C LASSIFICATION OF S OILS
4.6 ILLUSTRATIVE EXAMPLES
IDENTIFICATION AND CLASSIFICATION OF SOILS 105 Boundary Classification for Fine-grained Soils
The fine-grained soils whose plot on the plasticity chart falls on, or practically on A-line, wL = 35-lines, wL = 50 and lines shall be assigned the proper boundary classification. Soils which plot above the A-line, or practically on it, and which have a plasticity index between 4 and 7 are classified ML—CL.
Black Cotton Soils
These are inorganic clays of medium to high compressibility. These are characterised by high shrinkage and swelling properties. When plotted on the plasticity chart, these lie mostly along a band above A-line. Some may lie below the A-line also. ‘Kaolin’ behaves as inorganic silt and usually lies below A-line; thus, it shall be classified as such (ML, MI, MH), although it is clay from the mineralogical standpoint.
The classification procedures for coarse-grained soils and for fine-grained soils, using this system, may be set out in the form of flow diagrams as shown in Figs. 4.5 and 4.6.
Relative Suitability for General Engineering Purposes
The characteristics of the various soil groups pertinent to roads and airfields value as subgrade, sub-base and base material, compressibility and expansion, drainage characteristics, and compaction equipment (in qualitative terms), ranges of unit dry weight. CBR value percent, and sub-grade modulus—are also tabulated.
Characteristics pertinent to embankments and foundations—value as embankment material, compaction characteristics, value as foundation material, requirements for seepage control (in qualitative terms), ranges of permeability and unit dry weight—are also tabulated.
Characteristics pertinent to suitability for canal sections—compressibility, workability as a construction material and shearing strength when compacted and saturated are also given in relative or qualitative terms.
This information is supposed to serve the purpose of a guideline or an indication of the suitability of a soil based on the I.S. Classification System. Important and large projects will need detailed investigation of the soil behaviour—first-hand. A comparison between IS Classi-fication and Unified ClassiClassi-fication shows many points of similarity but only a few points of difference, especially in classifying fine-grained soils.
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Fig. 4.5 Flow chart for classification of coarse-grained soils
COARSE-GRAINEDSOILS 50%orlesspass75-issievem Runsieveanalysis GRAVEL(G) Morethan50%ofcoarsefraction retainedon4.75mmISsieve
SAND(S) Morethan50%ofcoarsefraction fractionpass4.75mmISsieve Lessthan5%pass 75-ISsievemBetween5%&12% pass75-ISsievemMorethan12%pass 75-ISsievemLessthan5% pass75-ISsievem Examine grain-sizecurveRunwandw onminus425- ISsievefraction
Lp m
Examine grain-sizecurveBorderlinetohavedouble symbol,appropriatetograding andplasticitycharacteristics Well gradedPoorly graded GPGWGMGM–GCGCSWSPSMSM–SCSC BelowA-lineor hatchedzoneon plasticitychart Limitsplotin hatchedzoneof plasticitychart AboveA-line& hatchedzonein plasticitychart AboveA-line& hatchedzonein plasticitychart
Well gradedPoorly gradedBelowA-lineor hatchedzoneon plasticitychart Limitsplotin hatchedzoneof plasticitychart
Morethan12%pass pass75-ISsievem Runand onminus425- ISsievefraction
wwLp m
Borderline,tohavedouble symbol,appropriatetograding andplasticitycharacteristics Between5%&12% pass75-ISsievem
IDENTIFICATION AND CLASSIFICATION OF SOILS 107
Fig. 4.6 Flow chart for classification of fine-grained soils
L Liquidlimitlessthan35H Liquidlimitgreaterthan50 BelowA-line orhatchedzone inplasticitychart
Limitsplotin hatchedzone inplasticitychart
AboveA-lineand hatchedzonein plasticitychart Colourodour, possiblyand onoven-drysoilwwLp AboveA-lineon plasticitychartBelowA-lineon plasticitychart Colourodour, possiblyand onoven-drysoilwwLp BelowA-lineon plasticitychart Colourodour, possiblyand onoven-drysoilwwLp
AboveA-lineon plasticitychart OrganicInorganicInorganicOrganic MIOICIMHOH
Organic OLMLML–CLCL
Inorganic CH
FINE-GRAINEDSOILS Morethan50%pass75-ISsievem Runandonminus425- ISsievematerialwwLpm I Liquidlimitbetween35&50
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(c) Which soil has better strength as a function of water content ? (d) Which soil has better strength at the plastic limit ?
(e) Could organic material be present in these soils ?
