Soil classification is very important to take basic overview of corresponding soil type. There are several soil classification methods available in the world. such as,
1). USDA (United states department of agriculture) classification
2). AASHTO (American Association of State Highway and Transportation Officials) classification
3). USCS (Unified soil classification system) classification etc.
USDA soil (United states department of agriculture) classification system
This is a very easy method to classify soil. But this is not adequate for engineering purposes.
Procedure
First question will give the sand, clay and silt consistency for us. if it is not given we need to do a laboratory testing and find the consistency.
Example 01:
For a given soil mass there are 40% of sand, 30% of clay and 30% of silt what is the soil type?
for this just go though above values (check below image). the answer is Clay loam.
Find the soil type which have 20% of gravel, 10% of sand, 30% of silt and 40% of clay.
Note: USDA classification is based only on particles smaller than 2 mm. therefore, if the soil has particles larger than 2mm, a correction should be applied.
correction
sand = 10/(100-20) x 100% = 12.5%
silt = 30/(100-20) x 100% = 37.5%
clay = 40/(100-20) x 100% = 50%
then using chart the answer is clay. but due to large percentage of gravel it is classified as gravelly clay.
AASHTO (American Association of State Highway and Transportation Officials) soil classification system
First we need to learn about GI (Group Index)
GI = (F-35)[ 0.2+0.005(LL-40)] + 0.01(F-15)(PI-10)
where,
F = Percentage passing #200 sieve
LL = Liquid limit
PI = Plastic index (PI = LL - PL)
If the equation gives a negative group index it is consider as 0.
GI is rounded off to the nearest intiger
The GI of soil group A-1-a, A-1-b, A-2-4, A-2-5 and A3 will always be 0 ( check below method)
For soil groups A-2-6 and A-2-7 the group index is estimated using the following equation
GI = 0.01(F-15)(PI-10)
Procedure
First of all there are 2 charts to classify soil in AASHTO classification system.
1. For granular materials (#200 passing 35% or less)
2.For silt and clay (#200 passing more than 35%)
GI is use to find the sub group.
1). First look at #200 passing percentage and select the chart.
3). If one value is ok then go directly down to check the others.
4). Using GI equation, get the value for GI and add that into classification name (inside parenthesis)
Example: 01
There is a soil type with sieve analysis data as below classify the soil type using AASHTO soil classification system.
#10 - 88%, #40 - 75%, #200 - 34% and LL = 39%, PI = 12%
#200 passing is 34% therefore use 'Granular materials' table.
consider A-1,
A-1-a
#10 passing 88% in the question, therefore A-1-a is not applicable (because #10 max is 50)
there are no values further for #10 passing
therefore let's consider #40 passing, for this question #40 passing is 75%
A-1-a is not applicable ( max 30)
A-1-b is not applicable ( max 50)
A-3 is okay for #40 passing ( min 50), go directly down but #200 max is 10 therefore neglect A-3
there are no values further for #40 passing
therefore let's consider #200 passing, for this question #200 passing is 34%
lets consider from A-2-4
A-2-4 is okay for #200 passing ( max 35), go directly down LL max is 40 (in our question LL = 39) okay, go directly down PI max is 10 (in our question PI = 12) therefore not applicable
lets consider from A-2-5
A-2-4 is okay for #200 passing ( max 35), go directly down LL min is 41 (in our question LL = 39) therefore not applicable.
lets consider from A-2-6
A-2-6 is okay for #200 passing ( max 35), go directly down LL max is 40 (in our question LL = 39) okay, go directly down PI min is 11 (in our question PI = 12) therefore okay.
because of that basic classification is A-2-6
lets consider sub group.
For A-2-6 and A-2-7 GI is the short equation,
GI = 0.01 (F-15) (PI-10)
= 0.01 (34-15) (12-10)
= 0.38
= 0 (rounding off to nearest integer)
therefore the final classification code is : A-2-6 (0)
Example: 02
There is a soil type with sieve analysis data as below classify the soil type using AASHTO soil classification system.
