Pile Design and Construction Practice, Fifth edition

the overhanging bows of a ship and the bridge pier or pile if the vessel should ride up the
slope of the island when drifting out of control in a fast-flowing river.
Piles can be strengthenedagainst buckling under direct impact by increasing the wall
thickness and a group of piles can be given lateral restraint by a diaphragm connecting them
at some point between the cap and bed level. The cylinder piles of the Bahrain Causeway
bridge were strengthened by the insertion of precast concrete elements to increase the thickness
over the zone of possible impact (Figure 9.22).
Fender pilesinstalled independently of the piers can be installed in deep-water locations.
Piles are required to protect the sides of the piers as well as the ends in case of impact at an
angle to the axis of the pier. The arrangement of fender piles capped by a massive reinforced
concrete ring beam to protect the piers of the Sungei Perak Bridge(9.36)in Malaysia is shown
in Figure 9.27. The ring beam was constructed by placing precast concrete trough sections
on the piles, sealing the joints between the sections, and placing the reinforcement and
concrete infill in dry conditions. The loading on fender piles is calculated in the same way
as fender piles for berthing structures (see Section 8.1.1).

9.6.3 Pile caps for over-water bridges

It can be advantageous to locate pile caps at or below low river or low tide level. It avoids
floating debris building-up between piles, and ensures that if collision by vessel does occur
the impact will be on a massive part of the substructure instead of directly on a pile. Also a
vessel is likely to sheer off at the first impact with a pile cap whereas it might become
trapped when colliding with a group of piles. A pile cap at or below water level is preferable,
aesthetically, to one exposed at low water. However, high-level pile caps are economical for
a bridge requiring a high navigation clearance, but such an arrangement would have to be
restricted to approach spans in water too shallow to be navigable by vessels which could
demolish piles supporting a high-level deck bridging the navigation channel.
Pile caps partly submerged or wholly below water level can be constructed within sheet
pile cofferdams (Figure 9.28a). The sheet piles can be cut-off at low water to give protection
against scour. Alternatively, if a heavy lifting barge is available a precast concrete cap in the
form of an open-topped box can be lowered on to collars welded to the heads of the piles and
prevented from floating by clamps. The annulus between the pile wall and the opening in the
box can be sealed by quick-setting concrete or by rubber rings. The box is then pumped out
and reinforcement and concrete is placed in dry conditions. The concrete seal is used in tidal

Miscellaneous piling problems 469

Figure 9.26Artificial Islands protecting the piers of the Penang Island Bridge Malaysia (after Chin Fung
Kee and McCabe(9.35)).

Two layers of armour rock

Under layer

Sand

Sea-bed bottom

Channel bottom

Sand fill

Quarry run

1.0 m OD 3 0.13 m wall thickness

prestressed concrete piles

2 – 4 m marine

clay dredged out

1:10

1:1.5

RL – 6.0

MHW RL + 1.08