Civil Engineering Reference
In-Depth Information
bridge. Consequently foundation type should not be determined by soil specialists, but
by the designer, with the benefi t of expert advice.
7.7.2 Pad foundations
Usually, if it is possible to adopt a pad, this will provide the most economical foundation.
An additional advantage is that the general bridge contractor does not need to bring
onto site a specialist foundation sub-contractor. There are times when, for instance,
all the piers except one can be founded on pads at a reasonable depth. It is worth
fi nding ways of avoiding mobilising a piling rig for a few piles under just one pier,
such as using cofferdams or mining techniques. However, if there is no alternative to
piling, once the cost of mobilising a rig has been incurred piles may be found to be
economical at other pier positions.
Pads give rise to stiff foundations that do not move under the effect of horizontal
loads, and have very limited rotation under overturning moments. This rigidity can
affect the articulation, as described later in this chapter.
There is no reason why pads and piles should not be mixed beneath the same deck,
as long as the relative settlements and differential stiffness of the foundations are
properly considered in the design.
7.7.3 Driven piles
Most driven piles used in the UK have a relatively small capacity, generally 1.5 MN or
less. A typical medium span concrete highway bridge weighs, including fi nishes, live
load and substructure, between 0.025 MN/m
2
and 0.035 MN/m
2
. Thus a continuous
bridge 13 m wide with spans of 52.5 m will give rise to pier reactions of some
18 MN, and would require 12 such piles at each pier to carry the vertical loads alone.
However, foundations are subjected not only to vertical loads, but also to longitudinal
bending moments caused by bearing friction and wind forces on the pier stem as well
as transverse moments due to live load eccentricity and wind forces on the deck and on
the pier stem. In a rectangular array of piles subjected to bi-axial bending and vertical
load, only the corner piles will be stressed to the maximum. Consequently, the average
pile load will be signifi cantly less than the rated load, increasing the number of piles
required.
Driven piles in steel or concrete are normally considered in design as pin-ended,
incapable of carrying bending moments. The consequence of this assumption is that
any horizontal loads have to be carried by a system of triangulation, using at least three
rows of piles, in which at least one row is inclined, Figure 7.5. This further increases
the number of piles required, and leads to a larger pile cap which, together with the
earth carried, increases the vertical loads and leads to the need for yet more piles. The
total number of piles required to carry a typical pile cap is likely to be two to three
times the number required for centred vertical loads alone.
Signifi cant savings can be made in number of piles and size of pile cap if it is possible
to use only vertical piles, with the horizontal forces carried in bending. However, there
are complex group effects, which may spread the bending unequally among piles, and
specialist literature should be consulted or expert advice sought. If the bridge is subject
to very high wind loading such as typhoons, to railway loading, to hydraulic loading,
