SOIL STRUCTURE, DIFFERENCE BETWEEN SILT AND CLAY

                                                     Difference between silt and clay:

1. Dispersion Test: Take a spoon of oven dried soil sample and mix in the glass of water. The soil settles early is silt and soil which takes more time is clay.                                                     Because : Size of silt is more than clay.
2. Dilatency Test: Take a moist/wet sampe of soil and placed it over palm, if upon remoulding moisture appears over the surface of soil, it is silt otherwise clay.                                        Because : Moisture holding capacity of clay is more than silt, because voids of clay are very small but huge in number.
3. Toughness Test: Prepare threads of 3mm dia. And rolled between palms, if cracks appears over the surface of the soil thread then soil is silt else clay.                                                                        Because : Plasticity of clay is more than silt. Also shear strength of clay is more than silt. 

 

SOIL STRUCTURE 

Soil may have following type of structure :

1. Coarse grained structure 

2. Honey - comb structure 

3. Flocculant & Dispersed structure 

1. COARSE GRAINED STRUCTURE :

• This type of structure is found in soils having particle size greater than 20micron.
• Examples : Gravel, Sand & Coarse silt. 
• In the formation of this structure gravitational forces plays predominant role & surface electrical forces are insignificant (negligible) as the result of which soil solid settles under the effect of their own weight during their deposition from suspension leading to particle to particle contact. 
• Such soil doesn't show swelling and shrinkage property and hence are less compressible (because of less voids). 

2. HONEY - COMB STRUCTURE :

• This structure is found in the soils having particle size in the range of (20 micron to 0.2 micron). 
• Example: Very fine soils (fine silt & clay). 
• In the formation of this structure gravitational forces & surface electrical forces both play equally important role as both are of almost same magnitude. 
• As the result of which when the soil solid settles down under the effect of their own weight, surface electrical forces hold these solids in contact with each other in the form of cluster of honey comb enclosing large volume of void in between them. 

      1.IF THE STRUCTURE IS UNBROKEN :

• Load carrying capacity is high
• Permeability is high(void size) 
• Compressibility (low) 

     2. IF STRUCTURE IS BROKEN :

• Load carrying capacity & permeability decreases 
• Compressibility increases. 

3. FLOCCULANT & DISPERSED STRUCTURE :

• These type of structures are formed when size the particles are less than 0.2micron.
• Example: Fine clay & colloids
• In the formation of this structure surface electrical forces plays a predominant role & the gravitational forces are insignificant. 
• These fine grained soils consists of tiny platelets which are electrically charged. 
• Generally positive charge at edges and negative charge at the centre. 
• And depending upon the nature of NET ELECTRICAL FORCES between the platelets during the deposition FLOCCULANT & DISPERSED structure may be formed. 

     A. FLOCCULANT STRUCTURE : If net electrical forces during deposition is ATTRACTIVE then FLOCCULANT structure is formed.In this structure soil platelets are arranged in EDGE TO EDGE or EDGE TO FACE orientation. 

     B. DISPERSED STRUCTURE : If net electrical forces during deposition is REPULSIVE then Dispersed structure is formed. In this structure the soil platelets are arranged in FACE TO FACE orientation.

NOTE: 1.With increase in concentration of dissolved minerals in water during deposition, tendency of formation of flocculant structure increases (sea water, marine clay). 

2. In fresh water (lakes) generally Dispersed structure formed. 

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