Grain Size Distribution - Hydrometer Analysis printable pdf download

Grain Size Distribution - Hydrometer Analysis

The University of Toledo

Soil Mechanics Laboratory

Grain Size Distribution – Hydrometer Analysis

Introduction

The hydrometer analysis, also called sedimentation method, is used to determine the grain

size distribution for the fraction of soil that is smaller than the No. 10 (2.00 mm) sieve. Fine soil

particles are dispersed by soaking the soil sample in a dispersing agent and by rapid stirring to

neutralize the charges between the soil particles. The test uses a Type 152H hydrometer

calibrated to give the mass of solids with specific gravity equal to 2.65 in suspension and the

settling velocity of the dispersed soil particles. The soil grain diameter D (mm) is computed by

application of Stokes’ Law, a theoretical equation for the terminal settling velocity of spheres in

a fluid, according to the following equations.

(1)

0 3

(2)

(

)

980

where

v = settling velocity of the fluid, water plus dispersing agent

= specific gravity of the soil solids

= specific gravity of the water and dispersing agent solution

h = absolute viscosity of the suspending fluid which depends on the temperature

Since the soil particles are particles are not spheres, the hydrometer analysis gives the diameters

of spheres that would be settling at the same velocity as the soil particles. This approximation to

the particle size distribution is useful for such engineering purposes as estimating the percentage

of fine grained particles and the percentage of clay sized particles.

Apparatus

Balance capable of measuring to 0.1 g

Evaporating dish

Mechanical mixer and mixing cup to assist in dispersing the soil

Type 152H hydrometer which reads grams of soil per 1000 ml. of slurry

2 - 1000 ml. cylinders, 1 - 1000 ml. glass sedimentation cylinder

Rubber stopper

Thermometer

ASTM D 422 – 1963 (Reapproved 1998)

Hydrometer Analysis – 1

Grain Size Distribution - Hydrometer Analysis