Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 197

Complete coverage of pressure relief around the tunnel is only ensured when the pressure-
relieving effect of two adjacent holes overlaps. The holes are connected to the open de-
watering system and normally remain in operation to relieve the shotcrete support layer
until the inner lining has hardened. They should be laid out so that tunnelling operations
are not obstructed.
The most favourable combination of spacing, length and diameter of the holes has to be
discovered experimentally, although economic aspects also have to be considered. It is
certainly cheaper to drill more or longer holes with the available drills than to drill fewer
holes of larger diameter if this entails importing new machinery.
Fig. 5-6 shows the effect on the water pressure distribution of measures like borehole
drainage, grouting and waterproof linings.
a Unaffected condition
b Pressure altered by
drainage boreholes
c Pressure altered by
grouting
d Pressure altered by
grouting and drainage
boreholes
e Pressure altered by wa-
terproof support layer
f 1 Boreholes for contact
grouting
f 2 Boreholes for ground
consolidation and
waterproofing grouting
g General layout of
grouting and pressure-
relief holes in the
Plöcken heading for
the Trieste-Ingolstadt
pipeline
Figure 5-6 Groundwater pressure distributions around tunnels [160].
Drainage boreholes ahead of the advance. (Horizontal drains) are sensible if
- heavy water inflow is expected,
- sudden but not predictable water inflows can be expected (for example when there is
standing water or when large-scale advance drilling in karst conditions is also intended
to detect open joints containing water under pressure),
- advance drainage can be expected to considerably simplify the subsequent excavation
and support works (for example in gravel),
- flow pressures can considerably impair structural stability (quicksand, running ground),
- water lenses may be present and are to be detected,
- high joint water pressures are to be relieved.