Soil Classification and Index Properties Department of Civil Engineering National Institute of Technology, Raipur Geotechnical Engineering - I

Soil Classification Why we wish to classify soil at all or what is the purpose of classifying soil? Because it provide us information about the expected engineering properties of a soil. The engineering properties of soil which are of greatest concern to the Consulting Geotechnical Engineer are: Colour Shear strength Sizes of particles Porosity Origin Stickiness Permeability Smell Water content Compressibility Sulphate contain Ability to sustain plant life

Shape of Soil Particles Shape of Coarse Fraction of Soil The orientation of particles in a mass depends on the size and shape of the grains as well as upon the minerals of which the grains are formed. The structure of soils that is formed by natural deposition can be altered by external forces. Figure gives the various types of structures of soil. Fig.(a) is a single grained structure whereas in a honeycomb structure, the particles will have face-to-face contact as shown in Fig. (e). Natural clay sediments will have more or less flocculated particle orientations. Marine clays generally have a more open structure than fresh water clays. Figs. 2.8(f) and (g) show the schematic views of salt water and fresh water deposits.

The orientation of particles in a mass depends on the size and shape of the grains as well as upon the minerals of which the grains are formed. The structure of soils that is formed by natural deposition can be altered by external forces. Figure gives the various types of structures of soil. Fig. (a) is a single grained structure which is formed by the settlement of coarse grained soils in suspension in water. Fig. (b) is a flocculent structure formed by the deposition of the fine soil fraction in water. Fig. (c) is a honeycomb structure which is formed by the disintegration of a flocculent structure under a superimposed load. The particles oriented in a flocculent structure will have edge-to-face contact as shown in Fig. (d) whereas in a honeycomb structure, the particles will have face-to-face contact as shown in Fig. (e). Natural clay sediments will have more or less flocculated particle orientations. Marine clays generally have a more open structure than fresh water clays. Figs. (f) and (g) show the schematic views of salt water and fresh water deposits. SOIL MASS STRUCTURE

Orientation of soil particles depends on Size Shape minerals Fig. Schematic diagrams of various types of structures (Lambe, 1 958a

Soil Fabric Soil particles are assumed to be rigid. During deposition, the mineral particles are arranged into structural frameworks that we call soil fabric. Each particle is in random contact with neighbouring particles. The environment under which deposition occurs influences the structural framework that is formed. In particular, the electrochemical environment has the greatest influence on the kind of soil fabric that is formed during deposition of fi ne-grained soils. Two common types of soil fabric—fl occulated and dispersed—are formed during soil deposition of fine-grained soils, as shown schematically Figs. A fl occulated structure, formed in a saltwater environment, results when many particles tend to orient parallel to one another. A fl occulated structure, formed in a freshwater environment, results when many particles tend to orient perpendicular to one another. A dispersed structure occurs when a majority of the particles orient parallel to one another.

Clay Structure dispersed structure: If there is net repulsion the particles tend to assume a face to face Orientation, this being referred as dispersed structure. Flocculated structure: If there is net attraction the Orientation of the particles tends to be edge-to-face or edge – to – edge, this is being referred as a flocculated structure. Bookhouse structure: If there is face to face orientation and combine to form assemblages than there is two possible forms: i) Bookhouse structure ii) Turbostratic structure * Example of a natural clay.

The spaces between the mineral particles are called voids, which may be filled with liquids (essentially water) gasses (essentially air), and cementitious materials (e.g., calcium carbonate). Voids occupy a large proportion of the soil volume. Interconnected voids form the passageway through which water flows in and out of soils. If we change the volume of voids, we will cause the soil to either compress (settle) or expand (dilate). Loads applied by a building, for example, will cause the mineral particles to be forced closer together, reducing the volume of voids and changing the orientation of the structural framework. Consequently, the building settles. The amount of settlement depends on how much we compress the volume of voids. The rate at which the settlement occurs depends on the interconnectivity of the voids. Free water, not the adsorbed water, and/or air trapped in the voids must be forced out for settlement to occur. The decrease in volume, which results in settlement of buildings and other structures, is usually very slow (almost ceaseless) in fine-grained soils because these soils have large surface areas compared with coarse grained soils. The larger surface areas provide greater resistance to the flow of water through the voids. Packing Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass. The surface area per unit mass (specific surface) of sands is typically 0.01 m2 per gram, while for clays it is as high as 1000 m2 per gram (montmorillonite)

