Lesson 1/Learning Event 1
A soil, in order to be classified as well-graded, must have a good range of all representative particle
sizes between the largest and the smallest. The soil in Figure 7 is well-graded.
FIGURE 7. WELL-GRADED SOILS
Poorly-graded soils are either those containing a narrow range of particle sizes or those with some
intermediate sizes lacking. Soils with a limited range of particle sizes are called "uniformly graded."
Soils which have some intermediate size or sizes not well represented or missing are called "gap
graded," "step graded," or "skip graded." Figure 8 shows poorly graded soils.
FIGURE 8. POORLY GRADED SOILS
The well-graded soil is preferred for construction because it can be easily compacted into a dense mass
with minimum voids. It has three advantages over poorly-graded soil:
The solid mass is denser because of the interlocking of the particles which enable it to support
heavier loads.
Since the particles are fitted, it realizes the best downward load distribution.
The tendency for displacement of individual grains by either loads or moisture is minimized
because they are locked in place.
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