platform to extend the shafts to depths of 114 to 117 m using bentonite to maintain hole
stability. Reinforcement cages were inserted and a batching plant rated at 100 m^3 /hour
moored downstream of the platform supplied concrete. Post-grouting of the pile tip was
carried out using methods similar to those shown in Figure 3.39, increasing pile capacity by
20% as indicated by before and after tests.
9.6.2 Imposed loads on piers of over-water bridges
In addition to the loadings listed in Section 9.5.2 the piles of over-water bridges are required
to withstand lateral forces from current drag and wave action, pressure from floating flood
debris or ice, and impact from vessels straying from the designated navigation channels.
Current dragand wave forcescan be calculated using the methods described in Sections
8.1.3 and 8.1.4. The profile of the current velocity with depth varying from a maximum at
the water surface to a minimum at bed level must be considered in relation to the bending
moments on piles in deep fast-flowing rivers. Current-induced oscillation can also be a
problem in these conditions. It is also necessary to calculate the lateral deflections in the
direction of the river flow at pile head level because these can induce bending of the bridge
superstructure in the horizontal plane.
The depth of scourbelow river bed around piles at times of peak flood must be estimated for
the purpose of calculating bending moments due to current drag forces and wave action on
piles. The scour consists of three components: (a) general scour from changes in bed levels
across the width of the channel, (b) formation of troughs in ‘sand waves’which move down-
stream with the passage of the flood and (c) local scour around the piles. Rip-rap, armouring,
cable-tied concrete block mats and grout bag mats are used to protect piers and abutment foun-
dations. Care has to be taken to prevent failure due to ‘winnowing’of sediments between the
mats and blocks, causing uplift and rolling up of the leading edge of the mat if not anchored.
May et al.(9.34)review the causes and effects of, and remedies for, scour around bridge piers.
An extreme example of the influence of bed scour on bridge foundations is given by the
design of the foundations of the multi-purpose bridge over the Jamuna river near Sirajgang
in Bangladesh(4.33,4.34). The bridge provides a dual two-lane roadway, a metre gauge railway,
pylons carrying a power connector and a high-pressure gas pipeline. At the bridge location
the river was 15 km wide. The waterway had a braided configuration with numerous deep
scour channels and shifting sandbanks. In order to limit the overall length of the bridge the
waterway was narrowed by constructing massive armoured training bunds on each bank
which reduced the width to 4.8 km. It was calculated that the result of constriction of flow
would cause the river bed to scour to a depth of 40 to 45 m below bank level at the time of
a 1 in 100 year flood discharging 63 000 m^3 /sec. An additional 1 m of scour was estimated
to occur around the foundation piles.
The bridge structure consists of 52 segmental box girder spans carried on piers, each pier
being supported by a pair of raking piles (Figure 9.24). The 3.15 m OD40/60 mm wall
thickness piles were driven with open ends and have outside diameters of 2.50 and 3.15 m
depending on their location relative to the training bunds. The piles were driven to a depth
of about 70 m below bank level into a loose becoming medium-dense to dense silty medium
to fine sand containing upto 5% of micaceous particles. Support to the piles is provided
partly by shaft friction and partly by base resistance. The maximum load in compression on
a 3.15 m pile was estimated to be 57.1 MN resulting from the bridge loading combined with
current drag forces caused by the 1 in 100 year flood and by earthquake forces. The
maximum lateral load on each pile was calculated to be 1.5 MN.
466 Miscellaneous piling problems