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Engineering properties of soil

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Engineering properties of soil comprises of physical properties, index properties, strength parameters (shear strength parameters), permeability characteristics, consolidation properties, modulus parameters, dynamic behavior etc. This module highlights most of the engineering properties of soils.

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Engineering properties of soil

  1. 1. 1 ENGINEERING PROPERTIES OF SOIL by Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science University of Mysore India
  2. 2. 2 1. Introduction: Engineering properties of soil comprises of physical properties, index properties, strength parameters (shear strength parameters), permeability characteristics, consolidation properties, modulus parameters, dynamic behavior etc.
  3. 3. 3 2. Methods of Determining Soil Properties Geotechnical soil and rock properties of geologic strata are typically determined using one or more of the following methods: • In-situ testing data from the field exploration program; • Laboratory testing; and • Back analysis based on site performance data.
  4. 4. 4 Laboratory Testing : Laboratory soil testing is used to estimate strength, stressstrain, compressibility, and permeability characteristics.
  5. 5. 5 3. Cohesion : It is the internal molecular attraction which resists the rupture or shear of a material. Cohesion is derived in the fine grained soils from the water films which bind together the individual particles in the soil mass. Cohesion is the property of the fine grained soil with particle size below 0.002 mm.
  6. 6. 6 Cohesion of a soil decreases as the moisture content increases. Cohesion is greater in well compacted clays and it is independent of the external load applied. 4. Angle of Internal Friction The resistance in sliding of grain particles of a soil mass depends upon the angle of internal friction.
  7. 7. 7 It is usually considered that the value of the angle of internal friction is almost independent of the normal pressure but varies with the degree of packing of the particles, i.e. with the density. The soils subjected to the higher normal stresses will have lower moisture contents and higher bulk densities at failure than those subjected to lower normal stresses and the angle of internal friction may thus change.
  8. 8. 8 The true angle of internal friction of clay is seldom zero and may be as much as 260 . The angle of internal friction fro granular soils may vary in between 280 to 500 . 5. Capillarity : It is the ability of soil to transmit moisture in all directions regardless of any gravitational force. Water rises up through soil pores due to capillary attraction.
  9. 9. 9 The maximum theoretical height of capillary rise depends upon the pressure which tends to force the water into the soil, and this force increases as the size of the soil particles decreases. The capillary rise in a soil when wet may equal as much as 4 to 5 times the height of capillary rise in the same soil when dry.
  10. 10. 10 Coarse gravel has no capillary rise; coarse sand has up to 30 cm; fine sand and soils have capillary rise up to 1.2 m but dry sand have very little capillarity. Clays may have capillary rise up to 0.9 to 1.2 m but pure clays have very low value. 6. Permeability : Permeability of a soil is the rate at which water flows through it under action of hydraulic gradient.
  11. 11. 11 The passage of moisture through the inter- spaces or pores of the soil is called ‘percolation’. Soils having porous enough for percolation to occur are termed ‘pervious’ or ‘permeable’, while those which do not permit the passage of water are termed ‘impervious’ or ‘impermeable’.
  12. 12. 12 The rate of flow is directly proportional to the head of water. Permeability is a property of soil mass and not of individual particles. The permeability of cohesive soil is, in general, very small. Knowledge of permeability is required not only for seepage, drainage and ground water problems but also for the rate of settlement of structures on saturated soils.
  13. 13. 13 7. Soil plasticity : Soil plasticity is a property that enables the moist soil to change shape when some force is applied over it and to retain this shape even after the removal of the force from it. The plasticity of soil depends on the cohesion and adhesion of soil materials. Cohesion refers to the attraction of substances of like characteristics, such as, that of one water molecule for another.
  14. 14. 14 Adhesion refers to the attraction of substances of unlike characteristics. Soil consistency depends on the texture and amount of inorganic and organic colloids, structure and moisture contents of soil. 8 . Elasticity : This elastic behavior is characteristic of peat.
  15. 15. 15 A soil is said to be elastic when it suffers a reduction in volume (or is changed shape & bulk) while the load is applied, but recovers its initial volume immediately when the load is removed. The most important characteristic of the elastic behavior of soil is that no matter how many repetitions of load are applied to it, provided that the stress set up in the soil do not exceed the yield stress, the soil does not become permanently deformed.
  16. 16. 16 9. Compressibility: Gravels, sands & silts are incompressible, i.e. if a moist mass of those materials is subjected to compression; they suffer no significant volume change. Clays are compressible, i.e. if a moist mass of clay is subjected to compression, moisture & air may be expelled, resulting in a reduction in volume which is not immediately recovered when the compression load is withdrawn.
  17. 17. 17 The decrease in volume per unit increase of pressure is defined as the compressibility of soil, and a measure of the rate at which consolidation proceeds is given by the ‘co- efficient of consolidation’ of the soil. Compressibility of sand & silt varies with density & compressibility of clay varies directly with water content & inversely with cohesive strength.
  18. 18. 18 10. Uniaxial Compressive Strength (UCS) Test Compressive strength is the capacity of a material to withstand axially directed compressive forces. The most common measure of compressive strength is the uniaxial compressive strength (a.k.a. unconfined compressive strength).
  19. 19. 19 Triaxial Compression : The triaxial compression test is a more sophisticated testing procedure for determining the shear strength of a soil. The test involves a soil specimen subjected to an axial load until failure while also being subjected to confining pressure that approximates the in-situ stress conditions.
