Civil Engineering Reference
In-Depth Information
The ballast will usually be either mass concrete placed within the tunnel
section or rocks placed in a ballast box on the tunnel roof. Again, a range
of possible densities must be considered to test what flexibility exists in the
design. The ballast does offer some possibility of rectifying the mistakes
made earlier in construction. For example, if the element is built too light,
then there is the possibility of increasing the density of the ballast to achieve
the required factor of safety against uplift. Heavyweight aggregate could be
used in the ballast concrete. Conversely, if the element is too heavy, light-
weight aggregate can be used in the ballast concrete.
For buoyancy analysis, the weight of the finishings that are considered
permanent loadings must be assessed on a case-by-case basis. Only finish-
ings that would not be removed during maintenance should be included.
Therefore, a typical list of permanent loadings would be as follows:
• Structural concrete and structural steelwork
• External waterprooing membrane and any associated protective
concrete
• Reinforcement and other permanently cast-in items
• Ballast concrete
• Any regulating course where used, but not the carriageway wearing
course
The weight of ventilation equipment and other M&E plants is not gener-
ally included in the permanent loadings as it has to be removed and replaced
as part of the maintenance regime. Anything that was to be included would
have to be clearly identified in the maintenance manual to ensure that it was
not removed without taking some compensatory action.
Load from the backfill should be included for structural design and settle-
ment analysis. For the buoyancy analysis, the amount of loading due to the
backfill that should be included in the permanent loadings is often open to
debate. The safest approach, and one taken by many authorities, is that the
frictional resistance of the backfill on the tunnel walls is not included when
considering the buoyancy of the element. However, when considering the set-
tlement of the elements, it is common to include the frictional downdrag of
the backfill on the tunnel walls. Any fill above the tunnel can be considered a
permanent loading, provided it is protected by a rock protective layer so that
it cannot be scoured away. Similarly, it is often admissible to include some,
say 50%, of the rock protection as a permanent load. Whether the weight of
any fill above the rock protection can be considered depends on the particular
circumstances. If the tunnel is particularly deep, then some of this backfill,
say up to 3 m below the finished level of the backfill, could be included. This
would have to be accompanied by a risk analysis investigating the probability
of the backfill that is being considered as permanent remaining in place. For
the structural and settlement analyses, all loads should, of course, be included.
