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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.
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.