lecture 4 soil consistency and atterberg limits

Al-Rafidain University Collage Civil Engineering Department

Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly

Ph. D Civil / Geotechnical Engineering

Lecture 4: Soil Consistency and Atterberg Limits 42

Lecture 4

Soil Consistency and Atterberg Limits

4.1 Plasticity of Soil

Plasticity is ability of soil to undergo certain deformation without cracking. The following

notes can be written on plasticity of soil:

❶ Plasticity is property of cohesive soil only, especially clayey soil and appears only in

the presence of water.

❷ Plasticity is due to clay minerals; the soil becomes plastic only when it has clay

minerals.

❸ Plasticity plays an important role in properties of cohesive soil, the shear strength and

compressibility of clay depends mainly on its plasticity.

4.2 Consistency of Soil

Consistency is property of cohesive soil, especially clayey soil; it is commonly used to

describe the state of cohesive soil with respect to its water content and can be taken as a

degree of stiffness of cohesive soil, which is directly related to its strength. Based on

consistency, the cohesive soil deposits (clay and silt) can be described as the following:

very soft, soft, medium stiff, very stiff, and hard.





4.3 Atterberg Limits (Consistency Limits)

Atterberg developed a method to describe the consistency of cohesive soils with varying

water contents. He mentioned that a cohesive soil could exist in four basic states based on

its water content: ❶ Soiled state ❷ Semi-solid state ❸ Plastic state ❹ Liquid state

The water content at which the soil changes from one state to other are known as Atterberg

limits. Refers to the Figure (4-1), Atterberg limits are divided into:

❶ Liquid Limit (L.L): is the boundary between the liquid state and plastic state. Defined

as the minimum water content at which the soil begins to behave as a liquid material and

flow under its own weight. Point (B) in Figure (4-1).

❷ Plastic Limit (P.L): is the boundary between plastic state and semi-solid state.

Defined as the minimum water content at which the soil begins to behave as a plastic

material and the soil can be shaped without surface cracks. Point (C) in Figure (4-1).

❸ Shrinkage Limit (S.L): is the boundary between the semi-solid state and solid state.

Defined as maximum water content at which no volume change occurs in soil when is

dried. Point (D) in Figure (4-1).

Figure (4-1): Different States of Soil and Atterberg Limits





NOTES ON ATTERBERG LIMITS & SOIL CONSISTENCY

❶ Atterberg limits are important parameters of cohesive soil; its use in soil

classification and helps the civil engineers to predict the shear strength and

compressibility of clayey soil.

❷ Atterberg limits depend on amount and types of clay minerals. Clay with Kaolinite

has much lower (L.L) than soil with Montmorillonite. Clay with high

Montmorillonite usually has high (L.L) and may be over (100 %).

❸ Soil with high (L.L) has higher (P.L) and higher plasticity and compressibility.

❹ The typical values of (L.L) and plastic limit of soil are shown in Table below:

Soil Type L.L % P.L % Sand - - Silt 30-40 20-25

Clay 40-150 25-50

❺ Atterberg limits can be determined experimentally from Atterberg limits test.

❻ The shear strength of cohesive soil (clayey soil) can be estimated approximately

depending on its consistency as the following:

Soil Consistency Shear Strength (kPa) Very soft 25

Soft 25 – 50 Medium stiff 50 – 100

Stiff 100 – 200 Very stiff 200 – 400

Hard 400





4.4 Determination of Atterberg Limits in the Laboratory

Liquid Limit Determination

The test is performed by Casagrande method according to (ASTM D-4318) as the

following (Figure 4-2):

1- Prepare the Casagrande liquid limit device.

2- Take about (150 gm) of dried soil specimen passing sieve No. 40.

3- Add (20 %) of water and mix.

4- Place a small sample of soil in the liquid limit device.

5- Cut a groove of (2 mm) width at the base.

6- Run the device and count the number of blows, (N).

