Environmental Engineering Reference
In-Depth Information
Laboratory tests aimed at distinguishing between durable and slaking rocks include
the ISRM Slake Durability Test (ISRM, 1979) and the Modified Jar Slake Index Test
(Czerewko & Cripps, 2001).
2.9.2
Crystal growth in pores
Where rock is exposed in saline environments, e.g. as rockfill or riprap on marine break-
waters and some tailings dams, some salt is absorbed by the rock either by periodic inun-
dation or from spray. During warm, dry conditions the rock dries out and salt crystals
grow in pores near its surface, disrupting mineral grains (as in the Sodium Sulphate
Soundness Test). Periodic wetting up with rainwater causes further disruption. It appears that
fresh water is drawn rapidly into the pores by osmotic and capillary suction, compressing
entrained air. The amount and rate of degradation which occurs by the above process
depends largely upon the porosity, texture and strength of the rock.
2.9.3
Expansion of secondary minerals
Some rocks appear fresh and strong but in fact have been chemically altered (see
Section
they can cause the rocks to be significantly weakened or even to disintegrate, on inunda-
tion, or on exposure to the weather. The rocks most commonly affected are basic igneous
rocks (basalt, dolerite and gabbro), and the most common expansive secondary mineral is
montmorillonite (see Chapter 3, Section 3.2.3).
2.9.4
Oxidation of sulphide minerals
Metallic sulphide minerals (e.g. pyrite) occur in small amounts in many rock types of all
ages. They can occur in many ways, the most common being:
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grains from microscopic to a few millimetres across, scattered through the rock;
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grains as above, concentrated within particular beds;
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veins of any size, and
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coatings on joints or other defects.
In outcrops and in weathered rock, sulphide minerals are invariably absent, having
been oxidised to form limonite or similar minerals. Due to their lack of surface exposure,
small grain sizes, and often complex distribution, the sulphide mineral content in rock is
usually difficult to quantify, and sometimes not recognised.
When exposed to oxygen and moisture, e.g. in excavations, or in rock processed for
rockfill, filters or concrete aggregates, sulphides can oxidise rapidly and release sulphuric
acid and metallic sulphates. Bacteria assist in the oxidation. The acid can attack other
minerals in the rocks, weakening or dissolving them and producing other sulphates which
can cause swelling, heave and sulphate attack on rock or concrete (see also Section 3.5.1,
Engineering properties of mudrocks).
The effects listed above can render sulphide-bearing rocks unsuitable for rockfill, filters
or concrete aggregates. Also, acid and metallic salts can be released into the environment.
Taylor (1998) describes serious problems of this kind, referred to in the mining industry
as “acid drainage”. This has occurred in many places due to oxidation in waste rock
dumps and tailings from mining operations. Abandoned mine excavations which exposed
sulphide minerals have also caused acid drainage problems.
The severe effects described by Taylor (1998) have come mainly from materials with high
contents of sulphide minerals. However the following examples indicate that oxidation of
