Geology Reference
In-Depth Information
Because the plasticity index of 24.3% is greater than 16.2%, this
soil is classified as A-7-6. (Also, see Figure 11-1.)
Next, the group index must be determined, using either
Eq. (11-1) or Figure 11-2. By Eq. (11-1),
Group
index
F
35
0.2
0.005
LL
40
1
2
1
2
0.01
F
15
PI
10
(11-1)
1
2
1
2
Group
index
58.8
35
0.2
0.005
46.2
40
1
2
1
2
0.01
58.8
15
24.3
10
11.8
1
2
1
2
By Figure 11-2, with a liquid limit of
46.2%
and with
58.8%
pass-
ing the No. 200 sieve, the partial group index for the liquid limit is
5.5. With a plasticity index of 24.3% and with
58.8%
passing the
No. 200 sieve, the partial group index for the plasticity index is 6.3.
The (total) group index is the sum of these (5.5 + 6.3), or 11.8.
Therefore, this soil is classified as A-7-6(12), according to the
AASHTO system.
The Unified Soil Classification System was developed by Casagrande
[3] and is utilized by the U.S. Army Corps of Engineers. In this system,
soils fall within one of three major categories: coarse grained, fine
grained, and highly organic soils. These categories are further sub-
divided into 15 basic soil groups. The following symbols are used in the
Unified System:
UNIFIED SOIL
CLASSIFICATION
SYSTEM [2, 3, 4]
G
Gravel
S
Sand
M
Silt
C
Clay
O
Organic
PT
Peat
W
Well graded
P
Poorly graded
Normally, two group symbols are used to classify soils. For example, SW
indicates well-graded sand. Table 11-3 lists the 15 soil groups, includ-
ing each one's name and symbol as well as giving specific details for clas-
sifying soils by this system.
In order to classify a given soil by the Unified System, its grain-size
distribution, liquid limit, and plasticity index must first be determined.
With these values known, the soil can be classified using Table 11-3 and
Figure 11-3. The soil can also be classified using the flowcharts of
Figures 11-4, 11-5, and 11-6. The Unified Soil Classification System is
published as ASTM D 2487-06.
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