Environmental Engineering Reference
In-Depth Information
the superior strength and durability of quartz. However sandstones often have significant
porosity (5% to 20%) and may also be slightly permeable.
Greywackes tend to be stronger than sandstones due to the angularity and grading of their
particles.
Silica cement usually occurs in strong, durable rocks and at the other extreme rocks
cemented by clay or gypsum are usually weak and non-durable.
If gypsum or anhydrite is proven or suspected as a cement in a sandstone forming all or
part of the foundation of a dam, its significance needs to be assessed carefully and special
testing may be required, as discussed in Section 3.8.
The metamorphic rocks when fresh are usually stronger and more dense and durable
than the equivalent sedimentary rocks.
3.6.2
Suitability for use as construction materials
Rocks in the sandstone group which lie in the strong to extremely strong range have been
used successfully as rockfill and rip-rap in many dams. They are widely used also as aggre-
gates in concrete.
However, in a few cases concrete containing quartzite or strong sandstone as aggregate
has suffered expansion and cracking due to alkali-silica reaction. The authors recommend
that any silica rich rocks intended for use as concrete aggregate be tested for reactivity (see
The quartzites are often extremely strong and this together with the high content of
quartz (Moh hardness
7) makes them highly abrasive. This can result in high quarrying
and handling costs. Also if the quarry-run rock is not well graded it can be difficult to
compact, as little breakdown occurs under the roller.
The weaker rocks tend to be more porous and usually lose significant strength on
saturation. Mackenzie and McDonald (1981, 1985) describe the use of sandstones and
siltstones, mainly medium strong when dry, as rockfill in the 80 m high Mangrove
Creek Dam in New South Wales. Both rock types lost about 50% of their strength on sat-
uration.
The rock was compacted with up to 5% water by volume. Higher water quantities
caused the material to become unworkable. The fills produced were of high density and
moduli but of generally low permeability. The latter was allowed for by the inclusion of
drainage zones of basalt and high quality siltstone.
At Sugarloaf (Winneke) Dam thinly interbedded siltstone and sandstone were used for
rockfill and random fill in the 85 m high concrete faced embankment (Melbourne and
Metropolitan Board of Works, 1981; Regan, 1980).
The rockfill zone material was fresh or slightly weathered and strong to medium
strong. Compacted with up to 15% water it produced a dense, free-draining fill.
The rock used in the random fill zone was slightly to highly (or distinctly) weathered
and medium strong to weak. Compacted at about 10% moisture content it produced a
dense fill which was not free-draining and contained up to 20% of silt and clay fines
which were dispersive. It was therefore underlain everywhere by a blanket of rockfill and
its outer surfaces were protected by a thin layer of the rockfill.
3.6.3
Weathering products
The main effect of chemical weathering on rocks of this group is weakening due to
removal or decomposition of the cement or matrix. As a result the rocks with silica or iron
oxide cements are the most resistant to weathering. The sandstones and conglomerates
become progressively weaker until at the extremely weathered stage they are usually sands
or gravels. Quartzites and quartz rich sandstones produce relatively clean quartz sands,
