Civil Engineering Reference
In-Depth Information
Table 9.2
Stability and buoyancy conditions to be checked in design (
Continued
)
Stage of
construction
Criteria to be achieved
Loading considerations
Sand flow
Minimum load on temporary support
to be maintained
Maximum jack capacity
Light condition:
Minimum structure weight
Maximum water density
Fixed ballast water volume
Sandflow uplift pressure
Heavy condition:
Maximum structure weight
Min water density
Max possible ballast volume
during operation
Ballast
exchange
Minimum FoS against uplift of 1.025 (or
min earth pressure of 1-2 kN/m
2
)
Light condition:
Minimum structure weight
Maximum water density
Reducing ballast water volume
with increasing ballast
concrete—each step in
exchange process to be checked
Permanent
condition
Minimum FoS of 1.06 (or min earth
pressure of 1-2 kN/m
2
)
Minimum FoS:
1.06 (minimum components considered)
1.1 (all fill and permanent parts
considered)
Light condition:
Minimum structure weight
Maximum water density
Final ballast concrete, installed
in stages
onto a gravel bed, after the immersion of the elements for the longer-term
temporary condition and during ballast exchange, when the water ballast
is replaced by concrete ballast. Finally, the permanent condition with all
loadings applied should be considered.
The factor of safety should be calculated for an individual tunnel element
to take account of any specific features within that element, for example,
drainage sumps, which may result in different concrete and air volumes in
the tunnel. For a concrete segmental immersed tunnel in the temporary con-
dition, it is normal to calculate the factor of safety for each individual tunnel
segment and ensure that the minimum factor achieved for a segment is 1.04,
provided that the overall factor for the tunnel element is 1.06. In calculat-
ing the factors of safety, only the weights of the permanent materials should
be used. This will be primarily the self-weight of the tunnel structure, the
weight of the ballast concrete (or ballast water in the temporary condition),
the weight of the backfill, and the weight of the displaced water. No live
loads are considered. Partial load factors of 1.0 should be used. The follow-
ing should be excluded from the factor of safety calculation:
• The effect of friction between the walls of the structure and the backill
• Cladding systems
