Particle Size Distribution | Properties of Soil | Soil Mechanics

  Particle Size Distribution  

Particle size distribution analysis or mechanical analysis is carried out in two systems: Sieve Analysis, Sedimentation Analysis. In Coarse Grained Soil: we study Gravel & Sand and In Fine Grained Soil: we study Silt & Clay.

Coarse-Grained Soil

• Gravel: 4.75mm to 80mm
• Sand 0.075mm to 4.75mm (75μm to 4.75mm)

Fine-Grained Soil

• Silt 0.002mm to 0.075mm (2μm to 75μm)
• Clay <2μm(.002mm)

The percentage of various sizes of a particle in a given dry soil sample is found by particle size distribution analysis or mechanical analysis which is carried out in two systems.

1. Sieve Analysis.
2. Sedimentation Analysis.

  Sieve Analysis  

• Sieve analysis is done for soil fraction size greater than .75 mm. it means particles which are retained over 75μ sieve.
• As per IS460:1962 sieves are designed by the size of the square opening in mm or μm.
• In this method, different sieves are arranged one over each other in descending order of their size.
• An oven-dried sample of soil is placed over the top of most sieves and sieving is done at least for 10 min either manually or in a sieve shaker.
• The weight of soil fraction retained over each sieve is noted to compute the percentage finer corresponding to given size of sieve.

         %finer(%N)= 100- cumulative % weight.

(a) Coarse Sieve / Gravel Sieving

• It is done for soil fraction having size greater than 4.75mm. it means retained over 4.75 mm sieve.
• Standard sieves used are 80mm, 40mm, 20mm, 10mm and 4.75mm.

  

(b) Fine Sieve / Sand Sieving

• It is done for soil fraction that passes through 4.75mm sieve  but are  retained over .075 mm sieve.
• Standard sieves used are 2mm, 1mm, 600μm, 425μ, 212μ, 150μ, 75μ.
• The result of the sieve analysis represented in term of size of the particle and corresponding % finer.
• It is generally preferred to wash the soil fraction that passes through 4.75 mm sieve before carrying out sieving in order to separate clay and silt particles present over sand this process is termed as wet sieving.

  Sedimentation Analysis  

• Sedimentation analysis is carried out for the soil fraction having size less than 0.075 mm or which passes through the 0.075 mm sieve.
•  Particles having size less than 0.2 u (0.0002 mm) can not be analyses even by sedimentation. For them special techniques like electron microscope and X-ray diffraction techniques are used.

• Sedimentation analysis is based on stoke’s law, according to which velocity of particle undergoing settlement within infinite medium is depend upon size, shape and mass of the particle.

• According to stokes law, terminal velocity is given as.

       Vs=\(\frac{(γ_s-γ_w)d²}{18μ}\)

         Vs=\(\frac{(G-1)γ_w·d²}{18μ}\)

WHERE μ= Dynamic viscosity of water, d=Particle size

• At 20°C, dynamic viscosity of water is 0.01 poise and suppose, G = 2.65 then

Vs = 899250 d² m/s (d in meter)

Vs = 0.899 d² m/s (d in mm)

• At 29°C, dynamic viscosity of water is 0.00855 poise, then

Vs = 1.07 d² m/s  (d in mm)

Limitations of Stoke’s law

1. Particles undergoing settlement is assumed to be spherical but in actuality, fine-grained particles are flaky in nature.

2. The medium in which settlement takes place is assumed to be infinite but in actuality, the sedimentation jar has a finite dimension.

3. The particles are assumed to be undergoing discrete settling. But in actuality fine-grained particles aggregates during the settlement.
4. Stoke’s law is valid for particles having a size range between 0.2 mm to 0.2 μm.

• If the size of particle is greater than 0.2 mm gravity acceleration sets up during settlement due to turbulent motion as a result of which constant velocity is not achieved throughout the settlement.
• If the size of the particle is less than 0.2 μm, then the Brownian motion setup does not allow settlement of the particle.
  

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  Generally, the procedure for sedimentation analysis can be carried out by:

• Pipette Method
• Hydrometer Method

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