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 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. Lecture 4: Soil Consistency and Atterberg Limits 42 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering 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 Lecture 4: Soil Consistency and Atterberg Limits 43 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering 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 Lecture 4: Soil Consistency and Atterberg Limits 44 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering 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). Lecture 4: Soil Consistency and Atterberg Limits 45 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering Figure (4-2): Liquid Limit Test Figure (4-3): Flow Curve for Liquid Limit Determination Lecture 4: Soil Consistency and Atterberg Limits 46 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering 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 Lecture 4: Soil Consistency and Atterberg Limits 47 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering 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: . = . − . … … … … … … … … ( − ) 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 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: P. I = 34 − 28 = 6 . Soil low plastic tend to be Soil has (L.L = 50 %) and (P.L = 22 %), the plasticity index of soil will be: P. I = 50 − 22 = 28 . Soil high plastic tend to be Lecture 4: Soil Consistency and Atterberg Limits 48 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering 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: − . . = … … … … … … … … … … … ( − ) . − . 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. 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: 15 − 20 L. I = = −0.25 . 40 − 20 Soil in solid state A soil sample has the following properties: Wc = 45 %, L.L = 40 %, P.L = 20 %. The liquidity index of soil will be: 45 − 20 L. I = = 1.25 . 40 − 20 Soil in liquid state Lecture 4: Soil Consistency and Atterberg Limits 49 Al-Rafidain University Collage Civil Engineering Department Soil Mechanics Lectures for Third Year Students By Dr. Ahmed Al-Adly Ph. D Civil / Geotechnical Engineering Problems and Questions 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