Soil Mechanics - I Lecture # 3,4 Chapter # 1. Introduction to Soil Mechanics (Part 2) Prepared by: Engr Mamoon Kareem Department of Civil Engineering Swedish College Of Engg & Tech Wah Cantt. Introduction to Soil Mechanics Weathering of Rocks Soil and its Types Physical Properties of Soil • • • • • • • Color Soil Structure Particle Shape and Size Specific Gravity Soil Phases Porosity Void Ratio • • • • • • Moisture Content Degree of Saturation Air Content Consistency Limit Particle Size Distribution Relative Density Significance: Identification Purposes Colour depends upon: Type of soil mineral Organic content Amount of coloring oxides Degree of oxidation Examples: Black color Green or Blue Red, Brown or Yellow Grey Manganese Compound Ferrous Compounds Iron Organic matter Soil Structure is defined as the grouping or arrangement of soil particles with respect to one another. Factors that affect the structure are: Shape and Size Mineralogical Composition Nature and Composition of Water Structures in Cohesionless Soil Single Grained Soil particles are in stable position The shape and size distribution of soil particles and their relative positions influence the denseness of packing. Irregularity in the particle shapes generally yields an increase in the void ratio Honeycombed Relatively small sand and silt form small arches with chains of particles. They can carry an ordinary static load because of large inter-particle spaces. Structures in Cohesive Soil Flocculent Structure: The clay minerals are extremely flaky in shape and have a large surface area-to-mass ratio. Flocculated structure is developed when the edge of one clay particle is attracted to the flat face of another Dispersed Structure: Develops when the edges and faces of the clay particles have similar electrical charge Also develops as a result of remolding by the transportation process (manmade earth fills ) Different shapes: Nomenclature of material (soil type) and range of sizes The ratio of the unit weight of a substance, to the unit weight of water at 4oC How many times a substance (or material) is heavier than water Significance: Used for determination and calculation of many other soil properties ,as Particle size analysis by hydrometer test Porosity and void ratio Unit weight Critical hydraulic gradient Degree of saturation or zero air void Specific Gravity of some Minerals and Soil types Any homogeneous part of a soil mass different from other parts in the mass and clearly separated from them is called a phase. Fundamental phases: 1. Solid phase, 2. Liquid phase 3. Gaseous or vapour phase. 4. Ice phase (in cold regions) The ratio of volume of all the voids “Vv” to the total volume of the soil mass “V” is known as the porosity. Porosity falls in the range of 0 n 100 Where V = Vs + Vv V = Total volume of soil mass Vs = Volume of solid particles of soil Vv = Volume of voids, which may be filled with air or water or both The ratio of volume of voids present in a soil mass to the volume of solid particles. It is denoted by “e”. Vv volume of voids in soil e volume of slids in soil Vs The void ratio is expressed as a number and the limiting values can be within the range. The ratio of the volume of air present in the voids to the total volume of a soil mass. Va Vv Vw Av or A V Vv Vs Since; Vv = Va + Vw Air content or Air Void Ratio fall within the range of 0 A 100 percent. The condition when voids are partially filled with water is expressed by the degree of saturation or relative moisture content. It is the ratio of actual volume of water in voids “Vw” to the total volume of voids “Vv”. Vw Ww m S Vv Wv msat Ww – is the weight of water actually present in the voids. Wv – is wt of water that can fill all the voids. m – actual moisture content. msat – moisture content when all voids are totally filled with water. The range of “S” 0 S 100. The amount of water present in the voids of a soil in its natural state. weight of water m 100 weight of dry soil The common range of moisture content for most soil is 20-40 percent. Oven dried soil has zero percent moisture and the soils which appear dry (i.e., air dried soil) often have 2 to 4 percent moisture content. The range of water content is: The moisture/water in the voids of a soil mass can occur in a variety of forms. Depending upon the form of occurrence they are given different names e.g., Hygroscopic Moisture Film Moisture Capillary Moisture Chemically Bound Moisture Hygroscopic Moisture: 1. Also known as adsorbed moisture, contact moisture or surface bound moisture. This form of soil moisture exists as a very thin film of moisture surrounding the surfaces of individual soil particles and is held by the forces of adhesion. It depends upon temperature and humidity. It is not affected by gravitational forces, capillary forces and air drying at ordinary ordinary temperature. The approximate values of hygroscopic moisture for various soils are as under: 123- Sand Silt Clay 1-2 % 7-9 % 17-20 % 2. Film Moisture: The moisture film attached to the soil particles, above the layer of hygroscopic moisture film, is known is film moisture. It is held by the molecular forces and is not affected by gravity. The amount of film moisture depends on the specific surface i.e., higher the specific surface higher will be the film moisture and vice versa. 