Concept:

• Plasticity index of soil, I_P = W_L – W_P
• Plasticity index of A-line, I_P = 0.73 ( W_L - 20 )

Calculation:

Given,

Liquid limit ( W_L) = 60%

Plastic limit (W_p) = 20%

I_P of soil = W_L – W_P = 60 - 20 = 40%

I_P of A-line = 0.73 (W_L -20) = 0.73 (60 - 20) = 29.2%

I_P of soil > I_P of A-line, So it will lie above A-line and also W_L > 50%,

∴ Soil is CH.

Explanation:

Suffix and prefix used in soil classification:

Additional Information  These symbols are used for classification of fine-grained soil in plasticity chart as given below:

Important Points 

The equation of "A" line for plasticity index, P.I = 0.73 ( WL - 20 )

The equation of "U" line for plasticity index, P.I = 0.9 ( WL - 8 )

As specific surface increases as size of grain decreases. Therefore Microscopic particles have a large specific surface.

Concept:

• Plasticity index of soil, I_p = W_L - W_p
• Plasticity index of A-line, I_p = 0.73 ( W_L - 20 )

Calculation:

Given,

liquid limit (  W_L ) = 60%

plastic limit (W_p) = 20%

I_p of soil = W_L - W_p = 60 - 20 = 40%

I_p of A-line = 0.73 ( W_L -20 ) = 0.73 ( 60 - 20 ) = 29.2%

I_p of soil > I_p of A-line, So it will lie above A-line

and also W_L > 50%, so soil is CH

Based on liquid limit and plasticity index, fine grained soil is subdivided.

By IS classification, silts and clays of low compressibility having a liquid less than 35, are represented by ?L?.

• Prepare a part of moist soil having a volume equivalent to a 25 mm cube by adding enough water to make soil soft but not sticky.
• Place pat in open palm of one hand and shake horizontally by striking against or hand several times.
• If reaction is positive, water appears on surface of pat giving a glossy appearance.
• On squeezing sample between fingers water and glossiness disappear from surface, and soil becomes stiff and cracks.
• The phenomenon of appearance of water on surface of soil on shaking and disappearance on squeezing, followed by cracking is called as “dilatancy”.
• The rapidity of appearance and disappearance of water from surface of soil help to identify character of fines in soil.

• Prepare a part of soil to consistency of putty by adding water. Allow pot to dry by oven, sun or air.
• The strength is tested by breaking and crumbling dry pat between fingers.
• The dry strength of soil increases with increasing plasticity.
• Clays have high dry strength and silts have slight dry strength.

• Take a part of soil to consistency of putty, add water or allow drying as necessary.
• Roll soil between palms into a thread of 3 mm diameter. Fold thread of soil and repeat procedure several times till thread starts crumbling when rolled into 3 mm diameter.
• The crumbled pieces are lumped toger and subjected to kneading until lump crumbles.
• The threads are stiffer and lumps are tougher at plastic limit for soils having higher clay contents.

• Pour a small quantity of soil in a jar of water. Shake jar containing soil and water and allow soil to settle.
• The coarser particles settle first followed by finer ones.
• Sands settle in about 30 to 60 sees, silts settle in 30 to 60 mins
• clay particles remain in suspension for at least several hours.

• Organic soils have darker colours like dark grey, dark brown etc. and a musty odour.
• The odour can be more noticeable by heating a wet sample. Inorganic soils have clean, bright colours like light grey, brown, red, yellow or white.

In particle soil classification system, soils are arranged to grain size.

For general engineering purposes, soil can be classified in terms of particle size, textural and unified soil.

Explanation:

• Plasticity chart gives relationship between plasticity index and liquid limit.
• Equation of A-line, I_p = 0.73 ( W_L - 20 ) , where I_p = plasticity index , W_L = liquid limit
• If point gets plotted above A-line, classify soil as clay.
• Furr,
  • If, 35 > W_L ; classify as CL ( low compressible clay)
  • If, 35 < W_L < 50; classify as CI ( intermediate compressible clay )
  • If, 50 < W_L ; classify as CH ( high compressible clay)
• If point gets plotted below A-line, n we need to find out wher soil is organic or inorganic. If it's inorganic below A-line n it is silt ( M )
• Furr,
  • If, 35 > W_L ; classify as ML (  low compressible silt ) or OL (  low compressible organic)
  • If, 35 < W_L < 50; classify as MI (  intermediate compressible silt ) or OI (  intermediate compressible organic )
  • If, 50 < W_L ; classify as MH ( high compressible silt ) or OH ( high compressible organic)