The spaces between the mineral particles are called voids, which may be filled with liquids (essentially water), gases (essentially air), and cementitious materials (e.g., calcium carbonate). Packing spheres are stacked one on top of another spheres are packed in a staggered pattern How to measure denseness or loosest condition of soil? The loosest state for a granular material can usually be created by allowing the dry material to fall into a container form a funnel held in such a way that the free fall is about one centimeter. The densest state can be established by a combination of static pressure and vibration of soil packed in a container

The density of granular soils varies with the shape and size of grains, the gradation and the manner in which the mass is compacted. If all the grains are assumed to be spheres of uniform size and packed as shown in Fig. (a), the void ratio of such a mass amounts to about However, if the grains are packed as shown in Fig. (b), the void ratio of the mass is about The soil corresponding to the higher void ratio is called loose and that corresponding to the lower void ratio is called dense. If the soil grains are not uniform, then smaller grains fill in the space between the bigger ones and the void ratios of such soils are reduced to as low as 0.25 in the densest state. If the grains are angular, they tend to form looser structures than rounded grains because their sharp edges and points hold the grains further apart. RELATIVE DENSITY OF COHESIONLESS SOILS (a) Loosest state(b) Densest state e = 0.90 e = 0.35

If the mass with angular grains is compacted by vibration, it forms a dense structure. Static load alone will not alter the density of grains significantly but if it is accompanied by vibration, there will be considerable change in the density. The water present in voids may act as a lubricant to a certain extent for an increase in the density under vibration. The change in void ratio would change the density and this in turn changes the strength characteristics of granular soils. Void ratio or the unit weight of soil can be used to compare the strength characteristics of samples of granular soils of the same origin. The term used to indicate the strength characteristics in a qualitative manner is termed as relative density which is already expressed by On the basis of relative density, we can classify sandy soils as loose, medium or dense as in Table

Relative Density The term relative density is commonly used to indicate the in situ denseness or looseness of granular soil. It is defined as D r = relative density, usually given as a percentage e = in situ void ratio of the soil e max = void ratio of the soil in the loosest condition e min = void ratio of the soil in the densest condition The values of D r may vary from a minimum of 0 for very loose soil, to a maximum of 1 for very dense soil. Soils engineers qualitatively describe the granular soil deposits according to their relative densities. Max. density is achieved by vibration. Min. density is achieved by pouring oven-dried soil into a container. (not authentic ) Smaller the range of particle sizes present (i.e. the more nearly uniform the soil) for Smaller the particles and more angular the particles = the smaller the minimum density. The greater the range of particle sizes present, the greater the max. density.

PHYSICAL STATES AND INDEX PROPERTIES OF FINE-GRAINED SOILS INDEX PROPERTIES: The properties on which distinctions are based are known index properties, and the tests required to determine the index properties are classification test. The physical and mechanical behavior of fine-grained soils is linked to four distinct states: solid, semisolid, plastic, and liquid, in order of increasing water content. Soil initially in liquid state It dries uniformly w.c and volume LS PL PS S oil becomes stiff and no longer to flow as a liquid. soil can be molded into any desired shape without rupture soil at this state is said to exhibit But if drying is continued beyond the range of water content for plastic behavior, the soil becomes a semisolid But if drying is continued beyond the range of water content for plastic behavior, the soil becomes a semisolid. The soil cannot be molded now without visible cracks appearing. soil continues to dry, it comes to a final state called the solid state no further volume change occurs since nearly all the water in the soil has been removed

The water content at which the soil changes from a plastic to a semisolid is known as the plastic limit, The range of water contents over which the soil deforms plastically is known as the plasticity index, PI: Definition The shrinkage limit is useful for the determination of the swelling and shrinking capacity of soils. The liquid and plastic limits are called the Atterberg limits The range of water content from the plastic limit to the shrinkage limit is called the shrinkage index (SI), Liquidity Index: The relative consistency of a cohesive soil in the natural state can be defined by a ratio called the liquidity index (LI): The values of the liquidity index for some of these soils may be negative

Description of the Strength of Fine-Grained Soils Based on Liquidity Index

w.r.t. change in water content Geotechnical Engineer is interested to know the strength and deformation behavior of material, Liquid state Low strength Higher deform. solid state largest strength Lowest deformation Two extreme A measure of soil strength using the Atterberg limits is known as the liquidity index (LI) and is expressed as The liquidity index is the ratio of the difference in water content between the natural or in situ water content of a soil and its plastic limit to its plasticity index The Atterberg limits depend on the type of predominant mineral in the soil. If montmorillonite is the predominant mineral, the liquid limit can exceed 100%. Why? Because the bond between the layers in montmorillonite is weak and large amounts of water can easily infiltrate the spaces between the layers.

