2) Silts. Silts have a low degree of dry strength and
toughness, and dilate rapidly on shaking so that water appears on the sample
surface.
3) Organic Soils. Organic soils are characterized by dark
colors, odor of decomposition, spongy or fibrous texture, and visible
particles of vegetal matter.
(f) Consistency. Describe consistency in accordance with Table
4 (Reference 3, Soil Mechanics in Engineering Practice, by Terzaghi and
Peck). Use a pocket penetrometer or other shear device to check the
consistency in the field.
(g) Assignment of Group Symbol.
Assign group symbol in
accordance with Table 3.
(h) Examples of Sample Description:
Very stiff brown silty CLAY (CL), wet
Stiff brown clayey SILT (ML), moist
Soft dark brown organic CLAY (OH), wet.
Section 4.
SOIL CLASSIFICATION AND PROPERTIES
1. REFERENCE. Soil designations in this manual conform to the Unified Soil
Classification (see Table 3) per ASTM D2487, Classification of Soil for
Engineering Purposes.
2. UTILIZATION. Classify soils in accordance with the Unified System and
include appropriate group symbol in soil descriptions. (See Table 3 for
elements of the Unified System.) A soil is placed in one of 15 categories
or as a borderline material combining two of these categories. Laboratory
tests may be required for positive identification. Use the system in Table
2 for field soil description and terminology.
a. Sands and Gravels. Sands are divided from gravels on the No. 4
sieve size, and gravels from cobbles on the 3-inch size. The division
between fine and medium sands is at the No. 40 sieve, and between medium and
coarse sand at the No. 10 sieve.
b. Silts and Clays. Fine-grained soils are classified according to
plasticity characteristics determined in Atterberg limit tests. Categories
are illustrated on the plasticity chart in Figure 2.
c. Organic Soils. Materials containing vegetable matter are
characterized by relatively low specific gravity, high water content, high
ignition loss, and high gas content. Decrease in liquid limit after
oven-drying to a value less than three-quarters of the original liquid limit
is a definite indication of an organic soil. The Unified Soil
Classification categorizes organic soils based on the plotted position on
the A-line chart as shown in Figure 2. However, this does not describe
organic soils completely.
7.1-16
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