246 Excavation principles
at the kerfed free face as a tensile stress wave, resulting in complete frag-
mentation (Fig. 15.7(b)). This is the ideal form of the free face for the
excavation geometry. Although this system has been used in chalk in the
United States, it is not generally practical because of the difficulty of cutting
the kerf and also because of the ’bulking factor’. Broken rock occupies a
significantly greater volume than intact rock, and hence allowance must
be made for dilation.
Another way of generating the free face would be to drill a large diameter
‘relieving‘ borehole at the centre of the face, and leave it uncharged. As
shown in Fig. 15.8, the geometry of the free face could then spiral out from
this initial small free face as successive blastholes are initiated, using delay
detonators. With reference to Fig. 15.8, blasthole 1 is detonated with the
initial uncharged borehole being its free face, and the process of frag-
mentation occurs as shown in Fig. 15.5, but on a smaller scale. This creates
a larger free face, at a different orientation, which can be used by blasthole
- The same process continues with blastholes 3, 4 and 5, demonstrating
the important concept of progressively increasing the free face by the use
of successive detonations.
From these fundamental concepts it is clear that part of the practical
optimization of blasting rounds should include strict control of the drilling,
to ensure correct geometry of the blastholes, and the use of precision delay
detonators, to ensure the correct initiation sequence.
The face of a tunnel is a free face, but it is not parallel to any practically
obtainable blasthole orientations because of drilling constraints: note from
Fig. 15.5 that the free face should be parallel, and not perpendicular, to the
blasthole.
However, as illustrated in Fig. 15.9, one can compromise by having a
sequence of inclined boreholes which are successively detonated. This is
known as the wedge cut, detonated in concentric cones from the centre of
the face to the periphery, gradually increasing the area and changing the
orientation of the free face as detonation proceeds. Note again, that the use
of delay detonators is critical: the fragmentation process would not operate
if the blastholes were detonated simultaneously.
(^5) Initial unchareed
3
Figure 15.8 Practical application of the free-face concept using one form of the
burn cut.