AND SOIL CLASSIFICATION
7. The soil strength is lowest at the liquid state and highest at the solid state
4.6.2 American Society for Testing and Materials (ASTM) Classifi cation System
The American Society for Testing and Materials classifi cation system (ASTM-CS) is nearly identical to the USCS. ASTM-CS uses the same symbols as USCS but provides a better scheme for mixed soils, i.e., soils consisting of mixtures of, for example, sand, gravel, and clay. Soils are classifi ed by group symbols and group names. For example, we can have a soil with a group symbol, SW-SM, and group name, which describes the soil, as “well-graded sand with silt” if the gravel content is less than 15%. Flowcharts to classify soils based on the ASTM-CS are shown in Figures 4.10a, 4.10b, and 4.10c.
Are 50% of particles > 0.075 mm?
No
No
Yes
Yes
Yes
Yes Yes
Yes No
No Soil is fine–grained
Proceed to flowchart of Fig. 4.9b Soil is coarse-grained
Soil is gravel First letter is G
Soil is sand First letter is S Is gravel fraction > sand fraction?
Is clay + silt fraction
> 12%? < 5%? Between 5% and 12%?
Is clay fraction > silt fraction?
Plastic clayey fines Second letter C
If first letter is G, Cu ≥ 4 and 1 ≤ Cc ≤ 3 If first letter is S, Cu ≥ 6 and 1 ≤ Cc ≤ 3 second letter is W, otherwise second letter is P second letter is W, otherwise second letter is P
Nonplastic silty fines
Second letter M
Is clay fraction > silt fraction?
If first letter is G, Cu ≥ 4 and 1 ≤ Cc ≤ 3 Classification is: GW-GC Otherwise,
Classification is: GP-GC If first letter is S, Cu ≥ 6 and 1 ≤ Cc ≤ 3 Classification is: SW-SC Otherwise,
Classification is: SP-SC
If first letter is G, Cu ≥ 4 and 1 ≤ Cc ≤ 3 Classification is: GW-GM Otherwise,
Classification is: GP-GM If first letter is S, Cu ≥ 6 and 1 ≤ Cc ≤ 3 Classification is: SW-SM Otherwise,
Classification is: SP-SM Yes
FIGURE 4.9a Unifi ed Soil Classifi cation System fl owchart for coarse-grained soils.
4.6 SOIL CLASSIFICATION SCHEMES 71
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Are 50% of particles < 0.075 mm?
Soil is coarse-grained Proceed to flowchart of Fig. 4.9a Soil is fine-grained
Is ___________________ LL (oven-dried) < 0.75?
LL (not dried)
Yes No
Yes No
Yes No
Yes No
Is clay fraction > silt fraction?
Is liquid limit > 50%?
Soil is organic First letter is O
Soil is clay First letter is C
Plasticity is high Second letter is H Soil is fat clay or elastic silt
Plasticity is low Second letter is L Soil is lean clay or silt
Soil is silt First letter is M
FIGURE 4.9b Unifi ed Soil Classifi cation System fl owchart for fi ne-grained soils.
GRAVEL
% gravel
% sand
>
<5%
fines
<15% sand
>_15% sand
>_15% sand
<15% sand Cu>_4 and 1<_Cc<_3
Cu<4 and/or Cc<1 or Cc>3
fines=ML or MH fines=CL or CH fines=CL-ML
GM GC Cu<4 and/or Cc<1 or Cc>3
Cu>_4 and 1<_Cc<_3
>12%
fines 5-12%
fines
Silty gravel Silty gravel with sand
<15% sand
>_15% sand fines=CL, CH, GP-GC
GC-GM
Poorly graded gravel with clay (or silty clay) Poorly graded gravel with clay and sand (or silty clay and sand)
<15% sand
>_15% sand
fines=ML or MH GP-GM Poorly graded gravel with silt
Poorly graded gravel with silt and sand
<15% sand
>_15% sand fines=CL, CH,
(or CL-ML)
GW-GC Well-graded gravel with clay (or silty clay) Well-graded gravel with clay and sand (or silty clay and sand)
<15% sand
>_15% sand fines=ML or MH GW-GM
GP GW
Well-graded gravel with silt Well-graded gravel with silt and sand
<15% sand
>_15% sand Poorly graded gravel Poorly graded gravel with sand
<15% sand
>_15% sand Well-graded gravel GROUP SYMBOL
Well-graded gravel with sand
Clayey gravel Clayey gravel with sand
>_15% sand
<15% sand Silty, clayey gravel Silty, clayey gravel with sand
GROUP NAME
SAND
