Page 266 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 261

the system was adopted for civilian uses by the U.S. Bureau of Reclamation and
the U.S. Army Corps of Engineers and became known as the Unified
Classification. The ASTM Designation is D-2487. The system applies not only
to fine-grained soils but also to sands and gravels, and is the most widely used
system for soil investigations for building foundations and tunneling.

12.10.2 ‘‘S’’ Is for Sand
One advantage of the Unified Classification system is its simplicity, as it uses
capital letters to represent particular soil properties: S stands for sand, G for
gravel, and C for clay. Because S already is used for sand, another letter, M, was
selected for silt, from the German word Moh.

A sand or gravel can either be well graded, W, or for poorly graded, designated by
P, respectively indicating broad or narrow ranges of particle sizes. Thus, SP is a
poorly graded sand, GW a well-graded gravel.

Fine-grained soils are characterized on the basis of liquid limit and the PI and
LL relationships to the A-line. A silt or clay with a liquid limit higher than
50 percent is designated by H, meaning high liquid limit, and if the data plot above
the A-line the soil is CH, clay with a high liquid limit. If the liquid limit is higher
than 50 percent and the data plot below the A-line, the designation is MH, silt
with a high liquid limit.

The Unified Classification system therefore distinguishes between silt and clay not
on the basis of particle size, but on relationships to the liquid limit and plasticity
index. In order to avoid confusion, clay and silt that are defined on the basis of
size now usually are referred to as ‘‘clay-size’’ or ‘‘silt-size’’ material.

If the silt or clay liquid limit is lower than 50, the respective designations are
ML and CL, silt with a low liquid limit or clay with a low liquid limit. However, if
the plasticity index is less than 4, silt dominates the soil behavior and the soil is
designated ML. This is shown in the graph in Table 12.2. Soils with a plasticity
index between 4 and 7 show properties that are intermediate and are designated
CL-ML.

12.10.3 Details of the Unified Classification System
Letter abbreviations for the various soil characteristics are as follows:

G ¼ Gravel O ¼ Organic
S ¼ Sand W ¼ Well graded
M ¼ Nonplastic or low plasticity P ¼ Poorly graded
C ¼ Plastic fines L ¼ Low liquid limit
Pt ¼ Peat, humus, swamp soils H ¼ High liquid limit

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.