Environmental Engineering Reference
In-Depth Information
TABLE 5.13
Characteristics of some Metamorphic Rocks with Foliate Fabric
Rock
Characteristics
Plate
Gneiss
Coarse-grained; imperfect foliation resulting; from
5.20
banding of different minerals (see
Figure 5.5)
.
The
foliation causes lenticular planes of weakness resulting in
5.21
slabbing in excavations (see
Figure 6.46).
Chief minerals
are quartz, and feldspar, but various percentages of
other minerals (mica, amphibole, and other
ferromagnesians) are common. The identification of
gneiss includes its dominant
accessory
mineral such as
hornblende
gneiss (see Plate 5.20), biotite gneiss (see
Plate 5.21) or general composition, i.e., granite gneiss (see
Figure 2.85)
Paragneiss
Derived from sedimentary rocks
Orthogneiss
Derived from feldspathic igneous rocks
Schist
Fine-grained, well-developed foliation, resulting from
5.22
the parallel arrangement of platy minerals (termed schistosity).
The important platy minerals are muscovite,
chlorite, and talc. Schist
is identified
by the primary
mineral as mica schist (see Plate 5.22), chlorite schist, etc.
Garnet is a common accessory mineral to mica schist and
represents intense metamorphism. Schists and gneisses
commonly grade into each other and a clear distinction
between them is often not possible
Amphibolite
Consist largely of amphibole and show more or less
5.23
schistose form of foliation. Composed of darker minerals
and in addition to hornblende, can contain quartz,
plagioclase feldspar, and mica. They are hard and have
densities ranging from 3.0 to 3.4. Association with
gneisses and schists is common in which they form
layers and masses that are often more resistant to erosion
than the surrounding rocks
Phyllite
Soft, with a satin-like luster and extremely fine
5.24
schistosity. Composed chiefly of chlorite. Very unstable
in cut slopes. Grades to schists as the coarseness increases
(see
Figure 5.6)
Slate
Extremely fine-grained, exhibiting remarkable planar
5.25
cleavage (see
Figure 6.35).
Generally hard plates split
from formations: once used for roofing materials
between uniaxial compressive strength and deformation modulus for various rock types
are given in
Figure 3.92.
Modulus of deformation reduction factor (Beta) vs. RQD is given in
Figure 3.75.
5.2.7
Rock-Mass Description and Classification
Importance
Systems that provide an accurate description and classification of rock mass are necessary
as a basis for the formulation of judgments regarding the response to engineering prob-
lems including:
Excavation difficulties
●
Stability of slopes and open and closed excavations
●
Capacity to sustain loads
●
Capacity to transmit water
●
