Civil Engineering Reference
In-Depth Information
Ground vibrations need to be considered when using drill and blast for
the excavation of a tunnel, especially in urban areas, as these can affect
surface and subsurface structures (and humans). It is therefore important
to make predictions of the likely effect (possibly by conducting blasting
trials as part of the site investigation) and to carry out monitoring during
the construction works (New 1990).
When carrying out drill and blast, the order of the processes generally
consists of the following cycle: drilling, charging (including adding the
detonator), stemming, blasting, ventilation, mucking (spoiling) and support-
ing. Each of these components is described in the following sections.
Further details on rock blasting and drill and blast tunnelling can be
found in Hopler (1998), Persson
et al.
(1994), Holmberg (2000 and 2003).
5.6.2 Drilling
In order to create the drill holes, which are needed to take the charges, rubber
tyred drilling carriages are, usually, used as these have relatively high driving
speed and are manoeuvrable, i.e. they need to drive to the face and then return
to a safe distance (Figure 5.34). Nevertheless, tracked drilling carriages are
also an option. From a practical point of view the same drilling carriage
should be used when placing the support anchors. Drill carriages carrying
two to four hydraulically operated booms are commonly used (Figure 5.35).
The speed of drilling is approximately one to five metres per minute. These
booms serve to provide exact positioning of the drill hammer. This is
essential in order to achieve a successful blast, as the exact position of the
boreholes is crucial. A computer navigated drilling vehicle (jumbo) is
therefore used as this makes a fast and exact positioning of the booms
possible, and it also guarantees a uniform depth of drill hole even for a
strongly jointed or uneven face. The drill holes are normally drilled 10%
(commonly 20 cm) longer than the desired advance length in order to ensure
that the advance length is achieved as the rock is not always completely
blasted over the total drill hole length. In very small tunnels, i.e. 4-5 m
2
,
Figure 5.34
Example of a rubber tyred drilling carriage 'jumbo' as used on the
2nd tube of the Katschberg Tunnel, Austria (details are provided in
section 4.3.3)
