Lesson 1/Learning Event 1
Particle Size
The shape of the particles influences the strength and stability of a soil. The two basic shapes are bulky
and platy.
Bulky shapes are subdivided depending on the amount of weathering which has acted upon
them. They may be angular, subangular, subrounded, or rounded (Refer to Figure 6). The angular shape
shows flat surfaces, jagged projections, and sharp ridges; whereas the rounded shape has smooth curved
surfaces and almost approaches a sphere. Cobbles, gravel, sand, and silt fall into the bulky shape group.
FIGURE 6. PARTICLE SHAPES
Platy grain shapes have one dimension relatively small compared to the other two. The faces at
approximately right angles to this dimension will have relatively large areas whereas the other surface
will be edges and present small surfaces. Clay exhibits this shape. Coarse-grained soil particles with
bulky shapes are observable by the naked eye, whereas fine grained particles with platy or bulky shapes
are not.
The angular shape is preferred for construction projects. The rough sides and sharp edges interlock and
provide friction for higher strength and greater resistance against penetration.
Smooth particles, even if bulky, slide over each other, providing low resistance to penetration. Platy soil
is often not desirable for construction because it loses strength and cohesiveness.
Soil Gradation
Gradation is the distribution of the different size groups within a soil sample. The soil may be well-
graded or poorly-graded.
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