Environmental Engineering Reference
In-Depth Information
Table 3.1.
Common characteristics of sandstones, arkoses and greywackes.
Minerals
Rock name
Particle shapes, grading
Most grains
Common matrix/cements
Sandstone
Usually rounded, one-size
Quartz, fragments of older
Silica, clay, iron oxides,
grains and less than 15%
rocks
calcite, gypsum
matrix or cement
Arkose
Sub-angular, often well
Quartz plus at least 25%
Clay, iron oxides, silica
graded, little matrix
feldspar; some mica
Greywacke
Angular, well graded down
Felspar, quartz hornblende,
Clay, and same as grains
to clay matrix which is
micas, rock fragments, iron
usually
15% of volume
oxides
- Unstable slopes (shallow, in weathered materials)?
- Unstable slopes (deep-seated, if bedding in folded rocks daylights)?
- Possibility of high pore pressures, in layered sequences?
- Suitability for rockfill, random fill, earthfill and haul roads?
3.6
SANDSTONES AND RELATED SEDIMENTARY ROCKS
The following are the main rock types considered under this heading. They will be
referred to as the sandstone group.
- Sandstones;
- Arkoses;
-Greywackes;
-Siltstones;
- Conglomerates.
Table 3.1 sets out some common characteristics of sandstones, arkoses and greywackes.
Siltstones and conglomerates have a similar range in composition to those of the other
rocks. All except the siltstones occur usually in thick beds. The siltstones may be thickly
or thinly bedded.
Also included under this heading are the lightly metamorphosed equivalents of the
above, e.g. quartzites, metasiltstones and metaconglomerates. If the original depositional
environment of the particular rocks at a site is known, then many generalisations can be
made about their mineral contents, fabrics, bed-thicknesses and sedimentary structures and
about rock types likely to be associated with them. Detailed discussion of these sedimento-
logical aspects is beyond the scope of this topic and only a few features of particular impor-
tance in dam engineering will be described. For more details readers are referred to
Pettijohn (1957), Pettijohn et al. (1972), Selley (1982) and Walker (1984).
3.6.1
Properties of the rock substances
When fresh the sedimentary rocks range from extremely weak, non-durable to very strong
and durable. The strengths and durabilities depend upon the strengths and durabilities of
the grains and of the cements or matrices and these can vary widely depending upon the
environment of deposition and subsequent histories of the rocks.
Quartz-sandstones (and conglomerates in which most grains are quartz) are often
stronger and more durable than arkoses and their conglomerate equivalents, because of
