GRAIN SIZE ANALYSIS
Why do grain size analysis?
1) Purely descriptive, quantitative measure of sediment
2) Grain size distribution may be characteristic of sediment deposited in certain environments
3) Size distributions may provide data about transport, deposition and diagenesis of the sediment.
4) Size is important in other properties like porosity and permeability

GRAIN SIZE ANALYSIS
We are primarily concerned with 3 things:
1) Measuring techniques and the expression of data in terms of a scale
2) Methods for summarizing and displaying grain size data.
3) Interpretation of the genetic significance of these data
1) We use the Udden – Wentworth Scale, also called the phi (

) scale.
It is a logarithmic scale because small changes are more important at small sizes than at large sizes.

GRAIN SIZE ANALYSIS
1) The Udden – Wentworth Scale phi (

) = - log
2
S

Why the negative log transformation?
1) If N= a x , then X = log a
N and
2) 1/N = a -x , then X = -log a
1/N
3) Since most grains are less than 1 mm we can avoid using negative phi numbers most of the time
4) A log scale makes phi increments more meaningful and useful at large grain sizes.
Example – An arithmetic increase from
1 mm to 2 mm is a doubling but a 1mm increase from 256 to 257 mm is tiny and isn’t useful in describing large particle differences

How does a Settling Tube Work?

The settling velocity of particles falling through a fluid medium depends on the size, shape, and density of the particle, and the density and viscosity of the fluid.

Note that particle shape is important – spherical particles fall faster than nonspherical particles.

This is known as Stokes Law
(
 s
-
 f
)g w = d 2
18

Where: w = settling velocity;

= dynamic viscosity;
 s
= sphere density;
 f
= fluid density; and d = particle diameter

Often Simplified to:
V = CD 2
Where: (
 s
-
 f
)g
= constant
18

Thus, settling velocities can be calculated for any particle of known diameter and density.

Grain Size Analysis of the fine fraction:
Hydrometer Analysis