneven surface, non-woven would probably be a better choice than a stiffer woven material. As with all types of geosynthetics, a professional who is knowledgeable in the field should be consulted to determine which product would produce the best results.

The AASHTO M288 specification for geotextiles provides three classifications of materials. The classifications are related to strength-like properties that are meant to provide a certain level of survivability during construction. This is important since installation is when the geotextile is most likely to be damaged. It must be noted that using a Class 1 geotextile does not mean that the Contractor can stop using proper handling and construction procedures.

A geosynthetic material that is shaped in an open, grid-like pattern (Figure 3) that is used primarily for reinforcement. Geogrids are normally manufactured by stretching plastic sheets in one or two directions.

They are three dimensional, honeycomb-like cells that are filled with soils or gravels. A geo-cell is typically used for erosion control and stabilization. Geo-cells provide cellular confinement to prevent the movement of granular materials, thereby creating a very rigid layer of material. Geo-cells can also be filled with concrete.

A geosynthetic material consisting of parallel sets of intersecting ribs that form a three-dimensional net-like material. They are used to improve drainage by creating a “thin” plane for water to travel through.

A geosynthetic material that is virtually waterproof when used as a fluid barrier. A common application of this is a landfill liner.

A material made up of a combination of geosynthetic materials that is used to improve performance by combining the benefits of two types of geosynthetics.

There are virtually hundreds of different types of geosynthetic products available on the market today. Please remember that all geosynthetic materials work in some type of application, but no geosynthetic works in all applications. Therefore, make sure that you have the right geosynthetic product for the right job.

One common question asked is, “Why do geosynthetics need to be used to improve the performance of the existing soils?” In many cases, the existing soil has many inherent flaws including:

Soils are made up of different types of particles such as gravel, sands, silt, clay and possibly organic materials. Many times, the consistency of the soil (types of particles) can vary throughout the length of the project. This can have a significant effect on such factors as drainage, settlement, frost heaves, etc., all of which can create problems.

In areas where soils consist of clays, silts and organics, especially in areas that drain poorly, the subgrade may be unstable. As a result, the unstable soil is not able to provide adequately support for a road or embankment.

Depending upon the consistency of the soil, the presence of moisture can create such problems as loss of strength, swelling/shrinking, and frost heave.

Most soils can resist forces that compress the material (to a certain level). However, soils cannot resist forces that pull the soil apart (tensile force). In situations where the existing soils exhibit one or more of the above-mentioned problems, traditional alternatives include: remove and replace poor soils, soil stabilization, use of piles or cassions, and/or installation of complex drainage systems. These solutions can be very costly and time consuming. Another potential solution is to use geosynthetics. Unlike soils, geosynthetics are manufactured specifically to provide consistent properties that can be designed by the manufacturer and specified by the user. Other benefits include ease of construction, increased life of the structure and reduced maintenance requirements.

In many cases, the use of geosynthetics will allow for the utilization of lower quality fill materials, less fill material, or reduce the amount of necessary excavation. In these cases, geosynthetics can reduce the overall cost of the project. However, not all applications of geosynthetics produce direct cost savings by lowering the initial cost of the project. In some situations, the cost savings result from an expected reduction in future maintenance costs and/or an increase in the life of the project (based on a life cycle cost analysis).

Since geosynthetics can increase the life of a road and reduce the long-term maintenance requirements, local agencies should consider amending their design and/or construction ordinances to require the use of geosynthetics in new road construction and rehabilitation projects, when feasible. Although this requirement will slightly increase the cost of the project, it should provide long-term cost savings to the agency when it has to maintain the facility (road, bridge abutment, embankment, etc.).

Four of the most common general uses of geosynthetics for local agencies are:

One of the most common uses of geosynthetics is to use a geotextile to provide separation of two layers with different soil properties. This is an extremely important function because separation will prevent the different soil types from mixing together, thereby maintaining the integrity and functionality of the soils. Using a road as an example, the separator will prevent the aggregate base course from sinking into weaker subgrade material (aggregate loss) and preventing fine material in the subgrade from pumping up into the aggregate base course (pumping). If aggregate loss or pumping occurs, the strength of the pavement can be drastically reduced as illustrated in Figure 6 which shows the reduced “effective” thickness of the aggregate base course.

a) Aggregate Loss due b) Separator prevents Aggregate Loss
to lack of separation

A recent visit to a Delaware municipality illustrates the need for separation. A newly constructed road with an aggregate base placed over a “selected fill” material was failing structurally. The select fill was used in the project to raise the elevation of the pavement in order to improve drainage. Unfortunately, the “select” fill material in the failed area contained a high percentage of clay soil. In failed areas, core borings showed that the “select” fill with a high clay content had pumped up into the aggregate creating a very weak supporting base. Due to the reduced strength of the base, the road failed in less than one year. In areas where the road was in good shape, the subbase consisted of granular material that did not pump into the base. This is one of many examples that illustrate the importance of separating dissimilar materials in order to assure the long-term performance of roads.

In this type of application, the geosynthetic acts as a filter by preventing material from washing out while allowing the water to flow through. The most common uses of this application are: geotextiles which wrap around an edge drain (see Figure 7), geotextiles placed under erosion control devices, and geotextiles used behind structures such as retaining walls.

Although filtering applications are commonly referred to as drainage applications, they are different. Drainage applications refer to situations where the water flows within the plane of the geosynthetic product (in-plane drainage). In filtration applications, the water flows across the plane of the material.

Although certain types of geotextiles provide some in-plane drainage, most drainage situations require a geo-composite drainage product such as prefabricated sheet drains that provide a much greater drainage capacity.

In this application, 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 tensile strength that helps to hold the soil in place. Reinforcement provided by geotextiles or geogrids allows embankments and roads to be built over very weak soils and allows for steeper embankments to be built.

A major benefit of using geosynthetic products is that the desired properties of the material can be engineered and manufactured. These “engineered” properties are essential to the product providing the desired results. To assure that these desired properties are achieved in the field, it is necessary to properly specify the correct geosynthetic material for the job, as well as, to correctly install the product. If not, chances are the geosynthetic material will not produce the desired results. Specifications should address such items as:

Should you have any questions, or want more information about geosynthetics, please call Kercher Engineering, Inc.