Plot the positions of these soils on the Casagrande’s plasticity chart and try to classify them as per IS Classification.
(a) Plasticity index, Ip for soil S1 = wL – wP = (38 – 18) = 20 Ip for soil S2 = wL – wP = (60 – 20) = 40 Obviously, Soil S2 is the more plastic.
As per Burmister’s classification of the degree of plasticity, S1 borders between low-to-medium plasticity and S2 between medium-to-high plasticity.
(b) Consistency index,
Ic for soil S1 = (w w) ( ) I
L p
− = 38 40−
20 = – 0.1 Ic for soil S2 = (60 50)
40
− = 0.25
Since the consistency index for soil S1 is negative it will become a slurry on remoulding;
therefore, soil S2 is likely to be a better foundation material on remoulding.
(c) Flow index, If for soil S1 = 10 If for soil S2 = 5
Since the flow index for soil S2, is smaller than that for S1, soil S2 has better strength as a function of water content.
(d) Toughness index, IT for soil S1 = Ip/If = 20/10 = 2 IT for soil S2 = 40/5 = 8
Since toughness index is greater for soil S2, it has a better strength at plastic limit. (e) Since the plasticity indices are low for both the soils, the probability of the presence of organic material is small.
These conclusions may be mostly confirmed from the following:
The soils are marked on Casagrande’s plasticity chart as shown in Fig. 4.7.
CH
w = 50L w = 50L
S2 S2
S1 S1 w = 35L wL= 35
MH or OH MH or OH CI
CI
CL
CL – ML
CL – ML M or
O II M or
O II
100 90 80 70 60 50 40 30 20 10
wL 60
50
40
30
20
10
0 IP
A-line
=0.73 (w
–20)
IP
L
Fig. 4.7 Plasticity chart, soils S1 and S2 plotted (Example 4.1)
IDENTIFICATION AND CLASSIFICATION OF SOILS 109 S1 and S2 are respectively in the zone of CI and CH (inorganic clays of medium and high plasticity).
Example 4.2: A soil sample has a liquid limit of 20% and plastic limit of 12%. The following data are also available from sieve analysis:
Sieve size % passing
2.032 mm 100
0.422 mm 85
0.075 mm 38
Classify the soil approximately according to Unified Classification or IS Classifiction.
(S.V.U.—Four year B. Tech.—June, 1982) Since more than 50% of the material is larger than 75-µ size, the soil is a coarse-grained one.
100% material passes 2.032 mm sieve; the material,passing 0.075 mm sieve is also in-cluded in this. Since this latter fraction any way passes this sieve, a 100% of coarse fraction also passes this sieve.
Since more than 50% of coarse fraction is passing this sieve, it is classified as a sand.
(This will be the same as the per cent passing 4.75 mm sieve).
Since more than 12% of the material passes the 75-µ sieve, it must be SM or SC.
Now it can be seen that the plasticity index, Ip, is (20 – 12) = 8, which is greater than 7.
Also, if the values of wL and IP are plotted on the plasticity chart, the point falls above A-line.
Hence the soil is to be classified as SC, as per IS classification.
Even according to Unified Classification System, this will be classified as SC, which may be checked easily.
SUMMARY OF MAIN POINTS
1. Certain generalised procedures have been evolved for identification of soils in the laboratory and in the field, and for classification of soils. The need for classification arises from the fact that natural soil deposits vary widely in their properties and engineering behaviour.
2. The requirements or desirable features of an engineerging soil classification system are so ambi-tious that it is almost impossible to evolve an ideal system satisfying all of these.
3. Preliminary classification procedures include descriptive and geological classifications, and also classification by structure.
4. Textural classifications are used as part of the more systematic and exhaustive systems such as the Unified Soil Classification.
5. The Indian Standard Soil Classification bears many similarities to the Unified Soil Classifica-tion although there are a few points of difference, especially with regard to the classificaClassifica-tion of fine-grained soils.
6. Grain-size is the primary criterion for the classification of coarse-grained soils, while plasticity characteristics, incorporated in the plasticity chart, are the primary criterion for the classifica-tion of fine-grained soils.
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REFERENCES
1. A Casagrande: Classification and Identification of Soils, Transactions of ASCE, Vol. 113, 1948.
2. IS: 1498-1970 (First Revision) : Classification of Soils for General Engineering Purposes.
3. A.R. Jumikis: Soil Mechanics, D. Van Nostrand Co., Princeton, NJ, 1962.
4. D.W. Taylor: Fundamentals of Soil Mechanics, John Wiley & Sons Inc., NY, U.S.A., 1948.
5. The Unified Soil Classification System. Appendix B, Technical Memorandum No. 3-357, March 1953, revised June, 1957. U.S. Army Engineer Waterways Experiment Station, Corps of Engi-neers, Vicksburg, Mississippi, U.S.A.
