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Plasticity of Soil

Plasticity of Soil:-

Plasticity of soil is its ability to undergo deformation without cracking or fracturing.

Engineering Properties:-

The main engineering properties of soils are permeability, compressibility and shear strength.

Permiability indicates facility with which water can flow through soils.

Compressibility indicates deformations produced in soils (C.L.).

Shear strength of soil is its ability to resist shear stress. It determines

  1. Stability of slope
  2. Bearing capacity of soil
  3. Earth pressure on retaining stress.

Index properties:

  • The tests required for determination of engineering properties are generally elaborate and time consuming.
  • Sometime Geotechnical Engineer is interested into the rough assessment of engineering properties without considering/conducting elaborate tests.
  • The properties of soils which are not of primary interest to the geotechnical engineer but which are indicative of the engineering properties are called as index properties.

Main Index Properties:

  1. Poor Grained Soil - Relative density particle size
  2. Fine Grained Soil - Atterberg's limits & consistency

The Atterberg limits are a basic measure of the critical water content of a fine grained soil. It is shrinkage limit, plastic limit and liquid limit.

Plasticity of Soil:

  • A plastic soil can be moulded into various shapes when it is wet
  • Plasticity is an important index property of fine grained soils especially clay.
  • Plasticity is due to presence of clay.

Consistency Limits:-

  • Consistency of soil means degree of firmness (backbone) of soil.
  • In 1911, a Swedish agriculture engineer Atterberg mentioned that fine grained soil can exist in four states. Namely, liquid, plastic, semi-solid, solid state.
  • The water content or moisture content at which the soil changes from one state to the other are known as consistency limits or Atterberg limit.

    1.Liquid Limit:-

  • The water content at which the soil changes from liquid state to plastic state or limit is water content at which soil

  • The liquid limit $\rightarrow$ determine

  • Casagrande's Apparatus

  • Cone Penitration Test

    • The liquid limits of soil depends upon class mineral present.

Cone Penitration Test has a several advantages.

  1. It is easier to perform
  2. The method is applicable to a wide range of soils
  3. The results are reliable and do not depend upon the judgement of the operator

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Plastic Limit:

The plastic limit is the water content below which the soil stop behaving as a plastic limit.

$S=\frac{V_w}{V_v} \rightarrow \frac{water}{soil}$

Complete saturation occur when $S \gt 1$

So partial saturated mean when value of S is between 0-1

partial saturate $\rightarrow (0 \le S \le 1)$

  • The water content at which the soil changes from one state to another known as consistency limit or Atterberg's limit.
  • A soil containing high water content is in liquid state.
  • It offers no shearing resistance and cam flow like liquid.
  • It has no resistance to shear deformation and therefore the shear strength is equal to zero.
  • The water content at which the soil changes from liquid state to plastic state is known as liquid state. $$OR$$
  • In other words, the liquid limit is the W.C. at which soil ceases to be liquid.

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1. Plastic State

  • The soil is in plastic state can be moulded into various shapes.
  • As the water content is reduced, the plasticity of the soil decreases.
  • Ultimately, the soil passes from the plastic state to semi-solid state when its stop behaving as plastic.
  • The water content at which soil become a semi-solid is known as plastic limit $$OR$$
  • Plastic limit is water content at which the solid just fails to behave plastically.

The numerical difference between liquid limit & plastic limit is known as plasticity index. Thus, PI=LL-PL

The soil remains plastic when the water content is between liquid limit to plastic limit. PI = LL-PL

1. Semi-Solid State:

  • When the soil sample reduce below the plastic limit, soil attains a semi-solid state. The soil cracks when molded.

  • In this (semi-solid state), the volume of soil decrease with a decrease in water content till the stage is reduced when further reduction of water content does not caused any reduction in the volume of soil.

  • The water content at which the soil changes from the semi-solid state to solid state is called as shrinkage limit.

  • Shrinkage limit may be also defined as lowest water content at which the soil is fully saturated.

  • Below the shrinkage limit soil does not remain saturated.

Shrinkage Limit :-

$W_s=\frac{(m_1 - m_2)-(v_1 - v_2) \rho _w}{m_s}$

where $m_1 - m_2 = mass$

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