#10 - 100%, #40 - 80%, #200 - 64% and LL = 47%, PI = 29%
#200 passing is 64% therefore silt/clay chart
Classification is A-7-6 (9)
Example: 03
There is a soil type with sieve analysis data as below classify the soil type using AASHTO soil classification system.
#10 - 90%, #40 - 76%, #200 - 34% and LL = 37%, PI = 12%
#200 passing is 34% therefore Granular chart
Classification is A-2-6 (0)
Example: 04
There is a soil type with sieve analysis data as below classify the soil type using AASHTO soil classification system.
#10 - 48%, #40 - 29%, #200 - 8% and LL = 0%, PI = 2%
#200 passing is 8% therefore Granular chart
Classification is A-1-a (0)
USCS (Unified soil classification system) classification
According to this classification system, soil is grouped as below,
Gravel - pass 76.2 mm retained 4.75 mm (#4)
Sand - pass 4.75 mm (#4) retained 0.075 mm (#200)
Silt & clay - pass 0.075 mm (#200)
This classification has two letters and the first letter of the group symbol is decided as follows,
1). Determination of first letter
Soil passing #200 sieve
If less than 50% ------------- Coarse grained soil ( Materials Gravelly or Sandy)
First letter in the group symbol may be,
S - for sand ( if sand fraction is more first letter 'S' )
G - for gravel ( if gravel fraction is more first letter 'G' )
If more than 50% ------------- Fine grained soil ( Materials Gravelly or Sandy)
First letter in the group symbol may be,
M - for silt ( if silt fraction is more first letter 'M' )
C - for clay ( if clay fraction is more first letter 'C' )
2). Determination of second letter
if the major soil type is
Sand use table 07
Gravel use table 08
Silt and clay use table 09
 |
| plasticity chart |
The U line is approximately the upper limit of the relationship between PI and LL for any soil found so far.
3). Determination of group name
Final section is the group name. For this there are 2 tables. Group name for sandy and gravelly soils could be obtained from A flow chart. For silty and clayey soils we can use B charts.
Example 01:
There is a soil sample with below percentage passing values took from sieve analysis laboratory test. 4.75 mm sieve 70% passing and 0.075 mm sieve passing is 20% LL = 52% and PI = 11% classify the soil using USCS.
1) #200 passing is 20% (less than 50%) therefore it's a coarse grained soil
2) Sand fraction is high therefore first letter is "S" (retained percentage)
3) Using LL = 52 and PI = 11 and the plasticity chart the point is locate below the A line.
criteria in SM - more than 12% passing #200 sieve (okay), Atterberg's limits plot below A - line ( okay) or PI less than 4 (not required - because there is "or" between 2 conditions)
5) According to group name table SM 2nd criteria is meets (retained gravel percentage is more than 15%). therefor final classification name should be : SM - Silty sand with gravel
Example 02:
There is a soil sample with below percentage passing values took from sieve analysis laboratory test. 4.75 mm sieve 70% passing and 0.075 mm sieve passing is 45% LL = 40% and PI = 22% classify the soil using USCS.
1) #200 passing is 45% (less than 50%) therefore it's a coarse grained soil
2) Gravel fraction is high therefore first letter is "G" (retained percentage)
3) Using LL = 40 and PI = 22 and the plasticity chart the point is locate above the A line.
4) Gravel is major therefore use Table no 08. by the table find the criteria that matches with our components. therefore the answer is 'GC'
criteria in GC - more than 12% passing #200 sieve (okay), Atterberg's limits plot above A - line ( okay) ; PI greater than 7 (okay).
5) According to group name table GC 2nd criteria is meets (retained sand percentage is more than 15%). therefor final classification name should be : GC - Clayey gravel with sand
very helpful! studying before exam. thank you.
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