__ DISCUSSION ON GRAIN SIZE ANALYSIS OF SOIL__

__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):-**

**Textural Classification:-**

**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**

**Procedure**

**Calculations and recording results**

**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**

**Computation of Zone of Sand of soil**

**(As per IS: 383-1970)**