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Geotextiles can be worth their cost in aggregate

Installed properly, these materials stabilize stone bases

Information for this article was provided by Amoco Fabrics and Fibers, Atlanta, Ga.

Geotextiles perform three basic functions in stabilizing aggregate sections: separation, drainage and reinforcement. Some agencies hesitate to specify geotextiles for these functions because of a belief that the material adds to the cost of a project. However, with a good design method for geotextile use, and proper installation, most projects realize a 30 percent to 40 percent drop in required aggregate base thickness. This leads to a drop in production costs because the nonwoven geotextile only costs a fraction of that saved from the reduction in required aggregate.

The use of a geotextile for the separation of aggregate and the soil subbase is easily justified to anyone who has placed an aggregate section and has seen it lose its effectiveness over time from the intermixing with underlying subgrade soil. Investigations of failures in unpaved and paved surfaces generally reveal the presence of fine-grained soils intermixed with the aggregate base.

As an aggregate layer is loaded, the bottom loosens with tension cracks allowing the underlying fines, under pressure, to migrate up into the aggregate. As little as 10 percent to 20 percent fines can completely destroy the structural strength of the aggregate by interfering with the hard, stone to stone contact. As fines infiltrate a portion of the structural section, flexure increases, fines migrate further upwards and the section deteriorates until complete structural section failure occurs. This process can quickly destroy the effectiveness of several inches (millimeters) of aggregate.

Geotextiles provide a separation layer between the aggregate and the subgrade soil, to prevent migration of fines and thus indefinitely preserve the original aggregate structural thickness.

The geotextile usually costs no more than 2 inches to 3 inches (50 mm to 75 mm) of compacted, in-place aggregate, but can save several inches (millimeters) of aggregate. The separate function is more dramatic over weak subgrade soils, but is economically practical in the long run to use even on more competent subgrades.

Geotextiles are recommended for this separation function because of their low cost, coefficient of friction, elongation and drape to conform to any surface, effective filtering even after elongation, abrasion and puncture resistance, and their high coefficient of permeability. Geotextiles are made of polypropylene, and, as such are basically inert and will last indefinitely in a buried application.

One extra benefit of using a geotextile for separation is that almost all the aggregate over the geotextile can be reclaimed and reused. This is particularly economical in temporary uses such as mine haul or logging roads or anywhere aggregate is expensive and equipment is available to reclaim the uncontaminated stone.

The drainage function of a geotextile can be critical to structural section performance.

If the subgrade soil is subject to persistent or even occasional wet conditions, the geotextile placed over it must be highly permeable to allow rapid drainage of water from the loaded subgrade soils up into the free draining aggregate base. Otherwise, under the rapid loading conditions from traffic, water pressures in the soil can fail the subgrade by soil liquification.

Geotextiles provide this critical permeability as they filter or keep the fines from migrating upward into the aggregate. Maintaining drainage of the aggregate base and subgrade soil is very important to prevent accelerated failure of the support system. A geotextile also allows the use of more open, free draining aggregates instead of those with fines, which are weakened by moisture and are freeze-thaw sensitive.

Geotextiles are used in reinforcement through mechanisms of restraint or confinement, friction, membrane effect and local reinforcement.

These reinforcement mechanisms, provided by all types of nonwoven and woven geotextiles, are widely recognized.

This design approach is based on the reinforcement function in general and years of experience gained with the use of geotextiles. According to most researchers, the reinforcement function of a geotextile comes into effect when the subgrade soil is weak, generally less than 12 psi shear strength, or CBR 3. However, most of the research to date has dealt with limited loading and the reinforcement function may well be effective in stronger soils when designing for very heavy wheel loads.

Fabrics are used in road construction with locally available aggregate such as a crushed stone, quarry or shotrock, sand, gravel, or sea shells to develop a structural layer. In reinforcement, fabric improves the performance of the aggregate-fabric-soil (AFS) system under repetitive vehicular loading from mechanisms including restraint on the aggregate and subgrade layer, membrane effect, friction developed at the fabric interfaces that creates a boundary layer, and local reinforcement.

These mechanisms are often measured by resistance to permanent deformation or rutting.

Two types of restraint should occur in the AFS system. The first is related to the reverse curve of the fabric outside the wheel path and the downward pressure on the soil that results. This effect increases the bearing capacity of the soil. A second type of restraint effect occurs when the aggregate particles at the soil-aggregate interface move from under the loaded area but are restrained or given a tensile reinforcement because of the presence of the fabric. The strength and modulus of aggregate material benefit from this increased confinement. The increased aggregate modulus decreases the compressive strength on the soil under the wheel load.

As the roadway undergoes large deformation the fabric is stretched and develops tensile stress, the magnitude of which depends on fabric strain and fabric modulus. The net effect is a reduction under the wheel load and an increase outside of the wheel path.

In order to develop fabric-induced stress, substantial vertical deformations, proper geometry, and fabric anchorage are required. Prestressing the fabric to reduce the system deformation to get the fabric in substantial tension is suggested.

Friction developed along the interface between aggregate-fabric and friction-adhesion of the fabric-soil interface create a boundary layer of aggregate and soil adjacent to the fabric. The composite material created contains more favorable properties of ductility and tensile strength. The effectiveness of this phenomenon is tied to the magnitude of friction-adhesion developed at the interfaces. Fabrics should develop high friction-adhesion.