3. Capillary Moisture: The moisture which in held within the voids of capillary size. The capillary moisture is continuously connected to the groundwater table. Capillary water can be removed from the soil by drainage 4. Chemically Bound Moisture: Moisture contained chemically within the mineral particles and can be removed only by chemical processes of the substance when the crystalline structure of the mineral breaks. Chemically bound moisture is not important for common soil engineering problems and therefore is not determined. The percentage of various particle sizes present in a soil is known as particle size distribution or gradation. Particle size analysis is made by sieving or by sedimentation. Sieving method – when particle size > .074 mm Sedimentation method – when particle size < .074mm The sieves normally required are as follows: Significance: Engineering classification of soils. Selection of the most suitable soil for construction of roads, airfields, levees, dams and other embankments. To predict the seepage through soil (although permeability tests are more generally used) To predict the susceptibility to frost action. Selection of most suitable filter material. The gradation curve: A gradation curve is drawn by plotting the percentage finer (%age passing) on ordinate against the particle sizes on abscissa. The gradation curves indicate the type of soil, and provide very important information related to the properties and behavior of soil The gradation curves have great importance in civil engineering and are extensively used for the following purposes. Determination of Effective Grain (Particle) Size. Determination of Uniformity co-efficient. Determination of co-efficient of Curvature. Determination of percentage of different soil types in a soil sample e.g., sand, silt, clay. Determination of percentage larger or finer than a given size. Classification of soil. Design of filters. Concrete mix design. Well-Graded Soil: A soil containing an assortment of particles with a wide range of sizes. A well-graded soil has following merits: 1. Higher shear strength 2. Higher density 3. Reduced Compressibility 4. Higher stability 5. Higher Bearing Capacity 6. Low permeability well graded uniformly graded Ideal packing, due to particles Loose packing, as smaller ranging from large to small particles to fill voids are sizes missing Uniformly-Graded Soil: A uniformly graded soil is defined as a soil containing particles having a limited range of sizes (Almost the same sizes) Poorly-Graded Soil: A poorly graded soil is defined as a soil containing particles of varying sizes with intermediate particle sizes missing. Such soils give lower density and lower strength. The gradation curve of a poorly graded soil show steps indicating an excess of certain particle sizes, and a deficiency of others The gradation curves: a) well graded soil b) b) poorly graded soil. uniformly graded soil Co-efficient of uniformity: When the value of Cu is less than 4, the soil is generally considered as uniformly graded. A higher value of Cu represents a wide range of particle sizes and the soil is termed as well graded. Cu D60 D10 Co-efficient of curvature: It is also known as coefficient of gradation (Cg) or Co-efficient of Concavity. ( D30 ) 2 Cc ( D60 )( D10 ) Cc = 1, represents that all the soil particles have the same size, and the soil is uniformly graded. Cc between 0.2 and 2.0 indicate well graded or poorly graded soil. The term relative density (also called density index, ID) is used to express the state of compactness of a granular soil. The following relationship between the void ratio values is termed as the relative density. The range of values for relative densities (Dr) and the commonly referred state of compaction for granular soil. The consistency of a soil means its physical state with respect to the moisture content present that time. Consistency states are: 1. Solid state 2. Semi solid state 3. Plastic state 4. Liquid state. Boundaries of the above four states are: Shrinkage Limit: It is the moisture content at which a soil changes from solid state to semi-solid state. Plastic Limit: It is the moisture content at which a soil changes from semi-solid state to plastic state. Liquid Limit: It is the moisture content at which a soil changes from plastic state to liquid state. 