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Subgrade Conditions in which Geogrids are Useful
Geosynthetics have a 30+ year history of successful use for the stabilization of very soft wet
subgrades. Based on experience and several case histories summarized by Haliburton,
Lawmaster, and McGuffey (1981) and Christopher and Holtz (1985), the following subgrade
conditions are considered optimum for using geosynthetics in roadway construction:
• Poor soils
(USCS: SC, CL, CH, ML, MH, OL, OH, and PT)
(AASHTO: A-5, A-6, A-7-5, and A-7-6)
• Low undrained shear strength
τ τ f = c u < 2000 psf (90 kPa)
CBR < 3 (Note: Soaked Saturated CBR as determined with ASTM D 4429)
R-value (California) ≈ < 20
M R ≈ < 4500 psi (30 MPa)
• High water table
• High sensitivity
Under these conditions, multiple functions are possible. Geosynthetics function as
separators to prevent intermixing of roadway aggregate and the subgrade. Filtration is
required because soils below a CBR of 3 are typically wet and saturated. This water must be
allowed to pass up through the geosynthetic into the aggregate, such that destabilizing pore
pressure in the subgrade generated from wheel loads can rapidly dissipate. Pore pressure
dissipation will also allow for strength gains in the subgrade over time. Some level of
reinforcement may also be provided through lateral restraint of the roadway aggregate placed
directly above the geosynthetic, which in turn reduces the stresses on the subgrade and
improves bearing capacity. If large ruts develop during placement of the first aggregate lift,
then some membrane reinforcing effect is also present.
Subgrade Conditions in which Geogrids are Useful
Geosynthetics have a 30+ year history of successful use for the stabilization of very soft wet
subgrades. Based on experience and several case histories summarized by Haliburton,
Lawmaster, and McGuffey (1981) and Christopher and Holtz (1985), the following subgrade
conditions are considered optimum for using geosynthetics in roadway construction:
• Poor soils
(USCS: SC, CL, CH, ML, MH, OL, OH, and PT)
(AASHTO: A-5, A-6, A-7-5, and A-7-6)
• Low undrained shear strength
τ τ f = c u < 2000 psf (90 kPa)
CBR < 3 (Note: Soaked Saturated CBR as determined with ASTM D 4429)
R-value (California) ≈ < 20
M R ≈ < 4500 psi (30 MPa)
• High water table
• High sensitivity
Under these conditions, multiple functions are possible. Geosynthetics function as
separators to prevent intermixing of roadway aggregate and the subgrade. Filtration is
required because soils below a CBR of 3 are typically wet and saturated. This water must be
allowed to pass up through the geosynthetic into the aggregate, such that destabilizing pore
pressure in the subgrade generated from wheel loads can rapidly dissipate. Pore pressure
dissipation will also allow for strength gains in the subgrade over time. Some level of
reinforcement may also be provided through lateral restraint of the roadway aggregate placed
directly above the geosynthetic, which in turn reduces the stresses on the subgrade and
improves bearing capacity. If large ruts develop during placement of the first aggregate lift,
then some membrane reinforcing effect is also present.