Concentrated stresses from vehicular loading can cause a punching at the points of contact between the aggregate and subgrade. Use of fabric between the aggregate and soft soil distributes the load, reduces localized stresses, and increases resistance to vertical displacement.

 

Installed properly, these materials stabilize stone bases

Preparation affects job success. The successful use of geotextiles in soil stabilization requires proper intallation. The four basic steps involved in placing geotextiles:

  • subgrade preparation
  • geotextile placement
  • aggregate placement
  • aggregate compaction

Careful planning and preparation for each installation step speeds construction and insures good performance and full benefit from nowoven geotextiles.

Follow the manufacturer's guidelines to determine the structural section thickness. The aggregate selected should, whenever possible, be compactible and non-moisture sensitive.

Usually, the geotextile is laid in the direction of construction traffic. However, specific project dimensions may alter this layout. Geotextile panels should be overlapped both side to side and end to end from 1.5 feet to 3 feet (0.5 to 1 meter), depending on subgrade strength.

Adjacent fabric edges can be sewn in the field with a portable sewing machine powered by a generator. Field sewing typically requires three or four laborers. Presewn panels can be supplied from the factory, and fabric can be sewn 2 to 4 inches (50 to 100 mm) from the fabric edges. Use of the presewn fabric minimizes the need for field sewing or overlapping. Sewing costs can be compared to cost of the geotextile lost in an overlap zone. Two laborers can easily handle a roll of nonwoven geotextile fabric.

In normal construction practices, trucks backdump aggregate onto the fabric. A tracked bulldozer works best to spread the aggregate. Lighter weight models are recommended for softer subgrades. Front-end loaders and motor graders exert greater pressure on the subgrade. Vibratory compactors can be used, but only after reasonable compaction and rut stability have been established by the bulldozer. Nonwoven stabilization geotextiles can be used in most weather and temperature conditions.

How to install geotextiles
Regardless of subgrade strength, the site should first be cleared of all sharp objects, tree stumps, and large stones that could puncture the fabric. Unless it is necessary to achieve final grade, the vegetative mat need not be removed, because it can provide extra support during aggregate placement until final compaction. Brush or cushion layers under the nonwoven fabric are usually necessery, since the fabric prevents soil fines from pumping into the aggregate layer.

Geotextiles should be rolled out onto the subgrade by two people, beginning at a point that allows easy access for construction equipment, yet is consistent with the layout plan. On very soft subgrades, the fabric layout and aggregate placement should begin on the firmest soil on the site perimeter, as an anchor point. From there the fabric can be rolled onto softer sections.

Fabric overlaps and seams should be made as specified. In windy weather, soil or rocks should be placed on the fabric to hold it down until aggregate is placed. Ground securing pins are sometimes used in the overlap sections of the geotextiles.

A compactible, non-moisture sensitive aggregate is then backdumped onto the fabric beginning on firm soil at a point just in front of the fabric. This should anchor the fabric firmly. The aggregate is then spread in one lift to a thickness greater than that needed for stabilization to allow for subsequent compaction. If the thickness from one lift is too great for satisfactory compaction, place more than one lift.

In any situation, the first lift should be as thick as necessary to prevent the compaction from overstressing the subgrade. The bulldozer must blade into the load and slightly upward during aggregate spreading for the same reason. This procedure is followed for each load until the fabric is completely covered.

Over very soft subgrade, care must be taken during aggregate placement to insure the fabric is not moved out of position nor the subgrade overstressed. The bulldozer operator can best determine which spots need additional aggregate for good stability by watching for rutting in the aggregate layer.

Over very soft soil conditions, mud waves may appear during aggregate placement or use. Normally, mud waves are not a problem if they do not heave above the surface of the aggregate base. Stress on the subgrade during fill placement causes suburface soil to move away and up from the loaded area. If you expect severe mud waves, contact your fabric manufacturer for information on construction procedures to minimize their adverse effects.

Vehicles should not be allowed to drive directly on the fabric. If the fabric is damaged during installation, the damaged section should be exposed and a patch of fabric placed over it. The patch should be large enough to overlap onto unaffected areas by 3 to 4 feet (1 to 1.25 meters). The aggregate is then replaced and compacted by the bulldozer.

For full stability, the aggregate must be compacted to required density for the design thickness. The surface is initially compacted by walking the tracked bulldozer back and forth over the aggregate while waiting for the next aggregate load. From that point on, construction traffic compacts the aggregate until stability is obtained.

Final compaction is achieved with a vibratory compactor, first without vibration for several passes, then with full vibration. Any weak spots found during final compaction usually indicate inadequate aggregate thickness at those spots. Do not grade ruts down. Instead, fill them with additional aggregate and compact. This rule applies to any future rut maintenance required.

It is important that the construction process be monitored If field conditions change from the design values, and cause a lower subgrade soil strength value, structural section thicknesses must be re-evaluated. Monitoring construction and early use of the aggregate section pinpoints weak areas missed in soil testing.

Equally important is monitoring the quality of the structural section materials and the placement method. The purpose is to detect changes so, if necessary, design adjustments can be made on site before excessive subgrade failures occur.

Final pavement construction should be delayed as long as possible to monitor the unpaved aggregate section's performance. If local areas require it, additional aggregate should be used to correct any rutting. Geotextile test sections provide a lot of insight into how the design structural section will perform.

After these steps are completed, the road or area is ready for use. Stability will increase as traffic and the confining action of the fabric continue to densify the aggregate and consolidate the subgrade.

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