Page 154 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
Particle Size and Gradation 149

that is, cut into four pie-shaped sectors and then combining opposing sectors
and returning the other half of the sample to the bag. This procedure is
repeated until the soil sample is small enough to be managed. A more rapid
method for quartering uses a ‘‘riffle-type’’ sample splitter that has parallel
shuts, with half directing the sample one way and the other half the other. Soils
are air-dried prior to quartering and sieving, but as discussed in Chapter 6,
if a soil contains halloysite clay mineral, it should be saved and sealed against
drying.

7.4.2 Sedimentation Analysis
Sieving is appropriate for measuring the amounts of sand and gravel in a soil, but
silt and clay sizes are too small to be separated by sieving. Also, clay particles
tend to be aggregated together into coarser particles and to occur as coatings
on coarser particles. Gravel is removed by sieving, and the rest of the soil
normally is soaked in water and then agitated and dispersed using a chemical
dispersing agent. The suspension then is tested by measuring sedimentation rates,
and finally the part of the soil that is retained on a fine sieve is dried and analyzed
by sieving.

The general procedure is as follows. After sieving to remove gravel and coarser
particles, the soil is soaked in water containing a small amount of a chemical
dispersing agent, usually sodium hexametaphosphate, a water softener that is
available in the detergent department of a supermarket. The dispersing agent
forces substitution of sodium ions for exchangeable calcium ions on the clay by
creating an insoluble phosphate precipitate.

The suspension then is agitated for a set amount of time with a standardized
mechanical or air-jet stirring device. Ideally this will separate but not break
individual soil grains. The soil suspension is diluted to 1 liter in a vertical flask and
stirred in preparation for starting the test.

The starting time is noted and the suspension is allowed to settle for various time
intervals. After each time interval, the density of the suspension is determined at a
particular depth with a hydrometer. An alternative method is to sample the
suspension with a pipette, then dry and weigh the sample.

The larger the weight of particles remaining in suspension, the denser the liquid,
and the higher the hydrometer will float. An engineering hydrometer is calibrated
to read directly in grams of soil per liter of suspension. Readings normally are
taken after 1 minute and at various time intervals to 1 hour and then after
24 hours.

After the sedimentation analysis is completed, the soil is washed on a fine sieve to
remove the silt and clay particles, then dried and the sand fraction analyzed by
passing through a series of sieves.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.