Backyard
Brainstormers
USGS Soil Classification System
In this system soils are classified by a group symbol that contains two letters. The first letter represents the significant particle size fraction and the second is a descriptive modifier.
​
Coarse-grained soils are divided into two major groups as being either:
-
G - gravels
-
S - sands
Once you have decided your coarse soil is classified as gravel or sand it is further defined by one of the following symbols:
​
-
W - well-graded, fairly clean
-
C - significant amounts of clay
-
P - poorly-graded, fairly clean
-
M - significant amounts of silt
​
Fine-grained soils are divided into three major categories:
​
-
M - inorganic silts
-
C - inorganic clays
-
O - organic silts and clays
-
PT - peat
​
Once defined by one of these three major groups, fine-grained soils are further classified as being:
​
-
​L - low compressibility (LL less than 50)
-
H - High compressibility ( LL 50 or greater)
​
Steps to USCS classification system
​
​ 1. Obtain your soil's grain size distribution, liquid limit, and plastic limit.
​
- Liquid Limit = ​moisture content at which the soil begins to behave as a liquid.
Value determined from lab tests such as Atterberg limits tests.
- Plastic Limit = moisture content at which the soil begins to behave as a plastic material. Value determined from lab tests such as Atterberg limits tests.
​
- Grain Size Distribution: Plot showing the percentages of different sized particles that make up a soil. It is constructed using data from sieve analysis. An example of a Grain Size Distribution is shown below:
​
2. Use the table below (Be sure to see the associated notes underneath):
Notes:
1. CU = D60/D10
2. CC = (D30)^2/(D10 x D60)
3. Gravels with 5% to 12% fines require dual symbols:
-
GW-GM: well-graded gravel with silt
-
GW-GC: well-graded gravel with clay
-
GP-GM: poorly-graded gravel with silt
-
GP-GC: poorly-graded gravel with clay
4. Sands with 5% to 12% fines require dual symbols:
-
SW-SM: well-graded sand with silt
-
SW-SC: well-graded sand with clay
-
SP-SM: poorly-graded and with silt
-
SP-SC: poorly graded sand with clay
5. If soil contains 15% or more sand, add "with sand" to the group name.
6. If the soil's fines classify as CL-ML, use dual symbol GC-GM or SC-SM.
7. If the soil's fines are organic, add "with organic fines" to the group name.
8. If soil contains 15% or more gravel, add "with gravel" to group name.
9. If Atterberg limits plot in the hatched area, soil is a CL-ML silty clay.
10. PI = LL-PL
-
LL (Liquid Limit) = water content (%) that divides a soil between liquid and plastic states.
-
PL (Plastic Limit) = water content (%) that divides a soil between plastic and semi-solid states.
11. If 15 to 29% of the soil's particles are retained on or before the No. 200 sieve, add "with sand" or "with gravel", whichever is predominant.
12. If the soil contains 30% or more particles that are retained on or before the No. 200 sieve, predominantly sand, add "sandy" to the group.
13. If the soil contains 30% or more particles that are retained on or before the No. 200 sieve, predominantly gravel, add "sandy" to the group.
14. PI is 4 or greater and plots on or above "A" line.
15. PI less than 4 or plots below "A" line.
16. PI plots on or above "A" line.
17. PI plots below "A" line
18. Coarse Fraction is the portion of the soil particles that have grain sizes larger than the No. 200 sieve, 0.003 inches (0.075 mm) in diameter or larger.
19. "Fines" are the soil particles that pass the No. 200 sieve, 0.003 inches (0.075 mm) in diameter or smaller.
20. Liquid Limit (LL) = Plasticity Index (PI) + Plastic Limit (PL) **see note 10 for definitions.
​
3. Use the Plasticity Chart below if directed by the Classification Chart in step 2:
AASHTO Soil Classification System
The AASHTO soil classification system is a three step system that groups soils into categories very similar to those of the USCS classification system. The last step of the AASHTO method shown below will leave you with a description of the soil components and their behavior.