1. Shrinkage Limit It is that moisture content at which a reduction in moisture will not cause a decrease in the total volume of soil mass, but an increase in moisture will result in an increase in volume of soil mass. At Shrinkage Limit The Degree Of Saturation is 100%. At certain point during drying process, air begins to enter the soil mass and the volume decrease becomes appreciably less than the volume of water lost. The shrinkage limit is not given much importance since it is not used in soil classification. 1. Shrinkage Limit Concept of surface tension forces and induced compressive stresses (a) Particle separated due to thick moisture film (b) Meniscus contracting due to drying process (c) Meniscus tending to tear off (d) Meniscus fully torn off allowing air entry Relationship between volume and moisture content: The soils which show higher shrinkage upon drying also swell more upon wetting and are known as expansive soils. Expansive soils are very dense and hard in dry state due to very high shrinkage stresses Shrinkage cracks at Rawal lake which dried due to drought Plastic Limit 2. The moisture content at which a soil can be rolled into threads of 1/8” (3.2mm) diameter without cracking and crumbling. Threads thinner than 1/8” (3.2 mm) diameter are possible, if the moisture is higher than the plastic limit. And if the moisture is less than plastic limit the thread will crumble before reaching the required diameter of 1/8” (3.2 mm). 2. Plastic Limit Liquid Limit 3. The moisture content at which 25 blows of Cassagrande apparatus closes a standard groove cut in the soil paste along a distance of 12.7 mm (0.5 in). The moisture content which gives a penetration depth of 20mm of the standard cone (fall cone test) into the soil, when the cone is released for 5 seconds. 3. Liquid Limit Plasticity Index Plasticity Index indicates the range of moisture through which a cohesive soil behaves as a plastic material It is the numerical difference between liquid and plastic limits. It is expressed as: Range of Plasticity Index P.I. = 0 The soil is non-plastic and non-cohesive. P.I. < 7 The soil is low plastic and partly cohesive. P.I. 7 - 17 The soil is medium plastic and cohesive. P.I. > 17 The soil is highly plastic and very cohesive. Change of liquid, plastic and shrinkage limits with plastic properties (not to scale, just to show comparison). Liquidity Index The ratio of difference between the moisture content and plastic limit to the plasticity index. m P.L m P.L L.I L.L P.L P.I L.I < 0, (i.e. negative value) the field moisture content is less than the plastic limit, and hence the soil is in a semisolid state. Consistency of a soil at its natural moisture content: ▪ L.I < 0, the soil is in a semi-solid or solid state (hard) ▪ 0.00 < L.I ≤ 0.25, the consistency is stiff or hard ▪ 0.25 < L.I ≤ 0.50, the consistency is medium ▪ 0.5 < L.I ≤ 0.75, the consistency is soft ▪ 0.75 < L.I ≤ 1, the consistency is very soft ▪ L.I > 1, the soil is in a liquid state Flow Index The slope of the flow curve (graph between log N and moisture content drawn for the determination of liquid limit) is known as the flow index and is equal to: F.I = F.I = Any two soils, although having the same plasticity indices and/or the liquid limits may have different values of flow index, and hence may possess varying degree of cohesiveness and shear strength. Flow Index The slope of the flow curve (graph between log N and moisture content drawn for the determination of liquid limit) is known as the flow index and is equal to: F.I = F.I = Any two soils, although having the same plasticity indices and/or the liquid limits may have different values of flow index, and hence may possess varying degree of cohesiveness and shear strength. Case-I: Two soils having the same values of plasticity index No. of blows are indicative of the resistance to deformation or shear strength. For the same drop of moisture ∆m, the No. of blows for flat curve increase very much, indicating higher shear strength. Therefore, the soils with same plasticity index may posses different shear strength. Case-I: Two soils having the same values of plasticity index No. of blows are indicative of the resistance to deformation or shear strength. For the same drop of moisture ∆m, the No. of blows for flat curve increase very much, indicating higher shear strength. Therefore, the soils with same liquid limit may posses different shear strength. Toughness Index Soils having same values of plasticity indices may vary in toughness. This property of a soil is expressed by the toughness index. Toughness and dry strength increases with increase in toughness index. T ..I . P.I . F .I . (1.31) Thank You … for paying your attention