7- Stop when the groove in the soil closes through a distance of 0.5in (12 mm).

8- Take a sample and find the water content.

9- Run the test three times [N ~ (10-20), N ~ (20-30), and N ~ (35-45)] and finds the

water content for each trail.

10- Plot number of blows versus water content on a semi-log paper (flow curve) and

determine the liquid limit (L.L) corresponding to N=25 blows (Figure 4-3).





Figure (4-2): Liquid Limit Test

Figure (4-3): Flow Curve for Liquid Limit Determination





Plastic Limit Determination

The test is performed according to (ASTM D-4318) as the following (Figure 4-4):

1- Take about (20 gm) of dried soil specimen passing sieve No. 40.

2- Add water, mixed thoroughly, and left for a suitable maturing time.

3- Prepare several ellipsoidal-shaped soil masses by quizzing the soil with hand.

4- Roll the ellipsoidal soil mass into a thread with (3 mm) diameter on a glass plate

with the finger of hand. Continue rolling until the thread crumbles into several

pieces.

5- The water content of the crumbed pieces of the thread is determined.

6- The test is repeated at least thrice to get the average water content. This average

water content is called Plastic Limit (P.L).

Figure (4-4): Plastic Limit Test





4.5 Atterberg Limits Indices

❶ Plasticity Index: is defined as a numerical difference between the liquid limit (L.L)

and plastic limit (P.L). Denoted as (P.I) and expressed as a percentage:

퐏. 퐈 = 퐋. 퐋 − 퐏. 퐋… … … … … … … … (ퟒ − ퟏ)

Importance of plasticity Index (P.I):

Important in classification of cohesive and is an indicator to soil compressibility. Measure the degree of plasticity of a soil, based on plasticity index, Atterberg classify

the soil as:

Plasticity Index (P.I) % Soil Description 0 Non-plastic 7 Low plastic

7-17 Medium plastic 17 High plastic

P. I = 34 − 28 = 6 퐏. 퐈 ퟕ Soil low plastic tend to be 퐒퐈퐋퐓

P. I = 50 − 22 = 28 퐏. 퐈 ퟏퟕ Soil high plastic tend to be 퐂퐋퐀퐘

IMPORTANCE NOTES

① Soil with a high (P.I) high plastic soil soil tends to be Clay.

② Soil with a lower (P.I) low plastic soil soil tends to be Silt.

③ Soil with a (P.I = 0) non-plastic soil (N.P) soil tends to be Sand.

For Example:

Soil has (L.L = 34 %) and (P.L = 28 %), the plasticity index of soil will be:

Soil has (L.L = 50 %) and (P.L = 22 %), the plasticity index of soil will be:





❷ Liquidity Index: is defined as the ratio of natural water content (Wc) minus plastic

limit (P.L) to plasticity index (P.I). It is denoted as (L.I) and expressed as a percentage:

퐋. 퐈 =퐖퐜 − 퐏.퐋퐋. 퐋 − 퐏. 퐋

… … … … … … … … … … … (ퟒ − ퟐ)

Importance of liquidity Index (L.I):

Measure the consistency of cohesive soil in natural state (in the field).

Measure the shear strength of cohesive soil using Atterberg limits.

L. I =15 − 2040 − 20

= −0.25 퐋. 퐈 ퟎ

Soil in solid state 퐒퐨퐢퐥 퐡퐚퐬 퐡퐚퐫퐝 퐜퐨퐧퐬퐢퐬퐭퐞퐧퐜퐲

L. I =45 − 2040 − 20

= 1.25 퐋. 퐈 ퟏ

Soil in liquid state 퐒퐨퐢퐥 퐡퐚퐬 퐯퐞퐫퐲 퐬퐨퐟퐭 퐜퐨퐧퐬퐢퐬퐭퐞퐧퐜퐲

IMPORTANCE NOTES

❶ If (L.I 0) Wc P.L Soil in semi-solid or solid state Very stiff to hard

❷ If (L.I = 0) Wc = P.L Soil at plastic limit Stiff consistency

❸ If (L.I 1) Wc L.L Soil in plastic state Medium stiff consistency

❹ If (L.I = 1) Wc = L.L Soil at liquid limit Soft consistency

❺ If (L.I 1) Wc L.L Soil in liquid state Very soft consistency

For Example:

A soil sample has the following properties: Wc = 15 %, L.L = 40 %, P.L = 20 %.

