Environmental Engineering Reference
In-Depth Information
Sulphuric acid formed by oxidation of the sulphides may attack the carbonate rock and
form calcium or magnesium sulphates, which could become deposited in adjacent filter zones.
Carbonate rocks commonly occur in association with mudrocks containing sulphide
minerals and carbonate rocks themselves often contain sulphide minerals. In Australia
sulphide minerals can occur in all rock types and have been found at the sites of many
dams. Quite small concentrations (much less than 1%) of these minerals can have delete-
rious effects.
The authors believe that it is best to avoid using carbonate rocks (of any age) as
embankment materials, if sulphide minerals are present in any of the above ways, as the
solution products could clog filters. In any rock or at any site, it can be difficult to deter-
mine the amount of sulphides present and their distribution.
3.7.9
Suitability of carbonate rocks for concrete and pavement materials
Dense carbonate rocks in the strong to extremely strong range have been used extensively
for the production of aggregates for concrete, bituminous concrete and pavements.
However, a few dolomitic rocks have been found to react with alkalis in Portland cement
causing expansion and cracking effects like those from alkali-silica reaction. Experiences
at sites where this has occurred are described by Luke (1963), Highway Research Board
(1964) and Huganberg (1987). Guillott (1975, 1986) describes petrographic work on
reactive carbonate rocks and concludes that only very fine grained dolomitic rocks con-
taining some clay are likely to cause expansion.
Some carbonate rocks contain nodules or beds of chert (extremely fine grained or glassy
silica). Alkali-silica reaction is likely to occur if these rocks are used as aggregate with high
alkali cement.
The authors recommend that all carbonate rocks intended for use in concrete be
assessed for reactivity.
3.7.10
Stability of slopes underlain by carbonate rocks
Natural landsliding is not common in areas underlain by pure carbonate rocks. In weath-
ered, solution affected carbonate rocks it is common to find that joints, faults and bedding
partings have been partly or wholly “healed” by redeposited calcite. This along with their
inherently high frictional strength of joints seems likely to be the reason for the low fre-
quency of landslides.
In the experience of the authors most slides in carbonate rocks have occurred along
interbeds of mudstone or shale.
Figures 3.34
to
3.36
show a landslide of approximately
2500 million m
3
in folded limestone of Tertiary Age in Papua New Guinea. The slide is
believed to have occurred into an abandoned valley of the Mubi River, due to daylighting
of a thin mudstone bed within the limestone (Figure 3.36). The slide was probably trig-
gered by earthquakes associated with fault movements, which displaced the Mubi River
about 400 m laterally, and by continued uplift and tilting of the limestone to the south of
the fault.
The Bairaman landslide, also in Papua New Guinea, occurred in a 200 m thick hori-
zontal bed of limestone (King et al., 1987) and its horizontal basal failure surface was
semicircular in plan, covering about 1 km
2
. Its basal surface must have been a mudstone
bed.
In detailed studies of carbonate rocks in the Vaiont landslide Hendron and Patton
(1985) have shown that clay-rich units and clay are present along and near most of the
failure surface.
James (1983) draws attention to situations in Sri Lanka where solution of near-horizontal
limestone beds near valley floors has caused collapse, undercutting and landsliding in
