DISCUSSION ON GRAIN SIZE ANALYSIS OF SOIL
The commonly used properties for the classification are the grain size distribution, liquid limit and plasticity index. Those properties have also been used in empirical design methods for flexible pavements and in deciding the suitability of sub grade soil.
The results of the test are of great value in soil classification. In mechanical stabilization of soil and for designing soil-aggregate mixtures the results of gradation test are used.
The grain size analysis of the coarser fraction soil is carried out using sieves that of finer fraction passing 0.075 mm sieve is carried out using the principal of sedimentation in water.
The mechanical analysis consists of two parts:
i) The determination of the amount and proportion of course material by the use of sieves; and
ii) The analysis for the fine grained fraction by sedimentation method like.
a) Pipette method
b) Hydrometer method.
The most widely accepted grain size classification system is the M.I.T. classification system. The Indian Standards Institution (I.S.I) has also adopted the limits similar to the M.I.T. system for the Indian Standard Classification system of soil grains. The limits of the grain size for each component as per this system are shown below:
M.I.T. Classification :–
I.S. Classification (as per IS: 1498, 1970):-
Silt is slightly plastic or non-plastic in property and gives no strength under addition of water in to it. But Clay is always plastic in property & gives strength under addition of water in it.
Textural Classification:-
The textural classification system is based on grain size distribution of the soil and is helpful in classifying a soil which contains different soil components such as sand silt and clay. A typical textural classification chart for sub-grade soils of sand and smaller grain sizes (suggested by U.S. Bureau of Public Roads) is given in Figure below.
Grain size analysis by sieve analysis (dry method)
Sieve analysis helps to determine the particle size distribution of the coarse and fine aggregates. We use different sieves as standardized by the IS code to (a) determine the percentage of passing, Fineness Modulus (FM), (b) determination of Zone of sand etc.
Fineness Modulus (FM) : The fineness modulus is define as A factor obtained by adding the total percentages of material in the sample that are coarser than each of the standard specified IS sieves (cumulative percentages retained), and dividing the sum by 100. Fineness modulus gives the idea about how course or fine the soil is. Lower FM indicates fine soil and higher FM indicates courser soil. In case concrete of cement-sand-aggregate , Fine sand requires more cement paste due to high surface area and tendency to develop crack. On the other hand course sand produces a concrete mixture that is harsh and difficult to finish and will cause segregation. Therefore recommended FM range is 2.3 to 3.1.
Following table shows fineness modulus for different types of sand
Computation of Fineness Modulus of soil
(As per IS: 2720 (Part-4)-1985, 2nd Revision)
Procure to determine Fineness Modulus for the soil for (a) retained on 4.75mm IS sieve & (b) passing on 4.75mm IS sieve .
1. Sieve the aggregate using the appropriate sieves 100 mm, 75 mm, 19 mm, 4.75 mm for test case (a) and 2.0 mm, 425 μ, 75 μ for test case (b).
2. Note down the weight of aggregate retained on each sieve.
3. Calculate the cumulative weight of aggregate retained on each sieve.
4. Find out the cumulative percentage of aggregate retained on each sieve.
5. Sum the cumulative weight of aggregate retained and divide the sum by 100.
FM = Σ Cumulative % retain on each Sieve / 100
Procedure
Air dry samples should be used for sieve analysis. This may be achieved either by drying at room temperature or by heating at a temperature of 100 °C to 110°C.
Starting with largest sieve, air dry samples should be weighed and sieved successively for the period of at least two minutes. Following precautions should be taken while sieving.
Material shall not be forced through the sieve by hand pressure. If lumps are present in fine aggregates, that may be broken by gentle pressure with fingers Under side of sieve may be clean by soft brush if require. On completion of sieving, the material retain on each sieve should be weighed.
Calculations and recording results
i) the cumulative percentage by weight of the total sample
ii) the percentage by weight of the total sample passing through one sieve and retained on the next smaller sieve, to the nearest 0.1 percent.
Graphical Method of Recording Results : The results of the sieve analysis may be recorded graphically on a semi-log graph with particle size as abscissa (log scale) and the percentage smaller than the specified diameter i.e. percentage finer as ordinate.
Example for calculation sheet to calculate Fineness Modulus
Fineness Modulus (F.M.) = 310/100 = 3.10
Hence the soil sample is coarse sand
For the determination of distribution of grain sizes smaller than 75μ, the pipette method is given as a standard method. For convenience we can adopt the “Hydrometer method” a wet sieve analysis of soil.
Computation of Zone of Sand of soil
(As per IS: 383-1970)
The grading of fine aggregates, when determined as described in IS: 2386 (Part I)-1963 shall bc within the limits given in Table 4 and shall be described as fine aggregates, Grading Zones I, II, III and IV:
Procedure
Test procedure is same as above for FM test. The necessary designated sieves for sieve analysis are mentioned in the bottom table (reference table No. 4 of IS: 383-1970 (clause 4.3);
Report:
Where the grading falls outside the limits of any particular grading zone of sieves other than 600-micron IS Sieve by a total amount not exceeding 5 percent, it shall be regarded as falling within that grading zone. This tolerance shall not be applied to percentage passing the 600- micron IS Sieve or to percentage passing any other sieve size on the coarse limit of Grading Zone I or the finer limit of Grading Zone IV.
Application of zone of sand:
Where high strength and good durability is required in concrete or other constructional work, fine aggregate conforming to any one of the four grading zones may be used, but the concrete mix should be properly designed. As the fine aggregate grading becomes progressively finer, that is, from Grading Zones I to IV, the ratio of fine aggregate to coarse aggregate should be progressively reduced. The most suitable fine to coarse ratio to be used for any particular mix will, however, depend upon the actual grading, particle shape and surface texture of both fine and coarse aggregates.
Use of S-curve from sieve analysis
From the sieve analysis result of grain size analysis, we can draw a S-curve taking size of the soil particle in X-axis by logarithmic scale and % finer along the Y-axis; This graph clearly indicates the soil grain sizes weather it is well graded, poorly graded or gap graded soil.
From the above S-curve, we can determine the D10, D30, D60 , D85 etc. values. Using these soil
grain size, we can obtain the followings important parameter as under:
Cu = Co-efficient of Uniformity = D60 / D10
Cc = Co-efficient of Curvature = D30^2 / (D10 x D60)
where, D60= particle size at 60% finer
D30= particle size at 30% finer and
D10= particle size at 10% finer.
Soils with Cu <= 4 are considered to be “poorly graded” or uniform. Cu Its value should be
more than 4 for well graded gravel and more than 6 for well graded sand.
While for well−graded soils, Cc~ 1; Coefficient of curvature (Cc) should lie between 1 and 3
for well grade gravel and sand.
(Source: Various Sources)