Civil Engineering Reference
In-Depth Information
that flexural crack widths of 0.2 mm are used for the design of all external
faces of the perimeter walls and slabs of the tunnel cross section. This has
proved satisfactory over the years, although the tendency of designers and
specifiers is to lower this criteria to 0.15 mm. This is somewhat conservative
as 0.2 mm has proved satisfactory. Internal faces are subject to less severe
conditions and the criteria could be relaxed. However, it should be noted that
the presence of deposits within the tunnel that result from exhaust fumes and
road salt brought into the tunnel by traffic create a highly corrosive environ-
ment and severe conditions should also be assumed. Specifications vary from
project to project, but a good guideline would be as follows:
• Internal faces of perimeter structure: 0.25 mm
• Faces of internal walls: 0.25 mm
• External faces of perimeter structure: 0.2 mm
These flexural crack width limits should be used whether the tunnel relies
on watertight concrete or has a membrane applied or even if the concrete is
all internal, as with a steel shell tunnel.
EARLY AGE CRACK CONTROL
The watertightness of a concrete tunnel will be compromised if the cracking
in the concrete penetrates through the full thickness of the walls or slabs
in the external perimeter of the tunnel structure. The risk of this occurring
should therefore be minimized such that cracks can be reliably eliminated.
Causes of cracking
If a concrete member is placed fully in tension across its complete thickness,
the resulting cracking, known as “through-section cracking,” would give
rise to leakage. Tension in the structure can be caused by
• Restraint to movement of the completed tunnel
• Longitudinal loading from seismic events
• Loading causing longitudinal bending moments in the tunnel
• Restraint to movement between concrete pours during the construc-
tion stage
The first three of these causes are dealt with by structural analysis and
design. This section deals with the last of these items—the restraint to
movement between concrete pours—which happens in the early age of the
structure while the concrete is curing. The others are in-service conditions
that must be dealt with in the structural design.
