The meaning of soil varies across disciplines. To geologists—probably those most familiar with soil—it is the product of past surface processes. To engineers, soil is a material that can be built on, built in, built with, or supported, and it can be crucial for tunnels, roads, retaining walls, building foundations, and other structures. Therefore, in engineering, a soil’s mechanical properties, such as strength or permeability, determine its usefulness.
Unified Soil Classification System in ASTM D2487-17
The Unified Soil Classification System, the system for classifying soils for engineering purposes, has its origins during WWII, when A. Casagrande developed the Airfield Classification System. In 1952, after several US government agencies adopted a modified version of this system, it became the Unified Soil Classification System.
The ASTM version of the Unified Soil Classification System is ASTM D2487-17: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System).
This classification system is based on particle-size characteristics, liquid limit, and plasticity index. According to ASTM D2487-17, there are three major soil divisions: coarse-grained soils, fine-grained soils, and highly organic soils. These are subdivided further into 15 basic groups, each with its own group symbol (e.g. GW).
Coarse-grained soils are broken up into gravels (more than 50% of coarse fraction retained on No. 4 sieve) and sands (50% or more of coarse fraction passes No. 4 sieve).
These soils are further placed into the following groups: well-graded gravel (GW), poorly graded gravel (GP), silty gravel (GM), clayey gravel (GC), well-graded sand (SW), poorly graded sand (SP), silty sand (SM), and clayey sand (SC).
These are defined broadly by the characteristic of having 50% or more pass the No. 200 sieve. Fine-grained soils are classified as silts or clays of a liquid limit either less than 50 or 50 or more.
Fine-grained soil groups include lean clay (CL), silt (ML), organic clay/organic silt with liquid limit less than 50 (OL), fat clay (CH), elastic silt (MH), and organic clay/organic silt with liquid limit 50 or more (OH).
Highly Organic Soils
These are primarily organic matter, dark in color, and have an organic odor. The only group classification for highly organic soils is peat (PT).
The ASTM D2487-17 Standard
The above information was drawn from Table 1 of ASTM D2487-17, and the group symbols are included in parentheses. These groups are based on laboratory tests performed on the portion of a soil sample passing the 3 inch sieve (see ASTM E11-20: Standard Specification for Woven Wire Test Sieve Cloth and Test Sieves for more information).
ASTM D2487-17 not only describes the Unified Soil Classification System for classifying mineral and organo-mineral soils for engineering purposes, but it also details the specific procedures for laboratory determination of the different soil classification groups.
ASTM D2487-17 revises and supersedes the 2011 version of the same standard. In the new edition, Appendix X3 for the wet preparation method and Appendix X4 for the dry preparation method were deleted, as they do not apply to the document’s tests. Furthermore, the Precision and Bias section was removed, as it also is not applicable.
Note 9 also was added to the standard to cover instances where it is not possible to perform the basic plastic limit or liquid limit test. In addition, all instances of “weight” have been changed to “mass” throughout the ASTM D2487-17 document.
ASTM D2487-17: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) is available on the ANSI Webstore.