Engineering Geology

E n g i n e e r i n g G e o l o g y
Slates are derived from argillaceous rocks that, because they were involved in major earth
movements, were metamorphosed. They are characterized by their cleavage, which allows
the rock to break into thin slabs. Some slates, however, may possess a grain that runs at an
angle to the cleavage planes and may tend to fracture along it. Thus, in slate used for
roofing purposes, the grain should run along its length. Welsh slates are differently coloured;
they may be grey, blue, purple, red or mottled. The green coloured slates of the Lake District,
England, are obtained from the Borrowdale Volcanic Series and are, in fact, cleaved tuffs.
They are somewhat coarser grained than Welsh slates but more attractive. As noted earlier,
the colour of slate varies. Red slates contain more than twice as much ferric as ferrous oxide.
A slate may be greenish coloured if the reverse is the case. Manganese is responsible for the
purplish colour of some slates. Blue and grey slates contain little ferric oxide.
The specific gravity of a slate is about 2.7 to 2.9, with an approximate density of 2.59 Mg m -3 .
The maximum permissible water absorption of a slate is 0.37%. Calcium carbonate may be
present in some slates of inferior quality that may result in them flaking and eventually
crumbling upon weathering. Accordingly, a sulphuric acid test is used to test their quality.
Top quality slates, which can be used under moderate to severe atmospheric pollution
conditions, reveal no signs of flaking, lamination or swelling after the test.
There is a large amount of wastage when explosives are used to quarry slate. Accordingly,
they are sometimes quarried by using a wire saw. The slate, once won, is sawn into blocks,
and then into slabs about 75 mm thick. These slabs are split into slate tiles by hand. Riven
facing stones are also produced in the same way (Fig. 6.4).
Today, an increasingly frequent method of using stone is as relatively thin slabs, applied as
a facing to a building to enhance its appearance. Facing stone also provides a protective covering.
Various thicknesses are used, from 20 mm in the case of granite, marble and slate in
certain positions at ground-floor level, up to 40 mm at first-floor level or above. If granite or
syenite is used as a facing stone, then it should not be overdried, but should retain some
quarry sap, otherwise it becomes too tough and hard to fabricate. As far as limestones and
sandstones are concerned, the slabs are somewhat thicker, that is, varying between 50 and
100 mm. Because of their comparative thinness, facing stones should not be too rigidly fixed;
otherwise, differential expansion, due to changing temperatures, can produce cracking
(Smith, 1999).
When fissile stones are used as facing stone and are given a riven or honed finish, they are
extremely attractive. Facing stones usually have a polished finish, then they are even more
attractive, the polished finish enhancing the textural features of the stone. Polishing is
accomplished by carborundum-impregnated discs that rotate over the surface of the stone,
successively finer discs being used to produce the final finish (Fig. 6.5). The discs are cooled
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