Environmental Engineering Reference
In-Depth Information
clay and light soils high in sand. In regions of the world with a predominance of sandy
soils, however, a soil with 1-2% clay might be called clayey. In other areas a soil with
20-30% clay might be called clayey. A similar sort of thing happens with sand. In areas
with clayey soils a significant sand content might be called sandy. These common local
usages need to be checked by referring to laboratory determinations of texture [17].
2.4.1.2. Texture—Engineering Definition
Engineers classify soil textures using a different classification scheme. They approach the
texture of the loose material on the Earth's surface from a broader perspective than does
the soil scientist, being concerned with a much larger range of sizes of separates. From an
engineering perspective the ability of a material to carry a load is most important. Also
from a load-carrying perspective, the uniformity of the material in terms of size is
important.
Engineers size soil components by noting the size of the sieve retaining the soil
component. The size can then be reported as the sieve number, which is sometimes
confusing because as the sieve number increases the size of the holes in the sieve
decreases. Table 2.2 gives a number of important sieve numbers and corresponding hole
sizes. Material not passing a #4 sieve is cobbles, which are larger than about 75 mm in
diameter. On the other hand, gravel is smaller, but is larger than 4.75 mm. Sand is
designated as coarse (smaller than 4.75, but larger than 2.00 mm), medium (smaller than
2.00 but larger than 0.425 mm), and fine (between 0.425 and
TABLE 2.2 Sieve Numbers and Hole Sizes
Hole size (mm) a
Sieve number
#4 b 4.75
#10 c 2.00
#40 0.425
#200 0.075
a Actually, the average diameter of the particles of gravel, sand, or
silt passing through it.
b There are larger hole sieves that do not have the # designation.
c This is the upper limit for sand in the soil science textural
designations.
0.075 mm in diameter). Material smaller than 0.075 mm is silt or clay. (See Table 2.2.)
Engineers use a combination of capital letters to indicate the texture of the material one
is working with. The letters and what they signify are given in Table 2.3. Combinations
of letters are used in pairs to indicate a particular type of material. For instance, GW
would be well-graded gravels with no fines (silt or clay). On the other hand, CL would be
clay with low or slight plasticity. Another possibility would be OH, which is organic
matter with high plasticity. In this way a wide variety of materials with varying size
components and plastic or organic matter contents can be described.
 
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