Pile Design and Construction Practice, Fifth edition

on shore as a single buoyant unit lowered on to the sea bed followed by driving piles through
peripheral skirts in a manner similar to the piled foundations of offshore drilling platforms.
Alternatively, the piles can be driven in the form of a raft with their heads projecting above
a rock blanket or geotextile mattress. A prefabricated pier unit is then lowered over the pile
group and the connection between the two formed by underwater concrete. The availability
of heavy-lift cranes on barges or jack-up platforms favours this type of design.
Prefabricated piers were used for 15 of the piers carrying the 3.3 km-long bridges between
the islands of Sjaelland and Falster in Denmark.(9.37)A group of forty-nine 700 mm tubular
steel piles in two concentric rings supported the deep-water piers in the navigable channel. The
first operation was to form a level bed by dredging with protection against scour by rockfill.
Then the piles were driven leaving their heads projecting about 6 m above the prepared
bed (Figure 9.29). During these operations a precast reinforced concrete conical base unit
weighing 440 tonne was being fabricated on shore. It was taken out to the pier site and low-
ered over the projecting pile heads on to three pinning piles. Concrete was then pumped into
place under water to about mid-height of the base unit. The next operation was to lower a
temporary circular steel cofferdam on to the top of the base unit to which it was locked by
stressed rods. The cofferdam consisted of an assembly of steel plate rings 10 to 11 m in diam-
eter and 3 m deep. The joints between the rings were sealed by rubber sheeting. This was fol-
lowed by dewatering the cofferdam and constructing the pier, after which the cofferdam was
flooded and lifted off the base unit by floating crane for transport to the next pier location.

Miscellaneous piling problems 471

Figure 9.29Construction of deep-water piers for the Sjaelland–Faro–Falster Bridge, Denmark (after
Levesque(9.37)).

High water

3.0 m deep rings

Pier constructed

in cofferdam

Temporary steel

plate cofferdam

Transition section

0.5 m

7.0 m

1 in 3.31 in 5

Dredged sea bed

700 mm OD 9.5 mm

WT steel tubular piles

Rockfill

Permanent RC

base unit

Concrete placed

under water

10 – 11 m dia.

18.0 m dia.