% sand
% gravel
>_
<5%
fines
<15% gravel
>_15% gravel
>_15% gravel
<15% gravel Cu>_6 and 1<_Cc<_3
Cu<6 and/or Cc<1 or Cc>3
fines=ML or MH fines=CL or CH fines=CL-ML
SM SC Cu<6 and/or Cc<1 or Cc>3
Cu>_6 and 1<_Cc<_3
>12%
fines 5-12%
fines
Silty sand Silty sand with gravel
<15% gravel
>_15% gravel fines=CL, CH,
(or CL-ML)
SP-SC
SC-SM
Poorly graded sand with clay (or silty clay) Poorly graded sand with clay and gravel (or silty clay and gravel)
<15% gravel
>_15% gravel
fines=ML or MH SP-SM Poorly graded sand with silt
Poorly graded sand with silt and gravel
<15% gravel
>_15% gravel fines=CL, CH,
(or CL-ML)
SW-SC Well-graded sand with clay (or silty clay) Well-graded sand with clay and gravel (or silty clay and gravel)
<15% gravel
>_15% gravel fines=ML or MH SW-SM
SP SW
Well-graded sand with silt Well-graded sand with silt and gravel
<15% gravel
>_15% gravel Poorly graded sand Poorly graded sand with gravel
<15% gravel
>_15% gravel Well-graded sand Well-graded sand with gravel
Clayey sand Clayey sand with gravel
>_15% gravel
<15% gravel Silty, clayey sand Silty, clayey sand with gravel (or CL-ML)
FIGURE 4.10a Flowchart for classifying coarse-grained soils more than 50% retained on No. 200 sieve.
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Inorganic
Inorganic
Organic LL<50%
LL> _50%
Organic
4<_Pl<_7 and plots on or above
“A”–line
Pl<4 or plots below “A”–line
Pl>7 and plots on or above
“A”–line
Pl plots on or above “A”–line
CL
% sand > _% gravel
Lean clay with gravel Lean clay with sand 15%<_+No. 200 <30%
<15% plus No. 200 Lean clay
% sand <% gravel
>
_15% gravel
Gravelly lean clay Sandy lean clay with gravel
% sand > _% gravel
% sand <% gravel
Sandy lean clay
<15% gravel
<30% plus No. 200
> _30% plus No. 200
<15% sand
Gravelly lean clay with sand
>
_15% sand
CL-ML
% sand > _% gravel
Silty clay with gravel Silty clay with sand 15%<_+No. 200 <30%
<15% plus No. 200 Silty clay
% sand <% gravel
>
_15% gravel
Gravelly silty clay Sandy silty clay with gravel
% sand > _% gravel
% sand <% gravel
Sandy silty clay
<15% gravel
<30% plus No. 200
> _30% plus No. 200
<15% sand
Gravelly silty clay with sand
>
_15% sand
ML
OL
CH
MH
% sand > _% gravel
Silt with gravel Silt with sand 15%<_+No. 200 <30%
<15% plus No. 200 Silt
% sand > _% gravel
Fat clay with gravel Fat clay with sand 15%<_+No. 200 <30%
<15% plus No. 200 Fat clay
% sand <% gravel
>
_15% gravel
Gravelly fat clay Sandy fat clay with gravel
% sand > _% gravel
% sand <% gravel
Sandy fat clay
<15% gravel
<30% plus No. 200
> _30% plus No. 200
<15% sand
Gravelly fat clay with sand
>
_15% sand
% sand <% gravel
>
_15% gravel
Gravelly silt Sandy silt with gravel
% sand > _% gravel
% sand <% gravel
Sandy silt
<15% gravel
<30% plus No. 200
> _30% plus No. 200
<15% sand
Gravelly silt with sand
>
_15% sand
Pl plots below
“A”–line LL––oven-dried
LL––not dried< 0.75 See Figure 4.10b
LL—oven-dried OH
LL––not dried< 0.75 See Figure 4.10c
% sand > _% gravel
Elastic silt with gravel Elastic silt with sand 15%<_+No. 200 <30%
<15% plus No. 200 Elastic silt
% sand <% gravel
>
_15% gravel
Gravelly elastic silt Sandy elastic silt with gravel
% sand > _% gravel
% sand <% gravel
Sandy elastic silt
<15% gravel
<30% plus No. 200
> _30% plus No. 200
<15% sand
Gravelly elastic silt with sand
>
_15% sand GROUP
SYMBOL GROUP NAME
FIGURE 4.10b Flowchart for classifying inorganic fi ne-grained soils (50% or more fi nes). (Source: Reprinted with permission from ASTM D 2487-10 Standard Practice for Classifi cation of Soils for Engineering Purposes, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.)
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74 CHAPTER 4 PHYSICAL SOIL STATES AND SOIL CLASSIFICATION