STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Geological Engineering 269
Plasticity is defined as the ability of such particle groups to deform rapidly without
cracking or crumbling. It also refers to the ability of such groups to change
volume with relatively small rebound when the deforming force is removed.
Silt, in one particle classification system, consists of rock particles from 0.005
to 0.05 mm in size.
Clay, in one particle classification system, consists of inorganic particles less
than 0.005 mm in size. In another system, clay is a fine-grained inorganic soil
that can be made plastic by adjusting the water content. When dried, clay
exhibits considerable strength (i.e., clay loses its plasticity when dried and its
strength when wetted). Also, it will shrink when dried and expand when
moisture is added.
Figure 2-60 shows a classification system developed by the Lower Mississippi Valley
Division, U.S. Corps of Engineers. Percentages are based on dry weight. A mixture
with 50% or more clay is classified as clay; with 80% or more silt, as silt; and with 80%
or more sand, as sand. A mixture with 40% clay and 40% sand is a sandy clay. A
mixture with 25% clay and 65% silt is a clay-silt (see intersection of dashed lines in
Figure 2-60).
100
0 10 20 30 40 50 60 70 80 90 100"
SILT, PERCENT
Figure 2-60. Classification chart for mixed soils (from Lower Mississippi Valley
Division, U.S. Corps of Engineers).