Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 218

Construction
Requirements. Any crack formation impairs the water-impermeability. The construction
must be designed to exclude any danger of crack formation due to settlement, movement
of the structure or static or dynamic loading. Waterproof in-situ concrete is often provided
with a waterproofing protection layer because any leaks can then be repaired more easily.
When it is used as a final lining without an additional waterproofing layer, DB guideline
853 requires the following:
- The water head may not exceed 30 m.
- Crack formation due to structural loading can be securely prevented to more than the
usual degree in a verification of crack width limitation.
- The surrounding ground and groundwater are free of content that is aggressive to con-
crete, or the degree of attack is at the worst “weakly attacking” according to DIN 4030.
- The type of construction delivers economic or other advantages over the use of a sepa-
rate waterproofing layer.
An additional waterproofing layer of appropriate waterproofing material should be provid-
ed for tunnel sections susceptible to frost penetration, for tunnels with special equipment,
for external walls with greatly different stiffnesses (like corners, box-outs, weak points,
strengthening) and for tunnels beneath roads with shallow cover. Waterproof concrete
without additional waterproofing may only be used under higher water pressures than
30 m head when a special exception has been issued.
Structural design. The forces and moments acting on a section are determined for water-
proof concrete as for normal concrete according to DIN 1045. Since the occurrence of
cracks endanger the serviceability and endanger the structural safety in the long term due
to increased risk of corrosion of the reinforcement, tension stresses in the concrete are to
be reduced to less than the tension strength. The crack width is limited through an appro-
priate selection of the degree of reinforcement, steel stress and bar diameter. Additional
verifications are to be preformed for the reduction of crack formation or the limitation of
crack width according to DIN 1045, Section 17.6.2.
Ground. If the geological investigation states that larger settlements are to be expected
and cannot be contained, then a structure with a waterproof layer is more appropriate.
This also applies to areas of mining settlement, for large cross-sections and for uneven
temporary support as connection surfaces.
Concrete sections, block joints. The cross-sections should have a simple, clear shape. In
order to avoid the formation of shrinkage cracks, block lengths less than 8 m should be
selected, in other words half the length of block lengths when waterproofing is provided.
Sections should by thick enough to enable successful placing and compaction of the con-
crete, normally more than 30 cm. Special low-shrinkage cements are often used in tunnel
construction. In order to ensure unrestricted shrinkage of the inner lining, large irregulari-
ties in the outer layer are to be levelled and when the surface is rough, an appropriate sepa-
rating layer should be provided. Measures known to have been used include limewash,
bitumen paint, foils, fleece and mortar levelling layers.
Working joints are unfortunately unavoidable. Joint surfaces should be prepared according
to specification before concreting. Steel waterproofing strips or waterstops should be cast
into the joints. Grouting hoses are necessary to enable subsequent grouting in the crown.