Activity : Skempton (1953) showed that for soils with a particular mineralogy, the plasticity index is linearly related to the amount of the clay fraction. He defined a term called activity (A) to describe the importance of the clay fractions on the plasticity index. The equation for A is Typical Atterberg Limits for Soils

Activity of Clay-Rich Soils Activity is one of the factors used in identifying expansive or swelling characteristics of soils.

Plasticity Chart Casagrande (1932) studied the relationship of the plasticity index to the liquid limit of a wide variety of natural soils. They proposed empirical A line = PI = 0.73(LL-20). It separates inorganic clay to inorganic silt

1. Fine-grained soils can exist in one of four states: solid, semisolid, plastic, or liquid. 2. Water is the agent that is responsible for changing the states of soils. 3. A soil gets weaker if its water content increases. 4. Three limits are defined based on the water content that causes a change of state. These are the liquid limit—the water content that caused the soil to change from a liquid to a plastic state; the plastic limit—the water content that caused the soil to change from a plastic to a semisolid; and the shrinkage limit—the water content that caused the soil to change from a semisolid to a solid state. All these limiting water contents are found from laboratory tests. 5. The plasticity index defines the range of water content for which the soil behaves like a plastic material. 6. The liquidity index gives a qualitative measure of strength. 7. The soil strength is lowest at the liquid state and highest at the solid state. THE ESSENTIAL POINTS ARE:

THE SHAPE AND SIZE OF PARTICLES The shapes of particles as conceived by visual inspection give only a qualitative idea of the behavior of a soil mass composed of such particles. Science particles finer than mm diameter cannot be seen by the naked eye. One can visualize the nature of the coarse grained particles only. Coarser fractions composed of angular grains are capable of supporting heavier static loads and can be compacted to a dense mass by vibration. The influence of the shape of the particles on the compressibility characteristics of soil are: 1. Reduction in the volume of mass upon the application of pressure. 2. A small mixture of mica to sand will result in a large increase in its The classification according to size divides the soils broadly into two distinctive groups, namely, coarse grained and fine grained. Since the properties of coarse grained soils are, to a considerable extent, based on grain size distribution, classification of coarse grained soils according to size would therefore be helpful. Fine grained soils are so much affected by structure, shape of grain, geological origin, and other factors that their grain size distribution alone tells little about their physical properties. However, one can assess the nature of a mixed soil on the basis of the percentage of fine grained soil present in it. It is, therefore, essential to classify the soil according to grain soil present in it. in it. It is, therefore, essential to classify the soil according to grain Soil particles >0.075 mm generally termed as coarse grained and the finer ones as silt, clay and peat (organic soil) are considered fine grained. From an engineering point of view, these two types of soils have distinctive ch

Soil particles >0.075 mm generally termed as coarse grained and the finer ones as silt, clay and peat (organic soil) are considered fine grained. From an engineering point of view, these two types of soils have distinctive characteristics 1.In coarse grained soils, gravitational forces determine the engineering characteristics 2.In fine grained soil Interparticle forces are predominant. The dependence of the behavior of sieves and sieve shaker, sieve

Soil classification is used to specify a certain soil type that is best suited for a given application. Classification of Soil WHY? Sieve analysis is widely used in classification of soils. Data obtained from particle-size distribution curves is used in the design of filters for earth dams and to determine suitability of soil for road, highway, construction, embankment fill of dam, airport runway etc. The distribution of particle sizes or average grain diameter of coarse-grained soils gravels and sands is obtained by screening a known weight of the soil through a stack of sieves of progressively finer mesh size. A typical stack of sieves is shown in Sieves are made by weaving two sets of wires at right angles to one another. The square holes thus formed between the wires provide the limit which determines the size of the particles retained on a particular sieve. The particle diameter in the screening process, often called sieve analysis, is the maximum dimension of a particle that will pass through the square hole of a particular mesh. sieve shaker, Let Wi be the weight of soil retained on the ith sieve from the top of the nest of sieves and W be the total soil weight. The percent weight retained is % retained on ith sieve =Wi/W *100 The percent finer is

