Civil Engineering Reference
In-Depth Information
Expansion joints that are marketed as waterproof are unlikely to remain so for
long. If surface water may fl ow across the expansion joint, either by design or because
gullies are blocked, it should be assumed that it will penetrate to the abutment shelf,
which should be provided with a drainage system to carry it away without overfl owing
and staining visible surfaces. It is absolutely pointless to provide an abutment drainage
system that is not accessible and maintainable, which requires sizing the abutment to
allow man access to the shelf. For shallow bridge decks, the abutment structure has
to be made deeper than the deck, attracting larger earth pressure forces, signifi cantly
increasing the cost of the bridge. Where decks are about 1.8 m deep or more, the
height for maintenance access already exists, and the additional expense is less.
Clearly, the design of the drainage system depends on the length and width of the
deck, on the longitudinal and transverse falls, and on the speed and intensity of the
traffi c. Drainage design is an important part of the designer's task that can affect the
choice of bridge deck, and should be considered early in the design process. It should
not be forgotten that the greatest risk to traffi c is from the neglect of maintenance
rather than from under-design for exceptionally intense rainfall.
9.7.2 Specifi c cases
a) Drain the water off the bridge
Wherever possible, the water should be allowed to fl ow off the bridge in the gutter.
A generously dimensioned gully and silt trap in the road embankment will be much
cheaper and easier to maintain than bridge deck gullies and pipes. Where the bridge
deck is integral with its abutments, there is no diffi culty in adopting this option. Where
there is a roadway expansion joint at the deck end, the abutment should be designed
for drainage, as described above.
b) Single gully placed just upstream of expansion joint
For bridge decks with expansion joints, another option is to provide a single gully,
generally without a silt trap, on the deck just upstream of the joint and to pipe the
water across the joint with a properly engineered system. Such a gully may be hidden
by incorporating it within the structure of the abutment diaphragm. The water would
then be collected in a manhole with a silt trap in the embankment off the deck. The
pipe expansion joint must be designed to accommodate any vertical movement due
to the elastic response of the bridge bearings or defl ection of the bridge deck, as well
as settlement of the abutment manhole. This latter can be avoided by suspending the
manhole from the back of the abutment structure.
c) Trough behind the kerb
For larger bridge decks or those where the falls make it essential to collect the water at
numerous points along the kerb, the most easily maintainable option is a trough in the
footpath behind the kerb. If the falls are appropriate, this trough can take the water
off the deck. Alternatively, the water may be collected in a gully placed just upstream
of the joint, as in (b). The trough may also be combined with a piped system, with the
water transferring from trough to pipe at regular intervals.
