Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 209

Figure 5-16 Basic princi-
ples of protecting a tunnel
against damp or water
ingress.

There are altogether three possibilities (Table 5-13). The tunnel can be constructed
- to resist water under pressure without drainage (watertight),
- with pressure-regulated drainage (drained) or
- with free drainage (drained).

Table 5-13 Basic systems.

Free drainage Pressure-regulated drainage Without drainage

Natural groundwater table Natural groundwater table Natural groundwater table

Water table
not lowered

Partially
lowered
water table

Lowered
water table

Water drainage until complete
pressure relief
Drained water quantity cannot
be regulated
Waterproofing against unpres-
surised water
Structurally unlimited scope of
application

Water drainage until selected
limit water level or
water drainage until selected
or permissible water drainage
quantity
Waterproofing against reduced
water pressure
Structurally unlimited scope of
application

No water drainage
Waterproofing against natural
water pressure
Structurally limited scope of
application

The decision between these basic principles has far-reaching consequences for the cross-
sectional shape, the constructional details of the inner lining, the excavated cross-section,
the waterproofing system and possibly also the drainage system and is thus one of the most
important decisions to be taken by a design team. All three variants have their advantages
and disadvantages so there is no simple right or wrong solution.
Construction without drainage has the advantage of no permanent impact on the natural
groundwater regime. The groundwater also plays no role in the operation and maintenance
of a watertight tunnel. The disadvantage of this principle is the higher construction cost
due to the larger excavated sections since an invert arch is always necessary, also thicker