Page 127 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
122 Geotechnical Engineering

electron microscope. More recent was the development of the scanning electron
microscope, which is a kind of subminiature video camera and is particularly
useful for viewing clay structures.

Both the electron and scanning electron microscope require that samples be in a
vacuum that desiccates the clay and can alter its composition and structure.

The most useful tool for identification of clay minerals is X-ray diffraction, which
does not require a vacuum and can be performed without pretreating the soil.
Even though this tool is not commonly used for routine soil analyses, the student
should know its capabilities. For example, a wet clay can be X-rayed as it is drying
out to determine if it is expansive. Furthermore, the extent of expansion and
contraction can be demonstrated to depend on the nature of the free ions that are
associated with the clay, such that changing the ions can change the expansive
properies. X-ray diffraction demonstrates that there is more to geotechnical
engineering than boring or punching holes in the ground.

6.2 X-RAY DIFFRACTION AND GEOTECHNICAL ENGINEERING

6.2.1 Not Your Usual X-Rays
If you ask a medical technician what is meant by X-ray diffraction, you most
likely will be met with a querulous look and be asked to repeat the question.
Instead of using penetration of X-rays to observe an inner structure, X-ray
diffraction defines relative positions of atoms in minerals or other solid objects.

The strongest diffractions are obtained from crystalline materials that have a
regular array of atoms arranged in planes so that the X-rays in effect bounce off as
if reflected from a mirror. A critical distinction is made from mirror reflections
because X-ray diffraction can occur only at specific angles that depend on
distances between the reflecting planes of atoms. By measuring the angles at which
reflections occur, one can decipher the distances and identify the minerals. It also
is possible to measure exactly how much a clay structure expands when it is wet
with water.

6.2.2 Production of X-Rays
X-rays are generated by an electron beam striking a metal plate that is aptly called
the ‘‘target.’’ The voltage pulling electrons to the target is high enough to knock
electrons out of target atoms, and as orbiting or shell electrons are replaced,
energy is given off as X-rays. The target in an X-ray tube therefore glows with a
brilliant but invisible light. The voltage is of the order of 50,000 volts. Such high
voltages are strictly prohibited in television or computer screens to prevent the
production of X-rays.

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