Geotechnical Engineering 1 (Soil Mechanics) PDF

Geotechnical Engineering 1 (Soil Mechanics) Module 3 – Part 1: Consistency of Soil Objectives: After studying these topics students will be able to: 1. Understand the importance of phase relationships, physical states and soil classification. 2. Know the significance and application soil consistency...

Geotechnical Engineering 1 (Soil Mechanics) Module 3 – Part 1: Consistency of Soil Objectives: After studying these topics students will be able to: 1. Understand the importance of phase relationships, physical states and soil classification. 2. Know the significance and application soil consistency and classification in determination of the strength of soil. Content: A. Introduction In the early 1900s, a Swedish scientist named Atterberg developed a method to describe the consistency of fine grained soils with varying moisture contents. At very low moisture content, soil behaves more like a solid. When the moisture content is very high, the soil and water may flow like a liquid. Hence, on an arbitrary basic, depending on the moisture content, the behavior of soil can be divided into four basic states-solid, semisolid, plastic and liquid. W a t e r c o n t e n t i n c r e a s e s MBV Geotechnical Engineering 1 Liquid State Liquid Limit, LL Plastic State Plastic Limit, PL Semisolid State Shrinkage Limit, SL Solid State Page | 1 Liquid Limit Test Set B. Definition of Terms and Formulas Consistency is the term used to describe the degree of firmness (e.g., soft, medium, firm, or hard) of a soil. The consistency of a cohesive soil is greatly affected by the water content of the soil. A gradual increase of the water content may transform a dry soil from solid state, into a liquid state to a semisolid state, to a plastic state, and after further moisture increase in to a liquid state. The water content at the corresponding junction points of these states are known as the shrinkage limit, the plastic limit, and the liquid limit respectively. 1. Liquid Limit (LL) is defined as the moisture content corresponding to the transition from liquid to plastic state. 2. Plastic limit (PL) is defined as the moisture content at which the soil crumbles, when rolled into threads of 1/8 in. 93.2 mm) in diameter and it is the lower limit of the plastic stage of soil. 3. Plasticity index (PI) is the difference between the liquid limit and the plastic limit of a soil. PI = LL − PL 4. Shrinkage limit (SL) is the moisture content corresponding to the final transition. SL = (M1 − M2 )(100) (V1 − V2 )(ρw )(100) − M2 M2 Where: M1 = mass of the wet soil pat in the dish at the beginning of the test M2 = mass of the dry soil in the pat V1 = initial volume of the wet soil pat V2 = volume of the oven-dried soil pat w= density of water 5. Shrinkage ratio SR = ( 1 M2 )( ) ρw V2 6. Specific gravity of solids Gs = 7. 1 1 SL − SR 100 Liquidity index (LI) is the ratio of the relative consistency of a cohesive soil in the natural state.  − PL LI = LL − PL MBV Geotechnical Engineering 1 Page | 2 8. Consistency index (CI) CI = LL −  LL − PI 9. Shrinkage index is the difference between the plastic limit and the shrinkage limit of a soil. SI = PL − SL 10. Activity of clay. Ac = PI  Where:  = percent of soil finer than 0.002 mm (clay size). Activity Classification Ac < 0. 7 Inactive 0. 7 < Ac < 1. 2 Normal clay Ac > 1. 2 Active clay Description of Clay in terms of Liquid Limit (LL) and Plasticity Index (PI) LL > 60% and PI is 25% LL is 50% – 60% and PI is 25% – 35% LL < 50% and PI < 25% Very High Medium Low Soil Indices Index Plasticity Liquidity Shrinkage Activity of clay Correlation Strength and compressibility Compressibility and stress rate Shrinkage potential Swell potential, and so forth Description of Soil Based on Liquidity Index LI < 0 0 < LI < 1 LI > 1 Semisolid state – high strength, brittle (sudden) fracture is exposed Plastic state – intermediate strength, soil deforms like a plastic material Liquid state – low strength, soil deforms like a viscous fluid Description of Soil Based on Plasticity Index PI 0 1 -5 5 – 10 10 – 20 20 – 40 >40 Description Nonplastic Slightly plastic Low plasticity Medium plasticity High plasticity Very high plasticity Atterberg’s Limits are also used to assess the potential swell of a given soil MBV Geotechnical Engineering 1 LL PI <50 50 – 60 >60 <25 25 – 35 >35 Potential Swell Classification Low Medium High Page | 3 Essential points: 1. Fine-grained soils can exist in one of four states: solid, semi-solid, plastic, and liquid. 