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Section 10: Geosynthetics in Pavement Structures

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10.1 Introduction

A geosynthetic, according to ASTM (1994) is a planar product manufactured from a polymeric material used with soil, rock, earth, or other geotechnical-related material as an integral part of a civil engineering project, structure, or system. Since this definition was established, geosynthetics have evolved to include many more complex (non-planar) formats. Also, the scope of applications now exceeds the traditional “geo-” applications.

For the purposes of this manual, we will define a geosynthetic as “A manmade material that consists of one or more products used to provide added benefit to the infrastructure.” In this manual, we address only roadway applications. Applications of geosynthetics for use in other structures are addressed in the Bridge Division’s Geotechnical Manual.

There have been relatively few studies that have offered conclusive evidence regarding geosynthetic material performance in the roadway. Although the department has conducted or is a participant in a number of studies, there is plenty of room for improvement in characterizing all geosynthetic materials to quantify their benefit to pavement performance.

Although improper usage or installation has contributed to early failure in some cases, the department’s geosynthetic usage has increased in recent years with good success. Applications have been in asphalt concrete overlays of existing asphalt concrete and hydraulic (Portland) cement concrete surfaces, unbound (flexible) base, soft subgrade, drainage, and encapsulation. Shared terminology exists between these applications; clarification of the terminology will hopefully alleviate the confusion.

This section begins with a description of materials for applications that are most frequently used in the department. Following this, a discussion of the materials frequently used in each application and the expected behavior of the material is presented.

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10.2 Description of Materials and Applications

Several materials are available for incorporation into pavement structures. The two geosynthetics primarily used are geotextiles and geogrids. Recently, manufacturers have combined the two materials creating a type of geocomposite (combination of a geosynthetic and another product).

10.2.1 Materials

The term geosynthetic is broad and encompasses numerous materials. A geotextile is a permeable geosynthetic made of textile materials; this material has uses in all applications. Numerous other materials may be used in combination with geotextiles to create a geocomposite—including grids, nets, meshes, and webs; geotextiles are most frequently used in HMA but do have applications with soil moisture barriers, unbound base confinement, and even soil erosion blankets.

Geogrids are primarily used for reinforcement and are formed from integrally connected and attached elements creating apertures in which adjoining material embed and are of sufficient size to interlock with it. The reference to geogrid is primarily reserved for the application in unbound bases and subgrades; although, reinforcing grids are available for use in HMA. Geocells can be thought of as a 3-dimensional grid allowing confinement of unbound low cohesion materials in-depth. Geomembranes are low-permeability geosynthetics used as moisture barriers.

10.2.2 Applications

Primarily, there are four applications of geosynthetics typically used by the department within pavement structures as given in Table 3-11:

Anchor: #i1010094Table 3-11: Geosynthetic Applications

Application

Name

Description

1

Pavement Surface Layer Reinforcement

The first application should be separated and not confused with any other application in a pavement structure. This application is specific to hot-mix asphalt concrete.

2

Geotechnical Reinforcement

As it is used in this manual, refers to pavement layers only inclusive of unbound bases, subbases, and subgrades. In this application, soils are restrained from movement, thus mechanically stabilizing the layer.

3

Drainage

Drainage and moisture control are features of the pavement structure, not reinforcements. Their functions are to enhance and lengthen pavement performance by reducing the influence moisture has on pavement materials.

4

Moisture Control


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10.3 Geosynthetics for Surface Layer Reinforcement

Geosynthetics used for HMA applications have been used by the department since the mid-1980s. Since then, there have been numerous products manufactured for various purposes to incorporate in the pavement surfaces. These products are used to:

Department research project 0-1777 investigated the use of several geosynthetics in HMA. Researchers developed a guidance document, “ Geosynthetic Guidelines,” available on TxDOT's Construction and Materials Tips webpage. This document discusses the advantages and disadvantages of using geosynthetics in HMA applications, guidance on the selection of materials, cost considerations, pavement design, as well as construction considerations.

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10.4 Geosynthetics for Geotechnical Reinforcement

10.4.1 General

Several research projects have been conducted in an attempt to quantify the benefits of geosynthetics in unbound pavement layers. Applications range from a passive use of materials creating a separation between pavement layers to an active role when relying on geosynthetics to take on some of the structure’s load.

Materials most often used in this application are geogrids and geotextiles. Many geosynthetics have multiple uses and can serve more than one function. For instance, geogrids are often used in a way to restrain base material during compaction or loading, but they also serve as a separation layer to prevent excessive migration and intermingling of pavement layers at interfaces.

