Characteristics of the Environment - Field Sampling: Principles and Practices in Environmental Analysis - page 39

As can be seen in Table 2.3, plasticity is an important characteristic of soils from an
engineering perspective. A plasticity index (PI) can be calculated for any soil or similar
material. It is the difference between the plastic limit (PL) and the liquid limit (LL).
When water is added to an air dry soil, individual soil peds are moistened first. At a
certain water content the soil becomes plastic (PL), which means that it can be molded
into a shape and will retain it. If more water is added it becomes liquid and runs; this is
the LL. The difference between these two limits is the PI, which is simply calculated
using the formula below.
PI=LL−PL
TABLE 2.3 Engineering Classification of Soils
Symbol Meaning
GW Gravel (G) larger than #4 sieve—well (W) sorted
GP Gravel poorly (P) sorted
GM Gravel containing silt (M)
GC Gravel containing clay (C)
SW Sand (S) smaller than #4 but larger than #200 sieve—well sorted
SP Sand poorly sorted
SM Sand containing silt
SC Sand containing clay
ML Inorganic silts with low plasticity (L)
CL Inorganic clay with low plasticity
OL Organic matter with low plasticity
MH Inorganic silts of high plasticity (H)
CH Inorganic clay of high plasticity
OH Organic matter of high plasticity
Pt High organic matter soils
Source: Taken from Ref. 18.
Soils with a high PI typically contain large amounts of clay. This gives an indication of
the water movement and the retention of contaminants spilled on soil.
There are several other terms or physical characteristics of soils and soillike media that
engineers use. One is the Attenburgh limits, which refer to the PL and LL of a particular
medium. There is also COLE, which is the coefficient of linear extensibility. As the name
implies, this is a measure of the amount of swelling and shrinking a material undergoes
between wetting and drying. It indicates the amount and type of clay in a soil. In working
with engineers one is likely to hear these terms used frequently.
Engineers will also talk of the A-line . This is a graph that relates the PI to the LL. Soils
with characteristics above the A-line and with a LL greater than 50 are inorganic clays of
high plasticity. In the same region but below the A-line are fine sands or silts and elastic