Civil Engineering Reference
In-Depth Information
may be more than one such sump. The sump is generally sized to cater for
a ruptured tanker or for fire hydrants running for one to two hours within
the tunnel and most highway immersed tunnel sumps have a storage capac-
ity of around 50 m
3
, but requirements will vary according to the nature
of the road and the operational philosophy for the tunnel. The challenge
with an immersed tunnel is finding space to locate this sizeable void. It is
undesirable to have protrusions below the bottom of a tunnel element as
this creates difficulty in construction. The base of the casting basin or dry
dock would need to be shaped to cater for any features below the element
and the dredged trench would have to be shaped similarly. If the tunnel
foundation is to be gravel, then this would need to be shaped to support the
tunnel element correctly. This adds complexity and cost to the construction
of the tunnel. The draft of the tunnel when floating will also be greater and
thus the water depth for access to the casting facility and the towing route
need to be increased. To avoid this unnecessary complexity, it is common
to form the sump within the depth of the ballast concrete and structural
base slab. This also has its challenges, but overall, is considered to be the
lower-cost, more manageable approach to design.
In a highway tunnel, the drainage carrier pipes usually are located in the
ballast concrete beneath the road surface. In order to form a low point for
the collection of liquids, the base slab can be locally thinned so that the top
surface steps down. Similarly, the ballast concrete that sits above the struc-
ture base slab is omitted at this location and a thinner walled structure can
be formed to create space for equipment and liquid storage and to support
access covers in the road surface. The same approach can be used in a rail-
way tunnel; however, there is typically less depth of ballast concrete and
drainage may be provided using surface channels. This is because drainage
inflows are likely to be less, which is useful because it means a sump can
still be created in the reduced depth of ballast and structure.
In order not to compromise the watertightness of the structure, it is com-
mon to ensure a minimum thickness of structural slab of 500 mm is pre-
served beneath the sump. Sump chambers tend to be quite long, typically
10-15 m along the length of the tunnel. This length is needed to generate
sufficient storage volume. However, a thin slab may have insufficient trans-
verse bending capacity if it extends over such a long length and it is usually
necessary to stiffen the slab with transverse beams. This has the effect of
dividing the sump chamber into a series of parallel chambers in between
the stiffening beams. These can be interconnected with pipes to create the
needed storage volume. The stiffener beams also serve as the support struc-
ture for the frames for access covers, which will be needed to open up the
sump chamber for periodic inspection and cleaning. A typical drainage
sump arrangement is shown in Figure 9.21.
Durability is of some concern for the reinforced concrete forming the
sump chamber as there is potential for aggressive materials to be stored.
