Civil Engineering Reference
In-Depth Information
undertaking analytical modeling that these were not significant to the tunnel,
but the rock protection over the tunnel where the tunnel was above the exist-
ing sea bed level needed to be of a more stable material than conventional
stone, and concrete armor units were used over the three tunnel elements
that were affected. The susceptibility to uplift was a function of the poros-
ity of the backfill and protection over the tunnel. This serves to remind the
designer that the local sea conditions must be fully understood and assessed
to ensure all possible loads on the tunnel structure can be accommodated.
Ice loads
Ice loading is a phenomenon that not many tunnels will experience. The
need to consider this will come from researching historical weather records.
If the watercourse beneath which the tunnel passes is prone to icing over,
then further research may be necessary to understand if there is a risk of ice
accumulation at the shoreline.
In marine conditions where the sea may freeze, the effect of the swell
means ice forms in pads on the surface of the water. These pads move inde-
pendently and may slide over one another with the motion of the swell and
in the same way as waves break to the shore, the ice gradually accumulates
at the shoreline. This can result in some considerable depth of ice, perhaps
several meters in extreme conditions, and the loading to the tunnel at the
shoreline can be significant. The shoreline is usually the point of highest
overburden to the tunnel, so this is an important maximum load condition
to consider, both in terms of structural capacity to carry the additional load
and for the short-term settlement to the tunnel.
The Øresund Tunnel considered the likelihood of ice buildup at the fore-
shores as freezing of the Øresund Strait has occurred many times and ice
at the shorelines is common. A height of 12 m of ice was considered the
additional loading on the flood protection bunds above the cut-and-cover
approach tunnels. A method of analysis was suggested that could be used
in lieu of the 12 m requirement that was proposed by F. T. Christensen in
the
Journal of Coastal Research
, Volume 10, Number 3, Summer 1994.
The ice pile-up was considered to apply a distance of 30 m seaward off the
shoreline and 24 m landward off the shoreline. The density of the ice pile
was taken as the volume multiplied by the specific weight of ice (9 kN/m
3
)
and factored by 0.7. This shows that in regions where this risk of ice build-
up exists, the loadings can be quite significant.
Accidental loads
Flooding
The tunnel should be assessed for the extreme event of it being flooded.
This is not so uncommon and many tunnels have suffered a degree of
