Grain Size Distribution
A Grain Size analysis (ASTM D422) which is sometimes referred as (Particle Size analysis), is the method of segregating different sizes of soil particle by using a series of standard sieve ranging from 300mm to 0.075mm (Per ASTM D2487). The large range between the top and bottom of sieve series is the reason, two laboratory tests were used: the sieve analysis and hydrometer analysis.
The sieve analysis (ASTM D422) consists of preparing a soil sample with a known weight of solids, Ws, and passing it through sieves from the coarsest one on top of the stack and decreasing the sieve sizes down the stack. The data collected is then expressed as a percentage of the total weight.
The following computations are necessary to calculate Percent Passed, which is plotted vs Sieve Size in mm. Furthermore, the Particle size distribution curve can be used to calculate Cc and Cu, which will be discussed further in the section about Coefficients of uniformity.
Thus to compute the data for 2 mm Sieve,
Thus, these plots point corresponding to 2 mm and 99.98%. The same process was followed for all other points in Table s1.
The Hydrometer analysis works for and is necessary to analyze all solids that pass the #200 sieve or the fines content. For this reason, it is very difficult to split silts and clays apart using conventional sieve, due to the fact that the sieve opening would have to be extraordinarily small and almost impossible to manufacture. This is why the procedure (ASTM D422) consists of using a 1000 mL graduated cylinder filled with water and hydrometer (151H) see figure s1.
The data collected was tabulated in table s2 and was necessary to calculate percent pass vs particle diameter. To obtain the following results in table s2 from plot point at the elapsed time of 2 min, it was necessary to compute the following:
Using table s1 to obtain % passing # 10:
Using table s3 to determine the effective depth for the actual hydrometer reading and table s4 to find K which is a constant.
Thus, the specific gravity was obtained using the pycnometer for the soil sample is 2.64, see (Specific Gravity Lab) which is in-between two known calculated K constant values, see table 4, and the method of interpolation was calculated using equation s1 with the temperature at 20°C:
Thus,
Thus, the K constant follows:
Thus, with the K constant, effective depth and the time interval T, can be using to calculate the particles diameters using equation s2:
Thus, the diameter of the particles at the 2 min interval was:
To quantify the Percent Finer, (P), at the 2 min interval equation s3 was used:
Refer to ASTM D422 for all descriptions of the variables in equation 3 to calculate (P)
Thus,
Thus, these plots point corresponding to Particle Size = 0.0387 mm, Percent finer 2.12% the same process was followed for all points in table s2.
Using table s1 and table s2 a plot was created, the Particle size distribution Curve, the x-axis is in log10 see figure s2.
An analysis of figure s2 was done to determine a given D-size, those that are important to soil engineers are the coefficient of uniformity, Cu, and the coefficient of curvature, Cc, which are based on D-size:
Thus, using equation s4 and equation s5, Cu and Cc can be determined for figure s2:
Therefore using table s5 to classify the soil, it classified as a poorly graded sand, SP, because of Cu < 6 and Cc is between 1 and 3.
