Building Design and Construction Handbook - Merritt - Ventech!

FIGURE 6.9 Plasticity chart.
SOIL MECHANICS AND FOUNDATIONS 6.33
mined in the laboratory by using a liquid limit device. The liquid limit is defined
as the water content at which a pat of soil, cut by a groove of standard dimensions,
will flow together for a distance of 12.7 mm (0.5 in) under the impact of
25 blows in a standard liquid limit device.
Plastic Limit (PL). The water content corresponding to the behavior change
between the plastic and semisolid state of a silt or clay. The plastic limit is also
determined in the laboratory and is defined as the water content at which a silt
or clay will just begin to crumble when rolled into a tread approximately 3.2
mm (0.125 in) in diameter.
The plasticity index (PI) is defined as the liquid limit minus the plastic limit
(i.e., PI � LL � PL). With both the liquid limit and plasticity index of the finegrain
soil known, the plasticity chart (Fig. 6.9) is then used to classify the soil.
There are three basic dividing lines on the plasticity chart, the LL � 50 line, the
A-line, and the U-line. The LL � 50 line separates soils into high and low plasticity,
the A-line separates clays from silts, and the U-line represents the upper-limit line
(i.e., uppermost boundary of test data).
As indicated in Table 6.8, symbols (known as ‘‘group symbols’’) are used to
identify different soil types. The group symbols consist of two capital letters. The
first letter indicates the following: G for gravel, S for sand, M for silt, C for clay,
and O for organic. The second letter indicates the following: W for well graded,
which indicates that a coarse-grained soil has particles of all sizes; P for poorly
graded, which indicates that a coarse-grained soil has particles of the same size, or
the soil is skip-graded or gap-graded; M for a coarse-grained soil that has silt-sized
particles; C for a coarse-grained soil that has clay-sized particles; L for a finegrained
soil of low plasticity; and H for a fine-grained soil of high plasticity. An
exception is peat, where the group symbol is PT. Also note in Table 6.8 that certain
soils require the use of dual symbols.
AASHTO Soil Classification System. This classification system was developed
by the American Association of State Highway and Transportation Officials (see
Table 6.9). Inorganic soils are divided into 7 groups (A-1 through A-7), with the