Page 160 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
Particle Size and Gradation 155

graded.’’ If coarse grains are in contact and voids between them are filled with
smaller particles, the soil must increase in the volume, or dilate, in order to shear.
This adds appreciably to the shearing resistance.

In many soils the silt and clay content are high enough to separate larger soil
grains so that shearing can occur through the silt-clay matrix without dilatancy,
which causes a marked reduction in the soil shearing strength. Artificial mixtures
of sand plus clay show that this property change occurs at about 25 to 30 percent
clay. The two distinct modes of behavior distinguish ‘‘granular soils’’ from
‘‘fine-grained soils.’’

7.5.5 Soil Mixtures
In Fig. 7.5 a poorly graded silt soil is combined with a poorly graded sand to
obtain a more uniform grading. In this example the mix is 50–50, and the
construction lines are shown dashed. A better grading could be obtained by
reducing the percentage of A and increasing that of B. The effectiveness of an
improved grading can be determined with strength tests. Geologists refer to a well-
graded soil as being ‘‘poorly sorted,’’ which means the same thing even though the
connotations are different.

Flat portions of a particle size accumulation curve indicate a scarcity of those
sizes, and a soil showing this attribute is said to be ‘‘gap-graded.’’ Gap grading
tends to give lower compacted densities and strength, and higher permeability.

7.5.6 Soil as a Filter
Filters are barriers that can transmit water while retaining soil particles that
otherwise would be carried along in the water. Filter soils usually are sands.
A common use of a filter is in the toe drainage area in an earth dam, where control

Figure 7.5
Combining two
poorly graded soils
A and B to obtain a
more uniform
grading A þ B.

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