Civil Engineering Reference
In-Depth Information
Figure 4.12
Schematic showing a typical arrangement of dowels used in the
tunnel face (after ITA/AITES 2007)
and potentially dangerous, ends when excavated due to the brittle manner
in which they break. Therefore, steel dowels can prove more useful even
though the excavation process needs more care. Steel dowels can be easily
cut using handheld cutters. Ideally, the dowels should provide a continuous
stability as the excavation advances. This is achieved by the dowels being
shortened as the face advances until a minimum length is reached. At this
point a new set of dowels is installed in the face. These new dowels overlap
with the previous dowels by a few metres.
Figure 4.13 shows an example of temporary face dowels being used
during the construction of the LT31 Tunnel in Vienna, Austria. In this case
the dowels were 12 m long and overlapped longitudinally by 5 or 6 m.
There was also a plate (12 cm
50 cm) attached to the end of the dowels
that distributed the stress and avoided overstressing the sprayed concrete,
which was relatively thin (approx. 10 cm) compared to the side walls
(approximately 30 to 35 cm thick).
4.2.7 Roof pipe umbrella
In the roof pipe umbrella method steel pipes are drilled from within the
tunnel in the direction of tunnelling around the perimeter of the tunnel
roof, as described in the artificial ground freezing section 4.2.1. The steel
pipes have a diameter of approximately 70 to 150 mm. After drilling, the
holes are filled with grout. The spacing of the pipes ranges from
approximately 20 to 50 cm. The length of the pipes is often 15 m or more.
The previous umbrella overlaps with the subsequent umbrella by at least
3 to 5 m. Roof pipe umbrellas act like forepoling, i.e. they are supposed
to protect the crown area against afterfall. However, due to their larger
diameter and length, roof pipe umbrellas are a lot more robust than
forepoling.
Figure 4.14 shows an example of forepoling (dowels) in association with
sprayed concrete lining being used as a roof pipe umbrella on the LT31
Tunnel, Vienna (see the case study in section 8.3 for further details of this
tunnel).
