Soil Classification

I.S. Particle size classification

• (i) Classification is done based on grain size only.
• (ii) Soil is given names like clay, slit, sand, Gravel, Coble, Boulder based on the grain size.                     
• Note : 1mm = 1000 microns.

American Association of State Highways and Transport officials (AASHTO) (or) Public Roads Administration (PRA) (or) Highway Research Board (HRB) Soil Classification: - 

>> Suitable for Highway (pavement) design. It is based on particle size distribution and plasticity characteristics (consistency limits). Soil is classified into groups like A1, A2 ......A7

• Black cotton soils fall under A7 group.
• Smaller the group number, better is the soil for pavement.
• Group Index (GI) is used to indicate quality of soil within its own group.

Group Index (GI) of a Soil :-

GI depends upon:

• % soil passing 75 micron IS sieve
• liquid limit
• plastic limit

If GI value worked out is —ve , it is reported as zero.

• 01 values varies from O to 20.
• A soil with GI value = O, is the best soil
• A soil with GI value =. 20, is the poor soil for pavement.
• Smaller the GI value, better is the soil for pavement.

Unified soil classification System :-

Developed based on ‘Air field classification system adopted by Casagrande. Criteria:

• Grain size distribution
• Plasticity Characteristics 
• compressibility

Commonly used in European countries.

IS Soil Classification :-

It is derived from unified soil classification system.

Criteria:

• Grain size
• Plasticity characteristics
• Compressibility

Soils are Primarily divided into 3 groups.

• Coarse grained soils
• Fine grained soils
• Highly organic soils (peat, Pt)

Coarse grained soils :- The given soil is classified as coarse grain if the % soil retained on 75 micron IS sieve is more than 50 %.

Fine grained soils: The given soil is classified as fine grained if more than 50 % of soil passes 75 micron IS sieve.

Coarse grained soils are divided into Gravel and sand

Gravel(G): A Coarse grained soil is classified as Gravel if more than 50% of coarse fraction (coarse fraction is the soil which is retained on 75 micron IS sieve) retains on 4.75  IS sieve.

Sand(S) : When more than 50 % of coarse fraction passes 4.75 mm IS sieve.

Gravel and Sand are divided into four types follows:

• Well graded Gravel (GW)
• Poorly graded Gavel (GP)
• Silty Gravel (GM)
• Clayey Gravel (GC)
• Well graded Sand (SW)
• Poorly graded sand (SP)
• Silty sand (SM)
• Clayey Sand (SC)

Fine grained Soils are divided as follows:

• Inorganic Silt (M)
• Inorganic clay (C)
• Organic silts atid clay (O)

The above fine grained soils are further divided as follows based on compressibility characteristics.

>>Liquid limit (LL) of the soil is taken is basis to express compressibility characteristics.

• If LL is les than 35 %, it is Low compressible
• If 35 <LL <50 , it is intermediate compressible
• If LL> 50 %, it is highly compressible

>>The fine gramed soils are classified based on plasticity chart developed by Casagrande.

• The equation of A line is Ip =O.73(WL—20)
• Inorganic clays are plotted above A — line
• Inorganic silts are plotted below A --- line
• Organic silts and organic clays are plotted below A --- line
• Black cotton soils lie along a band partly above and partly below A ---- line.

Gravel is divided into four groups as follows:

* Well graded gravel (GW): when fines< 5%, Cu >4 and Cclies between 1 and 3

* Poorly graded gravel (GP) : when fines< 5 % and not meeting all gradation requirements for GW.

*silty gravel (GM) : When fmes> 12 % and Atterberg limits below A — line or Ip <4.

*Clayey gravel (GC) : When fines>12 % and Atterberg limits above A — line with Ip> 7.

>>Sands are also divided into SW, SP, SM, SC on the above same lines expect that for a well graded Sand (SW), the Cu> 6.

>>When fines are between 5 % and 12 %, border line cases requiring dual symbol such as GP - GM, SW-SC, GW-GC, SP-SM, etc.

>>When Ip lies between 4 and 7, border line cases requiring dual symbol such as GM — GC, SM—SC etc.

>>Organic and inorganic soils are distinguished by:

(a) Colour test (b) Odour test (c) Liquid limit after oven drying

• Colour Test: Organic soils have a colour of dark grey to black.
• Odour Test: Organic soil has odour similar to that of a decaying organic matter.
• Liquid limit test: The liquid limit of oven dried organic soils will get reduced to more than75% of the original liquid limit before oven drying (i.e. decreases by more than 25 %                                            Important points:
• Silt and clay particles are not visible to naked eye.
• Rock dust particles even of clay size, are not plastic.