Lesson 1/Learning Event 3
fied as clay. And any material whose values of PI vs. LL plot below the A-line will be classified as silt.
Materials whose limits plot within the cross-hatched area will exhibit both clayey and silty
characteristics and therefore will be dually classified.
The B-line, of LL = 50 percent, separates the high compressible from the low compressible fine-grained
soils. If the limits plot on the B-line or the right, the material will be high compressible. If the limits
plot to the left of the B-line, the material will be classified as low compressible.
The amount of compressibility is an engineering characteristic which is critical to construction of roads
and airfields. Dense plastic soils, for example, having a high water content are subject to compression
and rebound under wheel load. Where this type of soil is encountered, steps must be taken to stabilize it.
These procedures will be covered later in this subcourse.
The soil characteristics determine its suitability to serve some intended purpose. Generally, a dense soil
withstands greater applied loads (has greater bearing capacity) than a loose soil. Particle size has a
relation to this capacity. From empirical tests, it has been found that well-graded, coarse grained soils
generally can be compacted to a greater density than fine grained soils, because the smaller particles
tend to fill the spaces between the larger ones.
The shape of the grains also affects the bearing capacity. Angular particles tend to interlock and form a
denser mass and are more stable than the rounded particles which can roll or slide past one another.
Poorly-graded soils with their lack of one or more sizes leave more or greater voids and therefore are a
less dense mass. Moisture content and the consistency limits also determine the soil's suitability. A
coarse-grained sandy or gravelly soil generally has good drainage characteristics and may be used in its
natural state. A fine grained clayey soil with a high plasticity index may require considerable treatment
especially if used in a moist location.