Reincement Knitted Gravel Geogrid

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.