Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 404

The most common petrographic investigations for engineering purposes are carried out
on 25 to 30 μm thin sections prepared from rock samples to give substrates transparent
for light, electrons and X-rays. Examination of thin sections under a microscope with
magnifi cations ranging from 5 to 1000, using polarized light, provides a qualitative and
to some extent also quantitative determination of the mineral content and allows state-
ments on the rock's structure. To obtain three-dimensional images of surface features
a scanning electron microscope can be used that enables magnifi cations up to 20,000.
To determine the content of clay minerals in fi ne-grained rocks such as shale or mud-
stone requires the application of X-ray diffraction techniques. Ultra-thin sections of 2
to 12 m are investigated by these methods. By means of X-ray diffraction, the type and

spacing of the crystal lattice can be identifi ed, which that are characteristic properties
of minerals.

μ

For a detailed description of these examination methods the reader is referred to the
corresponding suggested methods of the ISRM (ISRM 1978a) and related literature.

14.3 Water Content, Density, Porosity and Related Properties

Interrelations of physical properties
Physical properties of an intact rock sample are defi ned in terms of its constituents,
which are interrelated so that any property can be calculated if others are known. Fol-
lowing the corresponding suggested methods of the ISRM (ISRM 1979b) the determi-
nation of only three basic properties will be considered, namely water content w, dry
density ρ d and porosity n defi ned as

(14.1)

(14.2)

(6.1)

where m W and m S are the pore-water mass and the grain mass of the sample, and V P
and V are its pore volume and bulk volume, respectively.
Related properties that are often used and can be calculated if w, ρ d and n are known

are the bulk density ρ , the grain or particle density ρ s , the unit weight γ , the water por-

osity n w and the degree of saturation S r :

(14.3)