Grain size analysis

Principle:

Sand fractions are distinguished by passing the sample through shaker sieves.  Fines are separated by settling times through fluid, as given by Stokes' Law.

Equipment:

  • Shovels, ziplock bags, and sharpies for sampling
  • Sieves: Each of these sieves will hold the specified particle size and larger.  The first number is the U.S. Standard mesh size; the second is the Tyler mesh size.
    • Pebbles and larger (>2 mm): 10/9
    • Very coarse sand (>1 mm): 18/16
    • Coarse sand (>1/2 mm): 35/32
    • Medium sand (>1/4 mm): 60/60
    • Fine sand (>1/8 mm): 120/115
    • Very fine sand (>1/16 mm): 230/250
  • Weighing boats
  • Graduated cylinders
  • A stir rod
  • A thermometer

Sampling

You will need a large enough sample size to be able to accurately characterize the particle sizes.  As a rule of thumb, collect a cubic volume of soil or sediment with a dimension at least five times the maximum particle dimension in the sample.  Get more if you can.  This means your sample size gets larger rapidly in coarser sediments, and can eventually be difficult to carry!  For fine-grained samples you don't need much, but it doesn't hurt to collect a larger sample than you need.  If you have too little, you won't be able to weigh the different fractions precisely.

Preliminary steps

  1. Air dry your samples.
  2. Pass the samples through a 2 mm sieve.  Weigh and describe the coarse fraction, if any, then discard.
  3. (Note: carbonate minerals, organic carbon, soluble salts, and iron oxides can all bind particles together, making grain size appear too large.  However, none of these will be a problem in the samples you will be collecting, so we will skip the chemical pretreatments.)
  4. Separate the sand fraction from the fines: sieve through the 1/16 mm sieve over a funnel in a big 1 liter graduated cylinder (labeled!).  Use a squirt bottle with water to wash the sample through.
  5. Tare an oven-safe weighing boat, oven-dry the sand fraction at 105 degrees C for 24 hours, and weigh the total sand fraction.
  6. Disperse the fine fraction: pour it into a plastic container (not glass) and disaggregate using the probe sonicator on the 3rd floor (the first time you use the sonicator I need to be present.)
  7. Pour each sample back into the labeled graduated cylinder and make up to its full volume with water.  Let the water temperature equilibrate overnight.
Characterize the sand fraction with sieves
  1. Weigh each sieve individually and record its mass.
  2. Stack the sieves, finest at the bottom to coarsest on top
  3. Include a lid on the top and a solid container on the bottom
  4. Put your sample into the top sieve, close, and load into the Gilson.  Run the Gilson for 10 to 15 minutes.
  5. Reweigh each sieve and record the mass of each sieve with the retained soil.
Characterize the fine fraction by settling time
  1. Measure the temperature of the suspension.
  2. Mix the sample by stirring up and down for at least 30 seconds.
  3. After the appropriate time interval, pipet a 25 mL sample from the appropriate depth.  See the attached Excel spreadsheet to figure out your settling times based on the water temperature and sampling depth.
  4. Discharge the pipetted sample into a tared beaker, weighing bottle, or aluminum dish, rinsing the pipet tip into the container.
  5. Repeat this sampling for additional size fractions.
  6. Dry the samples in the oven until they are dry, then weigh the samples.
Presenting results

For each soil, graph Phi on the X axis vs. cumulative percent coarser on the Y axis.  Phi is equal to negative log base two of the particle diameter in mm.

Ĉ
Nick Bader,
Apr 3, 2015, 4:24 PM
Ċ
Nick Bader,
Apr 3, 2015, 4:19 PM
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