Classification of soil in civil engineering

 IS classification of soil:-

                                       it is classified into two categories.

• Coarse-grained soil:- (50% or more particles retained  on 75-micron sieve                         on the basis of % fineness coarse-grained soil can be further classified as:

1.  Gravel:-   50% or more than  retained on 4.75 mm sieve

case 1 :-   when fineness < 5% by weight 

           i) GW = well graded gravel

                                                  C_u > 4,  1<C_c<3

      ii) GP =  poorly graded gravel (in this values of C_u and C_c  are not satisfied)

                               

case 2 :- fineness in between 5% - 12%

     ( in this dual symbols are used. first for gradation and second for nature of fines. )

    

  i) GW - GC = well graded gravel containing clay 

 C_u > 4,  1<C_c<3

clay fraction > silt fraction

 ii) GP - GC  =  poorly graded gravel containing clay (in this values of C_u and C_c  are not satisfied)

clay fraction > silt fraction

iii) GW - GM = well graded gravel containing silt

 C_u > 4,  1<C_c<3

silt fraction > clay fraction

iv) GP - GM =  poorly graded gravel containing silt (in this values of C_u and C_c  are not satisfied)

silt fraction > clay fraction

case 3 :-   when fineness  > 12%  

 In this case, the soil is classified according to the I.S. plasticity chart (% I_p).

     i) GC = clayey gravel (I_p>7%)

clay fraction > silt fraction (because of % of I_p)

    ii) GM =  silty gravel (I_p<4%)

silt fraction > clay fraction (because of % of I_p)

2.  Sand:-   50% or more than  passed through 4.75 mm sieve

case 1 :-   when fineness < 5% by weight 

       i)  SW = well graded sand

 C_u > 6,  1< C_c<3

     ii) SP  =  poorly graded sand (in these values of C_u and C_c  are not satisfied)

case 2 :- fineness in between 5% - 12%

  i) SW -SC = well graded sand containing clay 

 C_u > 6,  1<C_c<3

clay fraction > silt fraction

  ii)  SP - SC  =  poorly  sand containing clay (in these values of C_u and C_c  are not satisfied)

clay fraction > silt fraction

 iii)  SW - SM = well graded sand containing silt

                 C_u > 6,  1<C_c<3

silt fraction > clay fraction

iv)  SP - SM =  poorly graded sand containing silt (in these values of C_u and C_c  are not satisfied)

silt fraction > clay fraction

case 3 :-  when fineness  > 12% 

in this case, the soil is classified according to the I.S. plasticity chart (% I_p).

    i)  SC = clayey sand (I_p>7%)

clay fraction > silt fraction  (because of % of I_p)

   ii) SM = silty sand (I_p<4%)

silt fraction > clay fraction (because of % of I_p)

Note:-  C_u  and  Cc are coefficient of uniformity and coefficient of curvature respectively. 

 % fineness means % fraction which pass through 75 micron sieve.

• Fine grained soil:-(50% or more particles  passed through 75 micron sieve)

1. silt (0.002 mm to 0.075 mm)
2. clay (size < 0.002 mm) :- it is classified as per % of liquid limit(LL)

  i) Low plastic soils (LL<35%)

CL= low plastic inorganic clay
ML= low plastic silt
OL= low plastic organic clay

ii) Medium plastic soils (35%<LL<50%)

CI= medium plastic inorganic clay

ML= medium plastic silt

OL= medium plastic organic clay 

iii) High plastic soils (LL>50%)

CL= high plastic inorganic clay
ML= high plastic silt
OL= high plastic organic clay

some clay minerals and their properties:-                                        

 

	CLAY MINERAL	BOND	EXAMPLE
1	Kaolinite mineral	Hydrogen bond	China clay
2	Illite mineral	Ionic bond
3	Montmorillonite	Water bond (weakest bond)	Black soil and Bentonite soil