​
Steps to AASHTO classification system
​
1. Obtain your soil's grain size analysis, liquid limit, and plasticity index.
​
-
Grain Size Distribution: Plot showing the percentages of different sized particles that make up a soil. It is constructed using data from sieve analysis. An example of a Grain Size Distribution is shown below:
-
LL (Liquid Limit) = water content (%) that divides a soil between liquid and plastic states.
-
PI (Plasticity Index) = the difference between the liquid limit and the plastic limit:
PI = LL - PL
- PL (Plastic Limit) = water content that (%) divides a soil between plastic and semi-solid states.
​
2. Use the AASHTO Classification Table below to determine a Soil Group:
**Plasticity Index of A-7-5 subgroup is equal to or less than LL minus 30. Plasticity index of A-7-6 subgroup is greater than LL minus 30.
- In other words:
-
Within the A-7 group, if the soil's Plastic Limit (PL) is 30 or greater, then the soil group is A-7-5.
-
Within the A-7 group, if the soil's Plastic Limit (PL) is less than 30, then the soil group is A-7-6.
​
3. Calculate the Group Index using the equation below (round to nearest whole number):
​
-
Group Index = (F-35) * [0.2+0.005(LL-40)] + 0.01(F-15)(PI-10)
- F = percentage (%) of soil passing the #200 sieve (percentage of soil finer than 0.075 mm)
​
**Once you have calculated to Group Index (and rounded it to the nearest whole number), it is added in parentheses after the Soil Group (which you obtained from Step 2).
​
Descriptions of AASHTO Soil Groups:
​
1. Granular Materials: 35% or less passing the #200 sieve (0.075 mm)
**Note - Classification of the granular materials below only applies to the particles that passed through the 3-inch sieve (75 mm). Anything retained on the 3-inch (75 mm) sieve are considered boulders and are not included in the following descriptions.
-
Group A-1: This group is generally a well-graded soil. It is commonly composed of stone fragments, gravel, coarse sand, fine sand, and a non-plastic soil binder.
- Group A-1-a: Predominately stone fragments and gravel.
- Group A-1-b: Predominately coarse sand.
​
-
Group A-3: This group generally includes fine sands with little or no silt/clay fines. It also includes stream-deposited mixtures of poorly graded fine sands with small amounts of coarse sand and gravel.
​
-
Group A-2: Generally includes a wide range of sediment sizes that includes granular materials along with some fines. It includes all materials containing 35% or less passing the #200 sieve (0.075 mm) which cannot be classified be classified as A-1 or A-3.
- Group A-2-4 & A-2-5: Includes gravel and course sand with silt contents in excess of the limitations of Group A-1, and fine sands with non-plastic silt content in excess of the limitations of Group A-3.
- Group A-2-6 & A-2-7: Includes materials similar to those outlined under A-2-4 and A-2-5 but their fine materials contain plastic clay that has the characteristics of the A-6 or A-7 group.
​
2. Silt-Clay Materials: More than 35% passing the #200 sieve (0.075 mm).
​
-
Group A-4: The material of this group is generally non-plastic or moderately plastic silty soil that commonly has 75% or more passing the #200 sieve (0.075 mm). It also includes mixtures of fine silty soil with sand and gravel.
​
-
Group A-5: This group is similar to group A-4 except it may be highly elastic with a high liquid limit.
​
-
Group A-6: This group is typically plastic clay soil that usually has 75% or more pass the #200 sieve (0.075 mm) but can also include clayey soils that contain a mixture of sand and gravel. The materials of this group commonly have high volume change between wet and dry states.
​
-
Group A-7: Similar to group A-6 but has the high liquid limit characteristic and may be elastic and/or subject to high volume change.
- Group A-7-5: Materials have moderate plasticity indexes when compared to their liquid limit,
may be highly elastic and subject to considerable volume change.
- Group A-7-6: Materials have high plasticity indexes when compared to their liquid limit, subject to extremely high volume change.