3. Classification of Soil (AASHTO) Flashcards
Two commonly classification system used are:
- American Association of State Highway and Transportation Officials (AASHTO) System (preferred by Transportation engineers).
- Unified Soil Classification System (USCS) (preferred by geotechnical engineers)
This system was originally developed by Hogentogler and Terzaghiin 1929 as the Public Roads Classification System. Afterwards, there are several revisions. The present AASHTO (1978) system is primarily based on the version in 1945.
AASHTO SYSTEM
The AASHTO SYSTEM is based on the following three soil properties:
i. Particle-size distribution (AASHTO T-11 andAASHTO T-27 test)
ii. Liquid Limit (AASHTO T-89 test).
iii. Plasticity Index (AASHTO T-90 test)
Large rock fragments that are generally unsuitable for most construction applications due to their size.
Boulder: >300 mm
Medium-sized rock fragments that are sometimes used in drainage and foundation applications.
Cobbles: 75 mm – 300 mm
4.75 mm – 75 mm
Gravel
19 mm – 75 mm
Coarse Gravel
4.75 mm – 19 mm
Medium Gravel
2.0 mm – 4.75 mm
Fine Gravel
0.075 mm – 4.75 mm
Sand
0.425 mm – 2.0 mm
Coarse Sand
0.075 mm – 0.425 mm
Fine Sand
<0.075 mm
These fine-grained soils are classified based on their Atterberg Limits rather than their particle size.
Silt and Clay
Separates fine gravel from coarse sand.
No. 10 Sieve (2.0 mm)
Separates coarse sand from fine sand.
No. 40 Sieve (0.425 mm)
The boundary between sand and silt/clay.
No. 200 Sieve (0.075 mm)
Grain Size: Fraction passing #10 sieve and retained #200 sieve
Sand
Grain Size: Fraction passing 75mm sieve and retained on #10 (2mm) US sieve
Gravel
Grain Size: Fraction passing #200 sieve.
Silt and Clay
Plasticity: Term ____ is applied when fine fractions have a PI ≤ 10
silty
Plasticity: Term ____ is applied when fine fractions have PI > 10
clayey
Groups: Soils are classified into eight groups, ____ through ____.
A-1 through A-8
Groups: The major groups A-1,A-2, and A-3 represent the _______ soils.
coarse-grained soils
Groups: The A-4, A-5, A-6, and A-7 represent _______ soils.
fine grained soils
Groups: The ____ are identified by visual inspection.
A-8
Granular materials: 35% or less of total sample passing No. 200
Excellent to good
Silt-Clay materials: more than 35% of total sample passing No. 200
Fair to poor
helps evaluate how good a soil is for use as a highway subgrade material (the foundation under roads)
Group Index
A lower GI = ____ soil quality (good for roads).
better
A GI of 0 = _____.
best quality
____ GI means the soil is weaker for subgrade use. (Higher or Lower)
Higher
Groups like A-1-a, A-1-b, A-2-4, A-2-5, and A-3 always have GI = ____.
0 (considered good soil)
If GI is negative, it’s still considered ___.
0
Well-graded mixtures of stone fragments or gravel ranging from coarse to fine with a non-plastic or slightly plastic soil binder. However, this group also includes coarse materials without soil binder.
Group A-1
Materials consisting predominantly of stone fragments or gravel, either with or without a well graded soil binder.
Subgroup A-1-a
Materials consisting predominantly of coarse sand either with or without a well-graded soil binder.
Subgroup A-1-b
This material consists of fine sands with little coarse material or soil binder, like beach or desert sand, and some non-plastic silt. It’s suitable for pavements when damp but prone to erosion. It can be compacted with vibratory, pneumatic-tired, or steel-wheeled rollers, but not sheepsfoot rollers.
Group A-3
This group includes a wide variety of “granular” materials that are borderline between the materials falling in Groups A-1 and A-3 and silt- clay materials of Groups A-4, A-5, A-6 and A-7. It includes all materials containing 35 percent or less passing the 75-μm (No. 200) sieve that cannot be classified as A-1 or A-3.
Group A-2
The typical material of this group is a non-plastic or moderately plastic silty soil usually having 75 percent or more passing the 75 μm (No. 200) sieve. The group includes also mixtures of fine silty soil and up to 64 percent of sand and gravel retained on the 75-μm (No. 200) sieve
Group A-4
The typical material of this group is similar to that described under Group A-4, except that it is usually of diatomaceous or micaceous character and may be highly elastic as indicated by the high liquid limit.
Group A-5
The typical material of this group is plastic clay soil usually having 75 percent or more passing the 75-μm (No. 200) sieve. The group includes also mixtures of fine clayey soil and up to 64 percent of sand and gravel retained on the 75-μm (No. 200) sieve.
Group A-6
The typical materials and problems of this group are similar to those described under Group A-6, except that they have the high liquid limits characteristic of the A-5 group and may be elastic as well as subject to high volume change.
Group A-7
Includes those materials with moderate Plasticity Indexes in relation to Liquid Limit and which may be highly elastic as well as subject to considerable volume change.
Subgroup A-7-5
Includes those materials with high Plasticity Indexes in relation to Liquid Limit and which are subject to extremely high volume change.
Subgroup A-7-6
Highly organic soils such as peat or muck are not included in this classification. Because of their many undesirable properties, their use should be avoided, if possible, in all types of construction.
Subgroup A-8