6. M. Whitney: Methods of the Mechanical Analysis of Soils, U.S. Department of Agriculture, Divi-sion of Agricultural Soils, Bulletin No. 4, Washington, D.C., Government Printing Office, 1896.
QUESTIONS AND PROBLEMS
4.1 (a) Four soil samples collected from a borrow area, to form a low earth dam, are classified as GW, CL, SC and SM. What is your inference ?
(b) The following data relate to five soil samples.
LL (%) ... 25 45 50 60 80
PL (%) ... 15 23 25 35 36
Plot these on Casagrande’s A-line chart and classify the soils.
(S.V.U.—B.Tech. (Part-time)—May, 1983) 4.2 The following data refer to a sample of soil:
Percent passing 4.75 mm IS Sieve = 64 Percent passing 75-µ IS Sieve = 6
Uniformity Coefficient = 7.5
Coefficient of Curvature = 2.7
Plasticity index = 2.5
Classify the soil.
4.3 A certain soil has 99% by weight finer than 1 mm, 80% finer than 0.1 mm, 25% finer than 0.01 mm, 8% finer than 0.001 mm. Sketch the grain-size distribution curve and determine the per-centage of sand, silt and clay fractions as per IS nomenclature. Determine Hazen’s effective size and uniformity coefficient.
4.4 (a) Write a brief note on Textural classification.
(b) Sketch neatly the Casagrande’s plasticity chart indicating various aspects. How would you use it in classifying the fine grained soils ? Give a couple of examples. How would you differ-entiate between organic and inorganic soils ? (S.V.U.—B.Tech., (Part-Time)—Sept., 1982) 4.5 (a) Bring out the salient aspects of Indian Standard Classification System.
(b) Write a brief note on the Textural classification.
(c) How would you distinguish if a material is:
(i) GW or GP or GM or GC
(ii) SW or SP or SM or SC (S.V.U.—B. Tech., (Part-time)—April, 1982)
IDENTIFICATION AND CLASSIFICATION OF SOILS 111 4.6 Describe in detail the Indian System of soil classification. When would you use dual symbols for
soils ? (S.V.U.—Four year B. Tech.—June, 1982)
4.7 (a) Draw neatly the IS plasticity chart and label the symbol of various soils.
(b) What are the limitations of any soil classification system ? (c) Explain the following tests with their significance.
(i) Dilatancy, (ii) Thread Test, (iii) Dry Strength Test.
(S.V.U.—B. Tech., (Part-time)—April, 1982) 4.8 (a) Why is classification of soils required ?
(b) What are common classification tests ?
(c) How do you classify a soil by the I.S. Classification system ? (d) How would you differentiate between SC and SF soils
(S.V.U.—B. Tech., (Part-time)—June, 1982) (Hint. The symbol ‘F’ was used in the older versions of the Unified classification, i.e., in the Airfield classification, to denote ‘Fines’. Thus, SF and GF were used in place of SM and GM).
4.9 What physical properties of soil distinguish between cohesive and cohesionless soils ? Also ex-plain the principle of sub-dividing cohesive and cohesionless deposits for the purpose of soil
classification. (S.V.U.—B. E., (R.R.)—May, 1975)
4.10 (a) Describe the U.S. Bureau of Soils Textural classification.
(b) Describe field identification tests to distinguish between clay and silt.
(S.V.U.—B.E., (R.R.)—November, 1994) 4.11 (a) Explain why soils are classified and outline the salient features of Casagrande’s airfield
classification. (S.V.U.—B.E., (R.R.)—November, 1973)
(Hint. Casagrande’s airfield classification was developed earlier and formed the basis for the Unified Classification. Symbols SF and GF were used in place of SM and GM, which were intro-duced later.)
4.12 (a) State the various classification systems of soils for general engineering purposes.
(b) Briefly describe the ‘‘Unified Soil Classification’’, (S.V.U.—B.E., (R.R.)—Dec., 1971) 4.13 (a) Describe the method of field identification of soils.
(b) How do you use the A-line to distinguish between various types of clays ?
(S.V.U.—B.E., (N.R.)—May, 1969) 4.14 How do you distinguish between clay and silt in the field ? State the purpose of identification and classification of soils. List any three important engineering classification systems and describe one in detail, clearly bringing out its limitations. (S.V.U.—B.E., (N.R.)—Sept, 1967)