Composite Polyfabrics Gravel Geogrid

As the geosynthetic provides multiple functions, which both benefit construction and allow

for subgrade improvement with time, AASHTO M288 has identified applications where the

undrained shear strength is less than about 2000 psf (90 kPa) (CBR about 3) as a form of

mechanical stabilization. From a foundation engineering point of view, clay soils with

undrained shear strengths of 2000 psf (90 kPa), or higher, are considered to be stiff clays

(Terzaghi and Peck, 1967) and are generally quite good foundation materials. Allowable

footing pressures on such soils can be around 3000 psf (150 kPa) or greater. Simple stress

distribution calculations show that for static loads, such soils will readily support reasonable

truckloads and tire pressures, even under relatively thin granular bases.

Construction loads, dynamic loads and high tire pressures are another matter. Some rutting

will probably occur in such soils, especially after a few hundred passes (Webster, 1993). If

traffic is limited, as it is in many temporary roads, or if shallow (< 3 in. {75 mm}) ruts are

acceptable, as in most construction operations, a maximum undrained shear strength of

approximately 2000 psf (90 kPa) (CBR = 3) for geosynthetic use in highway construction

seems reasonable. However, for soils that are seasonally weak (e.g., from frost heave) or for

high fines content soils which are susceptible to pumping, a geotextile separator may be of

benefit in preventing migration of fines at a much higher subgrade undrained shear strength.

This is especially the case for permeable base applications. Significant fines migration has

been observed with a subgrade CBR as high as 8 (e.g., Al-Qadi et al., 1998).

Base reinforcement in permanent roadway applications has also been found to be effective at

relatively high subgrade strengths, again with a subgrade CBR as high as 8 (e.g., Berg et al.,

2000). The application of a vehicular load to a flexible pavement results in dynamic stresses

within the various pavement components. As vehicular loads are repeatedly applied,

permanent strain is induced in the aggregate and subgrade layers and accumulates as traffic

passes grow, which leads to rutting of the pavement surface. Fatigue cracking of the asphaltic

concrete layer also results from repeated cycles of tensile lateral strain in the bottom of the

layer. The lateral restraint provided by the geogrid increases the confinement in the aggregate

and thus creates a stiffer system, especially in thin pavement sections. The influence of base

reinforcement does diminish as the pavement system itself becomes stiffer (i.e., thicker

asphalt, thicker base and stronger subgrade.) As discussed in Section 7, geogrids are most

effective in relatively thin base sections (12 in. {300 mm} or less) and weaker subgrade

conditions.

As a summary, the application areas and functions in Table 1 have been identified as

appropriate for the corresponding subgrade conditions.