Page 271 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
266 Geotechnical Engineering

clays, certain volcanic clays, and bentonite. The glacial clays of the northern U.S.
cover a wide band in the CL and CH groups.

ML-CL
Another type of borderline classification that already has been commented on is
when the liquid limit of a fine-grained soil is less than 29 and the plasticity index
lies in the range from 4 to 7. These limits are indicated by the shaded area on the
plasticity chart in Fig. 12.11, in which case the double symbol, ML-CL, is used to
describe the soil.

OL and OH
OL and OH soils are characterized by the presence of organic matter and include
organic silts and clays. They have plasticity ranges that correspond to those of the
ML and MH groups.

Pt
Highly organic soils that are very compressible and have very undesirable
construction characteristics are classified in one group with the symbol Pt. Peat,
humus, and swamp soils with a highly organic texture are typical of the group.
Particles of leaves, grass, branches of bushes, or other fibrous vegetable matter are
common components of these soils.

12.11 FIELD USE OF THE UNIFIED SOIL CLASSIFICATION SYSTEM

12.11.1 Importance of Field Identitication
A detailed classification such as indicated in Table 12.2 requires both a gradation
and plasticity analysis. Even after this information is available from laboratory
tests of soil samples, it is important to be able to identify the same soils in the
field. For example, if a specification is written based on the assumption that a soil
is an SC, and the borrow excavation proceeds to cut into ML, it can be very
important that somebody serves notice and if necessary issues a stop order. This is
a reason why all major construction jobs include on-site inspection. Suggestions
for conduct of a field identification using the Unified Classification system are in
ASTM D-2488.

12.11.2 Granular Soils
A dry sample of coarse-grained material is spread on a flat surface to determine
gradation, grain size and shape, and mineral composition. Considerable skill is
required to visually differentiate between a well-graded soil and a poorly graded
soil, and is based on visual comparisons with results from laboratory tests.

The durability of coarse aggregate is determined from discoloration of weathered
materials and the ease with which the grains can be crushed. Fragments of shale or

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