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Section 9: Geosynthetics in Pavement Structures

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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 the proffering of this definition by ASTM, the department has had a number of applications in materials other than 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 intend to address only those 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 concrete evidence regarding geosynthetic material performance in the roadway. The department has conducted or is a participant in a number of studies. Even so, there is plenty of room for improvement in characterizing all geosynthetic materials to quantify the benefit they lend to pavement performance.

Department geosynthetic usage has increased in recent years with good success, although improper usage or installation has contributed to early failure in some cases. 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. There is terminology shared 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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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).

Applications

This section will consider applications of geosynthetics as typically used by the department within pavement structures. Primarily, there are four applications:

  • pavement surface layer reinforcement
  • geotechnical reinforcement
  • drainage
  • moisture control.

The first application, pavement surface layer reinforcement should be separated and not confused with any other application in a pavement structure. This application is specific to hot-mix asphalt concrete. Geotechnical reinforcement, as it is used in this manual, refers to pavement layers only inclusive of subgrades and subbases. The third and fourth applications are not reinforcements, but are rather features of the pavement structure to enhance and lengthen its performance by reducing the influence moisture has on pavement materials.

Materials

The term geosynthetic is broad and encompasses numerous materials. A geotextile is a permeable geosynthetic made of textile materials; these have uses in all applications. Numerous other materials may be used in combination with geotextiles to create a geocomposite—including grids, nets, meshes, and webs; these are most frequently used in HMAC but do have applications with soil moisture barriers 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 HMAC. Geomembranes are low-permeability geosynthetics used as moisture barriers.

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Geosynthetics for Surface Layer Reinforcement

Geosynthetics used for HMAC 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. Among these are to:

  1. reduce reflective cracking from existing layers of HMAC or from cracks and joints in rigid pavements, and
  2. resist moisture intrusion into lower pavement layers. Fabric underseal has worked well in some parts of the state when placed on CRCP and overlaid with HMAC.

Department research project 0-1777 investigated the use of several geosynthetics for use in HMAC. Researchers developed a guidance document, “Geosynthetic Guidelines” that is available on TxDOT's Technical Data Exchange 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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Geosynthetics for Geotechnical Reinforcement

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

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. This research is due to conclude at the end of FY2006, so findings from the research will need to be incorporated at that time. Additional research on the contribution of geogrid to pavement performance is being conducted in a pooled fund study and will also conclude in late 2006. As the findings are made available, this section will be updated.

Current usage in the State has been for both restraint of pavement materials and for separation of materials. The department acknowledges the benefit of geosynthetics in pavement layers, but to date, there has been insufficient conclusive research to develop guidance with regard to reinforcement of unbound materials in pavement structures. As such, usage of geosynthetics is limited to separation and restraint and cannot be accounted for in FPS-19W design.

Separation

Separation of layers is intuitive. The mechanism is to place a physical barrier between two materials to keep 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 maybe 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 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 and prevent subgrade cracking from propagating to the pavement surface.

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; although, textiles must extend much more than geogrids to provide the same level of support. Grid is more likely to be used and can be very effective. A grid may be used for:

  • mitigation of reflective cracking (see mechanism in separation),
  • creation of a working platform on soft subgrades, sacrificial layer to obtain compaction of subsequent layers,
  • 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.
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Geosynthetics for Drainage Applications

Among the products used in drainage applications are:

Table 3-1: 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
  • 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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Specifications and Testing

The following table shows the current applicable testing specifications and test procedures used to qualify geosynthetics used in the department.

Table 3-2: Departmental Material Specifications

Specification

Title

Geosynthetic

Application

DMS-6200

Filter Fabric

Geotextile

Geotechnical

DMS-6210

Vertical Moisture Barrier

Geomembrane

Geotechnical

DMS-6220

Fabric Underseal

Geotextile

HMAC

DMS-6230

Sediment Control Fence

Geotextile

Drainage

DMS-6240

Geogrid

Geogrid

Geotechnical

DMS-6250

Grid/Fabric Composite for HMAC

Geocomposite

HMAC

DMS-6260

Reinforced Fabric Joint Underseal

Geocomposite

HMAC

DMS-6300

Waterproofing

Geomembrane

Drainage/Waterproofing


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