Materials Soil classification - Learn Civil Engineering Soil classification ... • Only a sieve analysis ... are those having more than 50% of soil being retained on the No. 200 sieve

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Materials

Soil classification

References:

1. Holtz, Robert, D., Kovacs, Williams, D. An Introduction to Geotechnical

Engineering. Englewoods Cliffs, New Jersey; Prentice-Hall, 1981.

2. Schroeder, W.L., Dickenson, S.E, Warrington, Don, C. Soils in Construction. Fifth

Edition. Upper Saddle River, New Jersey; Prentice Hall, 2004.

Background of soil classification:

Reason for classification: Soil classification is like a language between engineers.

The known soil classification communicates their probable engineering behavior,

and allows engineers access to the accumulated experience of other engineers.

Soil classification does not remove the need for detailed soil investigations or

testing of the soil, but it gives the engineer the general behavior of the soil.

The Unified Soil Classification System (USCS) and the American Association of

State Highway and Transportation Officials (AASHTO) system are the common

classification system we use in civil engineering practice.

Learning objectives:

1. Background of soil classification

2. How to classify soil using the Unified Soil Classification System (USCS)

3. How to classify soil using the American Association of State Highway and

Transportation Officials (AASHTO) System.

4. How to classify soil using US Department of Agriculture

5. Solve the most likely problems on the PE Exam.


Subsurface exploration and sampling

Soil classification - USCS

• Developed by Professor A. Casagrande in

1948 for airfield construction during World

War II, and modified in 1952 for the US

Bureau of Reclamation and US Army Corps

of Engineers to make the system applicable

to dams, foundations, and other

construction.

• The basis concept is “ soils in which the

“fines” do not affect the engineering

performance are classified according to

their grain size characteristics, and soils in

which fines do control the engineering

behavior are classified according to their

plasticity characteristics.”

• Only a sieve analysis and Atterberg limits

are required to completely classify the soil

using USCS.

• The four major divisions of soil are

1. course-grained

2. fine-grained

3. organic soils

4. Peat

• The highly organic soils and peat can be identified visually. The Course grained soils

are those having more than 50% of soil being retained on the No. 200 sieve. Fine

grained soils are those that allow more than 50% to pass through the No. 200 sieve.

• Gravel fraction = percent retained on the number 4 sieve = R4

• Sand fraction = percent passing through the number 200 sieve = F200

• The Course grained soils are then divided into Gravel (R4 > 50) and Sand (R<= 50)

are then divided into four secondary groups, GW and SW(<5% passing the No. 200

sieve), GP and SP (little or no nonplastic fines), GM and SM, GC and SC. This depends

on the grain size distribution and nature of fines in the soils.



Soil classification

• Well-graded (W) has good distribution of all particle sizes. Poorly graded (P) could be

uniform or skip a size. You figure out if the soil is well graded or not by the Coefficient

of uniformity (Cu ) and the Coefficient of Curvature (Cc ).

Cu = D60 / D10

Cc = D230 / ( D10 x D60)

D60 = grain diameter at 60 % passing

D30 = grain diameter at 30 % passing

D10 = grain diameter at 10% passing by weight

• The fine grained soils ( > 50% passing the No. 200) are subdivided in silts (M) and

clays(C). These are determined by the liquid limit and plasticity index.

• Plasticity Index (PI) = Liquid Limit (LL) – Plastic Limit (PL)

• Fine-grained soils are silts (M) if their liquid limits and plasticity indices plot below the

A-line on Casagrande’s plasticity chart.

• The Fines are clay(C) if the Liquid Limit and PI plot above the A-line.

• A-line separate the claylike material from those that are silty and the organics from the

inorganics. The exception to this rule is organic clays (OL and OH) which plot below the

A-line and behave similarly to soils of lower plasticity.

• L = low plasticity (liquid limit less than 50)

• H = high plasticity (liquid limit more than 50)



Soil classification

Use these following two charts and the formulas on the previous page to easily classify the

soil in accordance with USCS.



Soil classification


Question #1: The laboratory gradation analyses and Atterberg Limits were

performed on two soil samples. Results are summarized below. During testing

no organic odor or materials were noted in either sample.

Answer # 1

a. CL,SC

b. SL, OH

c. CH, GC

d. ML, SM


Soil classification

Soil A:

U.S. Sieve Size Percent Passing

No. 4 100

No. 10 91

No. 40 83

No. 200 51

liquid limit (LL) = 35%

plastic limit (PL) = 22%

Soil B:

U.S. Sieve Size Percent Passing

No. 4 80

No. 10 72

No. 40 35

No. 200 20

liquid limit (LL) = 37%

plastic limit (PL) = 20%

1a) What is the classification of Soil A according to the Unified Soil Classification System?

1b)What is the classification of Soil B according to the Unified Soil Classification System?


Answer #1: a = CL, SC

Easiest Way is to use the Use USCS chart

Step 1: Classify Soil A: use the USCS Flow chart to find classification:

Since 51% passing #200 sieve that is > 50 Fine-grained soil

LL is 35 which is < 50 follow the top row of chart

no organic odor or color follow the inorganic

Find PI ; PI = LL – PL = 35 – 22 = 13 so PI > 7 and plots above the “A” line.

So Soil A is classified as CL (Lean Clay)

Step 2: Classify Soil B: use the USCS Flow chart to find classification:

Since only 20% passes #200, more than 50 % retained Course-grained soils

80% passing #4 which is > 50 Sands

20 % passing #200 (i.e., % fines) > 12% Sands with fines

Plot on the plasticity chart to classify

- LL =37, PL = 20: PI = 17 falls above the A line in the region CL

So Soil B is classified as SC (Clayey Sand)


Soil classification


c.



Soil classification - AASHTO

• The American Association of State Highway and Transportation Officials(AASHTO)

and the Federal Highway Administration (FHWA) use the classification system to

indicate the behavior of materials used as highway sub-grades.

• The AASHTO System uses seven basic soil groups designated A-1 through A-7, the

soil is placed in the groups by performance characteristics. A-1 is the best and A-7 is the

worst. A-1 to A-3 are sands and gravels, A-4 to A-7 are silts and clays. There is a A-8

which is reserved for unusable peat. The following is the chart to use to find the

designated soil group.



Soil classification - AASHTO

• The American Association of State Highway and Transportation Officials(AASHTO)

and the Federal Highway Administration (FHWA) use the classification system to

indicate the behavior of materials used as highway sub-grades.

• The AASHTO System uses seven basic soil groups designated A-1 through A-7, the

soil is placed in the groups by performance characteristics. A-1 is the best and A-7 is the

worst. A-1 to A-3 are sands and gravels, A-4 to A-7 are silts and clays. There is a A-8

which is reserved for unusable peat.


This chart is used for basic identification of soil using US Department of Agriculture. To

read the chart you use the following convention. Clay is read straight across to the right,

sand is up and to the left and silt is down and to the left.

Example: The USDA soil classification of 60% clay, 20% sand and 20% silt is?

Answer: Clay


Soil classification - USDA

Documents

Materials Soil classification - Learn Civil Engineering Soil classification ... • Only a sieve analysis ... are those having more than 50% of soil being retained on the No. 200 sieve