Civil Engineering Reference
In-Depth Information
far, a crushing drill hole, i.e. a drill hole with no explosive, can be placed
in the middle of the wedge.
FAN CUT
The fan cut is also an angular cut. For this arrangement several drill hole
rows are placed in a fan shape. They have different lengths and are generally
positioned against the invert (Figure 5.43a). There are several other basic
forms of angular cuts which are randomly applied in tunnelling (for example
conical cut). Furthermore the basic forms can be combined (for example
a combined wedge and fan cut). Common to all angular cuts is the small
amount of explosive material required and the small number of drilling
metres, which is particularly true for the fan cut. Generally the drilling
works are more complicated for angular cuts as the angular positions of
the drill holes have to be precise and the drill carriage has to be frequently
repositioned. If the drill hammer on a drill carriage cannot be placed at
the desired position or if the advance length does not allow the required
angle due to the cramped conditions in the tunnel, a shorter advance length
has to be accepted.
PARALLEL DRILL HOLE CUT METHOD
The characteristic for this cut is that the drill holes are the same length
and obviously parallel to each other. The positioning and the distance of
the drill holes in the middle are important for a successful detonation result.
They should be arranged symmetrically and mirrored and, depending on
the type of explosive and ground, not lie further apart than approximately
30 to 50 cm (Figure 5.43b). The parallel drill hole method is applicable
for ground conditions of lower hardness and toughness.
Due to significantly simplified drilling works, parallel cut arrangements
are often used in tunnelling. The drill holes can be drilled from one position
using the drill carriage and are the same length for most parallel cut
arrangements. There is no interference between the drill hammers for a
multi-armed drill carriage due to angling. However, it is important that
the drill holes are exactly parallel in order to ensure the success of the
explosion. The drill carriages have to be constructed so that the drill
hammers can be positioned quickly and reliably in parallel. The blasting
effect for the parallel drill hole method relies mainly on a crushing effect.
Therefore there are higher requirements for the blasting material, smaller
pieces of rock are produced and the debris is thrown greater distances. The
last two points can be influenced by a suitable choice of millisecond deton-
ators, and in particular by omitting individual time steps. In order to achieve
an economic detonation effect when using a parallel cut it is necessary to
use millisecond or half-second detonators and the detonation should start
from the deepest end of the drill hole. With small cross sections there is