Particle size distribution curves. where D60 =is the diameter of the soil particles for which 60% of the particles are finer D10 = is the diameter of the soil particles for which 10% of the particles are finer Cu <4 contains particles of uniform size. Cu> 4 wider assortment of particle sizes Min. value of Cu = 1 and is corresponds to an assemblages of particles of the same size Cc = between 1 and 3 for well-graded soils absence of certain grain sizes, termed gap-graded, is diagnosed by a coeffi cient of curvature outside the range 1 to 3 and a sudden change of slope in the particle size distribution curve D10 is called the effective size of soil. Use for filter design PSD curve or Gradation curve use a logarithmic scale for particle size because the ratio of particle sizes from the largest to the smallest in a soil can be greater than 104 beach sands sorted by water and wind almost vertical Cu> 4 flat curve Cc = 1 to 3 indicate a wider assortment of particle sizes Sorted by water, but certain sizes were not transported produced by bulk transport processes (e.g., glacial till). Note: The uniformity coefficient and the coefficient of concavity are strictly applicable to coarse-grained soils

Gravel > 4.75 mm Sand > 4.75 to mm Coarse Sand= 4.75 to 2 mm Medium Sand= 2.0 to 0.42 mm Fine Sand = 0.42 to mm Silt = to mm Clay = to mm Soil Classification based on size The symbol for particle size groups are (Gravel (G), Sand (S), Silt (M) and Clay (C ). The letter denoting the dominant size fraction is placed first in the group symbol. If a soil has a significant content of organic matter the suffix O is added as the last letter of the group symbol.. A group symbol may consist of two or more letters, for example: SW - well graded SAND SCL – very clayey SAND (clay of low plasticity) CIS – sandy CLAY of intermediate plasticity MHSO –organic sandy SILT of high plasticity Q. A sieve analysis test was conducted on 650 grams of soil. The results are as follows. Determine (a) the amount of coarse-grained and fine-grained soils, and (b) the amount of each soil type based on the ASTM system. Ans: a 15.4% and 84.6 %

Soil Classification Based on Grain Size

Soil Classification As a Geotechnical Engineer soil is mainly classified two categories, they are Grain-size distribution and plasticity of soils. Grain-size distribution They are the American Association of State Highway Officials (AASHTO) classification system and the Unified Soil Classification System. AASHTO system is used in roads and highways, whereas Geotechnical engineers usually prefer to use the Unified System. AASHTO Classification System In this system, soil is classified into seven major groups: A-1 through A-7 A-1, A-2, and A-3 are granular materials, where 35% or less of the particles pass through the No. 200 sieve Soils where more than 35% pass through the No. 200 sieve are classified into groups A-4, A-5, A-6, and A-7. These are mostly silt and clay-type materials

Range of liquid limit and plasticity index for soils in groups A-2, A-4, A-5, A-6, and A-7

Particle size classification by various system

Unified Soil Classification System The USCS uses symbols for the particle size groups. These symbols and their representations are G—gravel, S—sand, M—silt, and C—clay. These are combined with other symbols expressing gradation characteristics—W for well graded and P for poorly graded—and plasticity characteristics— H for high and L for low, and a symbol, O, indicating the presence of organic material. A typical classification of CL means a clay soil with low plasticity, while SP means a poorly graded sand American Society for Testing and Materials (ASTM) Classification System The American Society for Testing and Materials classification system (ASTM-CS) is nearly identical to the USCS. ASTM-CS uses the same symbols as USCS but provides a better scheme for mixed soils, i.e., soils consisting of mixtures of, for example, sand, gravel, and clay. Soils are classified by group symbols and group names. For example, we can have a soil with a group symbol, SW-SM, and group name, which describes the soil, as “well-graded sand with silt” if the gravel content is less than 15%. Flowcharts to classify soils based on the ASTM-CS

For the evaluation of the quality of a soil as a highway subgrade material, a number called the group index (GI) is also incorporated with the groups and subgroups of the soil Group index (GI): GI = (F - 35)[ [ (LL-4)] (F - 15)(PI – 10) where F = percent passing the No. 200 sieve LL = liquid limit PI = plasticity index partial Partial group index determined from the liquid limit Partially group index determined from the plasticity index 1. If the GI Eq. yields a negative value for GI, it is taken as The group index calculated from above Eq. is rounded off to the nearest whole number (for example, GI = 3.4 is rounded off to 3; GI = 3.5 is rounded off to 4). 3. There is no upper limit for the group index. 4. The group index of soils belonging to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 is always When calculating the group index for soils that belong to groups A-2-6 and A-2-7, use the partial group index for PI, or GI = 0.01(F-15) (PI-10) The higher the group index, the lower the quality of the soil as a subgrade material. The GI should not exceed 20 for any of groups A-4 through A-7.

Unified Soil Classification System flowchart for coarse-grained soils

Unified Soil Classification System flowchart for fine-grained soils

You may ask: “How do I use a soil classification to select a soil for a particular type of construction, for example, a dam?” ? Geotechnical engineers have prepared charts based on experience to assist you in selecting a soil for a particular construction purpose