Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 230

secondary task is often considered of little importance [129]. The temporary support layer
typically consists of steel profiles, rock bolts and shotcrete; nonetheless, larger overbreak
occurs in all type of rock. The overbreak can have a magnitude of metres and is seldom
completely filled and thus often provide an inadequate waterproofing support .
There can also be considerable local deviations in the geometry of the temporary support,
redistribution of the ground pressure, shrinkage of the inner concrete construction. How-
ever it also has to be considered that a perfectly flat waterproofing support would often be
impossibly expensive.
The waterproofing support must be constructed so that its surface complies with the re-
quirements of the following process regarding geometry and its material properties for
fixing. Regarding the requirements to be met for geometrical imperfections of the surface,
two cases have to be differentiated [193]:
a) Areas, which are designed with a completely or nearly constant cross-sectional shape
with a slight curve of the waterproofing in one direction.
In such areas, the laying and joining of the waterproofing membrane sheets can nor-
mally be carried out without any special measures as long as the following require-
ments are complied with (Fig. 5-27).
- Adjacent high and low points of the waterproofing support should not differ by more
than 20 cm in their distance from the tunnel centreline.
- The deviation a measured in this way should not be more than 10 % of the relevant
spacing l of the adjacent high and low points.
- The radius of curvature r of the surface of the waterproofing support should not be
less than 5 a .
Figure 5-27 Permissible deviations
of the waterproofing support [193].
b) All cases, which do not fit the description in a), especially niches and changes of sec-
tion.
In these areas, the standard method of laying the membrane with deviations being sim-
ply passed over is not permissible. The membrane has to be adapted exactly to the
unevenness of the support and its final location by appropriate cutting to shape, fixing
and welding.
The criteria under a) for geometrical imperfections of the waterproofing support do not
have to be complied with on a large scale where ridges and valleys are inevitable due to
planned niches and changes of cross-section.
Ridges and valleys in the waterproofing support should be rounded as far as possible,
with a minimum radius of 20 cm (Fig. 5-28).