The liquidity index (L.I) of soil sample will be:

A soil sample has the following properties: Wc = 45 %, L.L = 40 %, P.L = 20 %.

The liquidity index of soil will be:





Q (1): Multiple-Choice Questions (MCQ) — Select the Correct Answer:

1- When the plastic limit of a soil is greater than the liquid limit, then the plasticity index is:

(a): Negative (b): Zero (c): Non-plastic (d): None of these

2- If the plasticity index of a soil is zero, the soil tend to be:

(a): Clay (b): Sand (c): Silt (d): Clayey sand

3- If the natural water content of soil a soil located between its liquid limit and plastic limit, the

soil mass is said to be in :

(a): Liquid state (b): Plastic state (c): Semi-solid state (d): Solid state

4- A soil has a liquidity index less than zero, the expected consistency of soil will be:

(a): Soft (b): Stiff (c): Very stiff (d): Hard

5- Which of the following soils has more plasticity index?

(a): Gravel (b): Sand (c): Silt (d): Clay

6- A soil sample has Wc = 20 %, L.L = 50 %, and P.L = 21 %, the consistency of soil will be:

(a): Hard (b): Soft (c): Stiff (d): Very soft

7- A soil sample has a plasticity index equal (20), the soil should be:

(a): Clay (b): Sand (c): Silt (d): Gravel

8- A soil sample has L.L = 45 % , P.L = 25 %, and S.L = 15 %, the plasticity index of soil will

be:

(a): 50 % (b): 20 % (c): 30 % (d): None of these

9- A soil sample has L.L = 53 % and P.L = 23 %, the soil considered to be as:

(a): High plasticity (b): Medium plasticity (c): Low plasticity (d): Non-plastic

10- A clay sample has Wc = 24 %, L.L = 44 %, and P.L = 24 %, the predicted shear strength

of clay will be:

(a): 25 (b): 400 (c): 100-200 (d): 200-400

Problems and Questions





Q (2): Answer with True (T) or False (F) and correct the false ones: 1- The activity of clay decrease with the increase of clay plasticity.

2- The consistency is property of coarse-grained soil.

3- The liquidity index cannot be more than (1).

4- Plastic limit is the water content of soil represents the boundary between the plastic state

and the semi-solid state.

5- The soil with soft consistency has high shear strength.

6- The soil at liquid state has a largest shear strength

7- If the liquidity index of a given soil equal to (1), then the soil is said to be in plastic limit.

8- The soil at solid state can be molded into any shape without cracking.

9- If the natural water content of soil equals its liquid limit, the soil will be at shrinkage limit.

10- Soil containing Montmorillonite has higher activity than soil containing Kaolinite.

11- The compressibility of soil decrease with the increase of liquid limit.

12- At liquid limit, the soil has stiff consistency.

13- The soil at solid state has largest shear strength.

14- Sandy soil considered plastic soil.

15- Clayey soil usually non-plastic soil.

16- A soil sample has a liquidity index greater than (1); the consistency of soil should be hard.

17- A soil sample has Wc = 20 %, L.L = 50 % and P.L = 21 %, the consistency of soil is soft.

Q (3): Describe briefly the states of clayey soils according to their Atterberge limits. Q (4): Define the terms liquid limit, plastic limit, and shrinkage limit and explain how we can

find the plasticity index of soil.

lecture 4 soil consistency and atterberg limits

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