360R-06 Design of Slabs-on-Ground

360R-8 ACI COMMITTEE REPORT
Identification procedures on fraction smaller than No. 40 (4.25 
μm) sieve
Dry strength 
(crushing
characteristics)
Dilatancy (reaction 
to shaking)
Toughness
(consistency near 
plastic limit)
Group 
symbol Typical names
Fine-grained soils 
(more than half of 
material is smaller 
than No. 200 sieve* 
[75 
μm])
Silts and clays (liquid 
limit less than 50)
None to slight Quick to slow None ML
Inorganic silts and very fine sands, 
rock flour, silty or clayey fine 
sands with slight plasticity
Medium to high None to very slow Medium CL
Inorganic clays of low to medium 
plasticity, gravelly clays, sandy 
clays, silty clays, lean clays
Slight to medium Slow Slight OL
Organic silts and organic-silt clays 
of low plasticity
Silts and clays (liquid 
limit greater than 50)
Slight to medium Slow to none Slight to medium MH
Inorganic silts, micaceous or 
diatomaceous fine sandy or silty 
soils, elastic silts
High to very high None High CH
Inorganic clays of high plasticity, 
fat clays
Medium to high None to very slow Slight to medium OH
Organic clays of medium to high 
plasticity
Highly organic soils 
Readily identified by color, odor, spongy feel; frequently by 
fibrous texture 
PT Peat or other highly organic soils
*
All sieve sizes herein are U.S. standard. The No. 200 sieve (75 
μm) is approximately the smallest particle visible to the naked eye. For visual classifications, the 1/4 in. (6.3 mm) size
may be used as equivalent for the No. 4 (4.75 mm) sieve size. Boundary classifications: soil possessing characteristics of two groups are designated by combinations of group symbols.
subgrade, although it may meet the criteria for a subbase or
even a base material. The Unified Soil Classification System
is predominantly used in the United States and is referred to
in this document. Table 3.1 provides information on classifi-
cation groups of this system and some important criteria for
each soil group. Visual procedures (ASTM D 2488) can be
used, but more reliable classifications can be made using
laboratory test results (ASTM D 2487). For example, the
plasticity chart of 
Table 3.2
 is used to classify the fine-
grained soils.
The following tests and test methods are helpful in the
proper classification of soils:
1. Moisture content: ASTM D 2216;
2. Specific gravity: ASTM D 854;
3. Liquid limits: ASTM D 4318; and
4. Plastic limit: ASTM D 4318.
The standard Proctor compaction test (ASTM D 698) and
modified Proctor compaction test (ASTM D 1557) are not
strictly classification tests, but their moisture-density
relationships are very useful in accessing a soil subgrade or
subbase.
A more detailed listing of the ASTM standards is
given in 
Chapter 14
.

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