  20. 20. 20 11. Moisture Content & Available water capacity The moisture content (w) is defined as the ratio of the weight of water in a sample to the weight of solids. Available water capacity refers to the quantity of water that the soil is capable of storing for use by plants.
  21. 21. 21 The capacity varies, depending on soil properties that affect the retention of water and the depth of the root zone. The most important properties are the content of organic matter, soil texture, bulk density, and soil structure. Available water capacity is an important factor in the choice of plants or crops to be grown and in the design and management of irrigation systems.
  22. 22. 22 Available water capacity is not an estimate of the quantity of water actually available to plants at any given time. 12. Atterberg Limits : When a clayey soil is mixed with an excessive amount of water, it may flow like a semi-liquid. If the soil is gradually dried, it will behave like a plastic, semisolid, or solid material depending on its moisture content.
  23. 23. 23 The moisture content, in percent, at which the soil changes from a liquid to a plastic state, is defined as the liquid limit (LL). Similarly, the moisture contents, in percent, at which the soil change from a plastic to a semisolid state and from a semisolid to a solid state are define as the plastic limit (PL) and the shrinkage limit (SL), respectively.
  24. 24. 24 These limit a referred to as Atterberg limits. The behavior of the soil is therefore related directly to the amount of water which is present. In 1911, A. Atterberg defined the boundaries of four states of consistency in terms of limits. 13. The consistency limits of the soil are controlled by the pore fluid pressure.
  25. 25. 25 The fluid in a unconsolidated material promotes inter-granular cohesion. Fluid in a soil will promote excess pressure to cause fluid like behavior of the soil. A. Plastic Limit(PL): The plastic limit (PL) is the moisture content at which a soil transitions from being in a semisolid state to a plastic state.
  26. 26. 26 B. Liquid Limit (LL) : The liquid limit (LL) is defined as the moisture content at which a soil transitions from a plastic state to a liquid state. C. Plasticity Index : The plasticity index (PI) is defined as the difference between the liquid limit and the plastic limit of a soil , PI = LL − PL.. The PI represents the range of moisture contents within which the soil behaves as a plastic solid.
  27. 27. 27 PI Range Description 0 Nonplastic 1 – 5 Slightly Plastic 5 – 10 Low Plasticity 10 – 20 Medium Plasticity 20 – 40 High Plasticity > 40 Very High Plasticity The liquid limit, plastic limit, and shrinkage limit are extremely useful in correlating anticipated soil behavior with previous
  28. 28. 28 experience on soils in similar consistency states. Each limit represents a water content at which the soil changes from one state to another. 14. Activity : The degree of plasticity related to the clay content is called the Activity of the soil. In most natural clays, the actual clay-sized material (smaller than 2 μm) comprise only a fraction of the total.
  29. 29. 29 15. Specific Gravity of Soils The specific gravity of soil, Gs, is defined as the ratio of the unit weight of a given material to the unit weight of water. Weight-Volume Relationships: In nature, soils are three-phase systems consisting of solid soil particles, water, an air (or gas). To develop the weight-volume relationships for a soil, the three phases can be separated.
  30. 30. 30 16. Soil Strength : The shear strength is the internal resistance per unit area that the soil can handle before failure and is expressed as a stress. There are two components of shear strength; the cohesive element (expressed as the cohesion, c, in units of force/unit area) and the frictional element (expressed as the angle of internal friction, φ).
  31. 31. 31 Soil strength tests are performed on high quality, relatively undisturbed in-situ specimens. However, it is difficult and frequently impossible to sample, transport, extrude and set- up testing for granular, cohesionless soils (Sand or Gravel) without excessively disturbing or completely obliterating the soil specimen.
  32. 32. 32 It is difficult to obtain good strength values through lab testing of disturbed (remolded) specimens since the soil matrix (i.e., cohesion/ bonding of soil particles) is destroyed and the in-situ density and moisture content are very difficult to recreate.
  33. 33. 33 17. Consolidation Test : The amount of settlement induced by the placement of load bearing elements on the ground surface or the construction of earthen embankments will affect the performance of the structure. The amount of settlement is a function of the increase in pore water pressure caused by the loading and the reduction of this pressure over time.
  34. 34. 34 The reduction in pore pressure and the rate of the reduction are a function of the permeability of the in-situ soil. All soils undergo elastic compression and primary and secondary consolidation. 18. Shrinkage and Swell : Certain soil types (highly plastic) have a large potential for volumetric change depending on the moisture content of the soil.
  35. 35. 35 These soils can shrink with decreasing moisture or swell with increasing moisture. Shrinkage can cause soil to pull away from structure thus reducing the bearing area or causing settlement of the structure beyond that predicted by settlement analysis. Swelling of the soil can cause an extra load to be applied to the structure that was not accounted for in design.
  36. 36. 36 Therefore, the potential for shrinkage and swelling should be determined for soils that have high plasticity. 19. Bulk density : Bulk density data are used to compute shrink- swell potential, available water capacity, total pore space, and other soil properties. The moist bulk density of a soil indicates the pore space available for water and roots.
  37. 37. 37 A bulk density of more than 1.6 can restrict water storage and root penetration. Moist bulk density is influenced by texture, kind of clay, content of organic matter, and soil structure. 20. Erodibility: It refers to the ease with which soil materials can be removed by wind or water. Easily eroded materials include unprotected silt, sand and other loosely consolidated materials,
  38. 38. 38 Cohesive soils (with more than 20% clay) and naturally cemented soils are not easily removed from its place by wind or water and, therefore, have a low erosion factor.

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