Civil Engineering Reference
In-Depth Information
separated by a thin fatty
layer which makes extrac-
tion very simple. In the tra-
ditional method, splitting is
carried out directly on the
exploded shelf within the
quarry. In industrial extrac-
tion larger pieces are split
with a hydraulic hammer
and then transported for
further splitting.
The secondary working of
slate is usually carried out
close to its place of extrac-
tion. Even at this stage, each
slate has its own characteris-
tics and requires its own
particular working meth-
ods. Slate is typical of a
material that requires manu-
al labour; machines are not
very useful for processing it.
Generally slates should be no thinner than 6 mm, although this varies with
type. Thin slates are easily broken during transport. Once laid on either a floor
or a roof, slates will not support high impacts.
If slate is knocked along its natural line, straight or curved, the structure of the
stone is crushed to a certain depth inwards, and the stone divides itself. Pouring
water over the slate makes the job even easier. This principle was used in manu-
al splitting with a hammer to produce slates. During one working day a crafts-
man could produce 60 to 80 slates. With the introduction of slate 'scissors' (see
Figure 7.4) which dominated production at the turn of the century, the number
went up to 400 slates a day. A small wooden block is used to position the notch-
es for the fixing nails, which are knocked out with a pick hammer or cut out with
an angle grinder. The working bench is a trestle with slate lying on it. It is possi-
ble to knock two slates at the same time.
Figure 7.4: Slate 'scissors'. One piece at a time is cut from the edge inwards
to the predetermined point. Source: Stenkontoret 1983
Crushed stone or stone block
Crushed stone is the only stone used today in foundations and structural work,
either as aggregate in concrete or as levelling or loose fill under concrete foun-
dations. In his essay 'Stone Technology and Resource Development' (Shadmon,
