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SUBGRADE CLASSES
soils have been divided into three classes: good (G), Moderate (M), and poor (P), CBR design values are assigned to these different subgrade classes.
Good
Good subgrade soils retain a substantial amount of their load-supporting capacity when wet. Included are the clean sands, sand gravels, and those free of detrimental amounts of plastic materials. Excellent subgrade soils are relatively unaffected by moisture or frost and contain less than 15 percent passing a No. 200 mesh sieve. A soil classified as good will have a CBR value of 9 or greater.
Moderate
Moderate subgrade soils are those that retain a moderate degree of firmness under adverse moisture conditions. Included are such soils as loams, silty sands, and sand gravels containing moderate amounts of clays and fine silts. When this soil becomes a cohesive material, it should have a minimum proctor density of 110 pounds per square inch. A soil classified as moderate will have a CBR value of 6 to 8.
Poor
Poor subgrade soils are those that become quite soft and plastic when wet. Included are those soils having appreciable amounts of clay and fine silt (50 percent or more) passing a No. 200 sieve. The coarse silts and sandy loams may also exhibit poor bearing properties in areas where deep-frost penetration into the subgrade is encountered for any appreciable periods of time. This also is true where the water table rises close to the surface during certain periods of the year. A soil classified as poor will have a CBR value of 3 to 5.
Very poor soils (those with a CBR of 3 or lower) often perform poorly as pavement subgrades. However, to improve their performance, these soils can be stabilized with granular material or a geotextile. Lime, fly-ash, asphalt cement, Portland cement, and combinations of cement stabilizers also can be added to improve the subgrade support.
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