2. Water is the agent that responsible for changing the states of soils. 3. A soil gets weaker if its water content that causes a change of state. These are the liquid limit- the water content that caused the soil to change from a plastic state; the plastic limit- the water content that caused the soil to change from a plastic to a semi-solid; and the shrinkage limit- the water content that caused the soil to change from a semi-solid to a solid state. All these limiting water contents are found from laboratory tests. 4. The plasticity index defines the range of water content for which the soil behaves like a plastic material. 5. The liquidity index gives a measure of strength. Problems 1. A saturated soil has the following characteristics: initial volume = 19.65 cm 3, final volume = 13.5 cm3, mass of wet soil is 36 g and mass of dry soil = 25 g. Determine shrinkage limit and shrinkage ratio. 2. The following are results from the liquid and plastic limit test for a soil: Number of Blows (N) 15 20 28 Moisture Content (ω%) 42 40.8 39.1 The plastic limit is 18.7%. Determine the a. b. c. d. Liquid limit using table. Plasticity index of the soil. Liquidity index of the soil if the water content is 24%. Consistency index. 3. The following data were obtained from the Atterberg Limits test for a soil: Liquid Limit = 52. 3% Plastic Limit = 26. 5% Determine the a. Plasticity index of the soil. b. Liquidity index of the soil if the in situ moisture content of the soil is 32%. c. Nature of the soil. 4. The following are the results of a shrinkage limit test: Initial volume of soil in saturated state = 24. 6 cc Final volume of soil in dry state = 15. 9 cc Initial mass in a saturated state = 44g Final mass in a dry state = 30. 1g Determine the a. b. c. d. e. f. Dry density of the soil in g/cc. Saturated density of the soil in g/cc. Void ratio of the soil. Shrinkage limit of the soil. Shrinkage ratio of the soil. Specific gravity of the solids. MBV Geotechnical Engineering 1 Page | 4 6. Laboratory results for a sample of clay soil for the purpose of evaluating the potential for volume change, swelling or expansion are as follows: Liquid limit = 68% Plastic limit = 24% Particles smaller than 0. 002 mm = 44% Determine the a. Plasticity index. b. Activity classification of clay. c. Rate of the volume change potential. 7. In a liquid limit test using penetrometer, the following readings were recorded and tabulated as follows: Plastic Limit Test Results γwet (kN⁄ 3 ) m 128.6 141.4 132.6 134.5 136.0 Trial Number 1 2 3 4 5 γd (kN⁄ 3 ) m 105.4 116.8 109.6 111.2 113.4 Liquid Limit Test Results Moisture Content (ω%) 42.5 47.5 58.1 60.0 Cone Penetration (mm) 16.0 17.5 22.8 26.0 Determine the a. Liquid limit of the soil. b. Plasticity index of the soil. c. Liquidity index, if the natural moisture content of the soil is 38%. References: 1. 2. 3. 4. 5. 6. 7. Images are Retrieved from https://www.google.com Geotechnical Engineering (Revised Third Edition) by C. Venkatramaiah, 2012 Principles of Geotechnical Engineering (Seventh Edition) by Braja M. Das, 2010 Soil Mechanics and Foundations (Third Edition) by Muni Budhu, 2011 Soil Mechanics 7th Edition, R.F. Craig, 2004 Basic Fundamentals of Geotechnical Engineering by Venancio L. Besavilla Jr., 1998 Fundamentals of Geotechnical Engineering by Diego Inocencio T. Gillesania, 2006 MBV Geotechnical Engineering 1 Page | 5