The three major groups in the Indian Standard Soil classification system (ISSCS) are coarse grained, fine grained, peat/organic. Index properties like grain shape, distribution, consistency and plasticity index help us classify soils.
How is soil classified?
Index properties like grain shape, distribution, consistency, and plasticity help us classify soils. Sieve analysis can be used to determine grain size distribution.
Coarse grained soil classification by Grain Size Distribution
Fine grained soil classification by Consistency and Plasticity Index
Methods of Soil Classification
Unified soil classification system(USC)
American Association of State Highways and Transport office system(AASHTO)
Indian Standard Soil classification system(ISSCS)
Find all details about USC and AASHTO here
The three major groups in ISSCS are coarse grained, fine grained, peat/organic.
Sieve Analysis for grain size distribution
Sieve analysis is used for grain size distribution. Different sizes of sieves are used to classify the soil.
Sieve analysis by mechanical shaking (10-15min).
% retained on a sieve = (Weight of soil retain ×100)/(Total weight taken)
Cumulative % retained = sum of % retained on all the sieves of larger size and that of a particular size.
% finer than the sieve under consideration=100-cumulative %retained
Coarse Grained (ISSCS)
If the percentage retained on 75micron is >50%, then it comes under this class.
Gravel
If the percentage retained on 4.75 mm is >50%.
If percentage fines pass 75 microns <5% and Cu > 4 and 1 < Cc < 3 then GW (Well graded Gravel). If any of this condition fails then it comes under GP (Poorly graded Gravel).
If percentage fines range between 5-12 % and Cu >4 and 1<Cc<3 then GW-GC/GW-GM. If any of this condition fails then GP-GC/GP-GM.
If % fines>12% then if PI<4% then GM
If % fines>12% then if PI>7% then GC
If % fines>12% then if 4< PI<7% then GM-GC
Sand
If % retained on 4.75 mm is <50%, then the category is sand.
If %fines passing 75 microns <5% and Cu>6 and 1<Cc<3 then SW (Well graded Sand). If any of this condition fails then SP (Poorly graded Sand).
If % fines 5-12% and Cu>4 and 1<Cc<3 then SW-SC/SW-SM. If any of this condition fails then SP-SC/SP-SM.
If %fines>12% then if PI<4% then SM
If %fines>12% then if PI>7% then SC
If %fines>12% then if 4< PI<7% then SM-SC
Fine-Grained (ISSCS)
It is fine-grained when % retained in 75-micron sieve is less than 50%. Further classification depends on the liquid limit, plasticity index and A-line.
If WL <35% and Ip>7% then CL; If 35%<WL <50% and Ip>7% then CI; If WL>50% and Ip>7% then CH.
If WL <35% and Ip<4% then ML/OL; If 35%<WL <50% and Ip<4% then MI/OI; If WL >50% and Ip<4% then MH/OH.
If WL <35% and 4<Ip<7% then CL-ML; If 35%<WL <50% and 4<Ip<7% then CI-MI; If WL >50% and 4<Ip<7% then CH-MH.
Subscribe to the APSEd blog to get such articles instantly as they release.