Environmental Engineering Reference
In-Depth Information
Current Classification Systems
A general summary of classification systems defining grain size components is given in
Table 5.27.
American Association of State Highway Officials (AASHO M-145)
Given in
Table 5.32,
the system is a modification of the U.S. Bureau of Public Roads sys-
tem dating from 1929, which is commonly used for highway and airfield investigations.
Unified Classification System (ASTM D2487)
The unified system
(Table 5.33)
appears to be the most common in current use. It was
developed by A. Casagrande in 1953 from the Airfield Classification system (AC or
Casagrande system, 1948) for the U.S. Army Corps of Engineers and has been adopted by
the U.S. Bureau of Reclamation, and many other federal and state agencies.
American Society for Engineering Education System
The ASEE or Burmister system, presented in 1940, is given in
Table 5.34.
It is not univer-
sally used, but is applied in the northeastern United States, particularly for the field
description of granular soils, for which it is very useful in defining component percent-
ages.
MIT Classification System
Presented by Gilboy in 1931, the MIT system was the basic system used by engineering
firms for many years, and is still used by some engineering firms in the United States and
other countries. Summarized in
Table 5.27,
it is similar to the British Standards Institution
system.
Field Identification and Description
Important Elements
Field descriptions of soils exposed in cuts, pits, or test boring samples should include gra-
dation, plasticity, organic content, color, mineral constituents, grain shape, compactness or
consistency, field moisture, homogeneity (layering or other variations in structure or fab-
ric), and cementation.
Significance
Precise identification and description permit preliminary assessment in the field of engi-
neering characteristics without the delays caused by laboratory testing. Such an assess-
ment is necessary in many instances to provide data of the accuracy required for thorough
site evaluation.
Granular soils
: Undisturbed sampling is often very difficult, and disturbed sample han-
dling, storage, and preparation for gradation testing usually destroy all fabric. Test results,
therefore, may be misleading and nonrepresentative, especially in the case of highly strat-
ified soils. Precise description provides the basis for estimating permeability, frost suscep-
tibility, height of capillary rise, use of materials as compacted fill, and general supporting
capabilities.
Clay soils
: Unless a formation contains large particles such as those found in residual
soils and glacial tills, precise description is less important for clay soils than for granular
formations because undisturbed samples are readily obtained.
