Page 37 - ebook_soil engineering practical workbook_15112021

4 GRADATION TEST

4.1 Experiment outcomes
 Determine the percentage of different grain sizes contained within a soil.
 Determine the distribution of the coarser, larger-sized particles.
 Determine the soil classification

4.2 Theory

The most important single piece of information to know about an agricultural

soil is the distribution of particle sizes, often referred to as texture. Soil
textures have been categorized into soil types such as sands or clays for
easier use. Particle-size distribution has an important influence on a soil's

permeability or water intake rate, its water storage capacity, its ability to
aggregate and propensity for crusting, and the chemical makeup of the soil

water. Particle-size distribution is also important in the design of gravel
filters for drainage tile or well, and for numerous engineering applications

involving construction with earth materials.

Particle-size (or grain-size) analysis is a procedure to determine the relative
proportions of the different particle (or grain) sizes which make up a given

soil mass. Two techniques will be used in this exercise to separate the soil
particles into particle-size ranges. Coarse particles (sands and gravels) can be

separated with mechanical sieves. The distribution of fine particle sizes (silts
and clays) are determined by uniformly dispersing the soil in water and

measuring how quickly the particles fall in the mixture. In this exercise, a
hydrometer will be used to measure the fall rate, which can be related to
particle size by Stokes' equation. Other sedimentation methods, such as the

pipette method, involve sampling the soil water mixture to determine the
density of the mixture at different depths over time.

29 | Soil Engineering Practical Workbook