Ongoing departmental research will attempt to better describe the interaction between pavement layers and geosynthetics used in pavement layers. Additional research on the contribution of geogrid to pavement performance has been conducted in pooled fund studies.

Current usage in Texas has been for both restraint of pavement materials and for separation of materials. The department acknowledges the benefit of geosynthetics in pavement layers; however, there has been insufficient conclusive research to develop guidance with regard to reinforcement of unbound materials in pavement structures at this time. As a result, usage of geosynthetics is limited to separation and restraint and cannot be accounted for in FPS 21 design. An exception to quantifying the structural contribution might be in the use of geocells that confine unbound base material, but no use has been documented in Texas.

10.4.2 Separation

Separation of layers is intuitive. This mechanism places a physical barrier between two materials to prevent them from intermixing. Mixing of fine-grained soil particles into the overlying flexible base can create a significantly finer gradation over time, increasing the base suction properties and greatly decreasing its strength. The result of this placement is to maintain a viable structure for a longer period of time.

Another more recent application, and possibly the most quantifiable, is grid used as both a separator layer and a restraint to base movement. It has been used to prevent or reduce reflective cracking due to differential relative movement between pavement layers. An example of this use is between subgrades (or stabilized materials) with high plasticity index (PIs) that exhibit large volumetric shrinkage when moisture is drawn out due to weather, drainage, or vegetation demands, and unbound flexible base. The grid holds the unbound material in a tight matrix allowing the shrinking subgrade (or stabilized material) to move while preventing subgrade cracking from propagating to the pavement surface.

10.4.3 Restraint

Restraint is the mechanism of preventing or reducing the lateral movement of materials. This application is useful when soft subgrades are present and a platform for subsequent construction is required. Both grid and geotextiles have been used in this application. Grid is more likely to be used and can be very effective. When textiles are used, note that textiles must extend much more than geogrids to provide the same level of support.

A grid may be used for:

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  • mitigation of reflective cracking (see mechanism in separation),
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  • creation of a working platform on soft subgrades, sacrificial layer to obtain compaction of subsequent layers, or
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  • a substitute in lieu of lime, e.g., sulfate laden soils, where lime is detrimental, or urban areas, where lime may not be tolerated, or combinations of these and soft soils.

Geocells have been used to confine low cohesion base materials, allowing these materials to be used more effectively in a pavement structure.

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10.5 Geosynthetics for Drainage Applications

Among the products used in drainage applications are:

Anchor: #i1010118Table 3-12: Geosynthetics for Drainage

Material

Application

Geotextile
  • Transmission of moisture to pavement edge.
  • Trench lining to prevent intrusion of fine soil into drainage layers and structure.
  • Wrapping drainage pipe to prevent siltation of the drain.
  • Wrapping aggregate to provide confinement and prevent fine soil intrusion.
  • Silt fencing.
  • Erosion control logs.

Geomembrane
  • Vertical moisture barrier for pavement edges.
  • Creation of retention/detention ponds.
  • Encapsulation provision for confining and waterproofing material in the roadbed.

Geowebs
  • Composite material that provides both a drainage structure and separation with a geotextile.

Vertical Drains
  • Composite material that transmits water from the roadway, through a geotextile and down to a drainage structure.


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10.6 Specifications and Testing

Table 3-13 shows the applicable Departmental Material Specifications (DMS) and test procedures used to qualify geosynthetics used in the department. Governing construction special specifications are located at: http://www.dot.state.tx.us/apps-cg/specs/ShowSD.asp?year=4&type=SS&number=5.

Anchor: #i1010137Table 3-13: Departmental Material Specifications

Specification

Title

Geosynthetic

Application

DMS-6200

Filter Fabric

Geotextile

Geotechnical

DMS-6210

Vertical Moisture Barrier

Geomembrane

Geotechnical

DMS-6220

Fabric For Underseals

Geotextile

HMA

DMS-6230

Temporary Sediment Control Fence Fabric

Geotextile

Drainage

DMS-6240

Geogrid for Base/Embankment Reinforcement

Geogrid

Geotechnical

DMS-6270

Biaxial Geogrid for Environmental Cracking

Geogrid

HMA

DMS-6260

Reinforced Fabric Joint Underseal

Geocomposite

HMA

DMS-6300

Waterproofing

Geomembrane

Drainage/Waterproofing



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