The first method of mechanical analysis is sieve analysis. It is pretty straight forward and involves straining a soil sample through a series of sieves. Generally the sieves are stacked as seen in the above diagram with a US Standard Sieve No. 4 at the top (This sieve size has a 4.75 mm opening) and a Sieve No. 200 at the bottom (with a 0.075 mm opening). The stack usually consists of 5 to 7 sieves and has a pan at the bottom. As you can see, the opening in the sieve gets smaller as you go down. Therefore larger particles will be captured while smaller particles will pass through. This allows us to measure how much soil of each size was in the original soil sample.
A few important steps:
- the soil sample needs to be oven-dried and all clumps broke.
- the weight of the empty sieves needs to be obtained.
Once this is done, the soil sample is placed into the top sieve, the lid is placed on the stack and the stack is placed in a mechanical shaker. The stack will then be shaken vigorously for approximately 5 minutes to ensure proper distribution of the particles.
The next step is to calculate what is called the percent passing or the percent finer. This is simply the percentage of the entire soil sample that passed through a particular sieve. For example, the percent passing the No. 200 sieve would be the mass of the soil in the bottom pan (since that is the soil that passed through the No. 200 sieve) divided by the total mass of the original soil sample. So in general we can write:
- %P is the percent passing
- i represents the ith sieve
- n represents the number of sieves
- m is the mass
Once the percent passing is determined for each sieve, a plot is made of sieve or particle size versus percent passing as seen below. The percent passing is on the ordinate and the sieve or particle size is on the abscissa. Also notice the abscissa is reverse logarithmic.
In future posts we will look more carefully at what information this distribution curve can provide, but for now we are highlighting the process. If you have any questions, please let me know in the comments.