Pile Design and Construction Practice, Fifth edition

on the river bed, through which the pile casings were driven. On completion of piling,
protection was provided by layers of quarry-run filter and 1 m of rock armour with a density
of 2.65 t/m^3. The outer zone, 20 m around the inner, consists of a layer of sandbags topped by
a filter layer and 1 m rock armour. A ‘falling apron’, in which the material in the apron is
intended to fall down a scoured slope to produce a stable profile, forms the next variable width
zone, set at 1.5 times the expected scour depth, and comprises quarry-run stone overlain by
armour with a D 50 of 0.4 to 0.6 m (Figure 9.25). Dumping of the materials was monitored by
echo sounders.
Impact by shipscan be a severe problem in the design of bridge support piles in situations
where impact cannot be absorbed by massive structures such as caissons or piers constructed
inside cofferdams. It is difficult to achieve an economical solution to the problem particu-
larly at deep-water locations. The incidence of random collisions between ships straying
from the navigable channel and bridge piers has not decreased since the introduction of
shipborne radar. In fact, it may have increased because of the false sense of security given
by such equipment.
Three possible methods of protecting piled foundations may be considered. In shallow
water not subject to major bed changes and with a small range between high and low water
the pile group can be surrounded by an artificial islandprotected against erosion by rockfill.
Figure 9.26 shows a cross-section of one of four islands protecting the piers of the Penang
Island Bridge(9.35). The Muroran Bay Bridge in Hokkaido features a 67 m diameter man-
made island formed by placing self-setting fly-ash slurry under water on the soft sea bed
within a cofferdam. These forms of protection have the added advantage of preventing local
scour around the foundations. The island must be large enough to prevent impact between

468 Miscellaneous piling problems

Figure 9.25Sutong Bridge, scour protection at main pylons (after Bittern et al.(9.33)). Copyright Deep
Foundations Institute 2005.

Assumed scour slope

1 in 2

• 43 m


• 15 m river bed

0.0 River level

42 m 40 m 20 m

Permanent protection:

2 layers of 0.5 m quarry stone

0.07 m quarry run

Temporary protection during piling:

3 layers of 0.8 m filled geotextile bags

2 layers of 0.4 m quarry stones

0.75 m quarry run

1 layer of 0.8 m filled geotextile bags

FALLING APRON:

2 – 3 layers 0.6 m quarry stones

0.75 m quarry run

20 m 40 m 24 m

113.8 m × 48.1 m pile cap

over pile group

28

m

Piles exposed

below cap

131No 2.8 m diameter piles in group

• 121 m or –124 m