Pile Design and Construction Practice, Fifth edition

At the time of a major flood more than one-half of the shaft friction available from the soil
below river bed level under dry conditions could be lost due to scour. Furthermore, the fric-
tional resistance in the upper part of the piles could be reduced as a result of relief of over-
burden pressure (see Section 4.3.6). These conditions could not be reproduced at the site of the
pre-construction trial piling, nor could loading tests to failure be contemplated on piles with
such large diameters. Accordingly, the tests were made on 762 mm tubular piles instrumented
to measure the distribution of shaft resistance during driving and test loading. The driving test
measurements were analysed by the CAPWAP method (see Section 7.3) to confirm that the
hammer selected to drive the piles was adequate for the purpose. This was a Menck 1700T
hydraulic hammer with a 102 tonne ram delivering 1700 kJ of energy per blow. The damping
constants and other characteristics obtained from the driving tests were used to correlate the
dynamic measurements made at the time of driving the permanent piles. The results of the
measurements of shaft friction resistance on the trial piles are discussed in Section 4.3.7.
On completion of driving the permanent piles the sand within the shafts was cleaned-out
by reverse-circulation drilling to within 3 m of the toe. A grid of tubes-à-manchette was
placed on the levelled sand surface, and the pile was filled with concrete followed by
grouting with cement through the tubes at a pressure of 50 bar.
Scour protection at the main piers is a major feature of the Sutong Bridge(9.33)where the steel
casings for the piles are exposed above river bed level. The initial inner protection zone, extend-
ing 20 m around the piles, comprises sand-filled geotextile bags (1.61.60.6 m) dumped

Miscellaneous piling problems 467

Figure 9.24Two-pile bent supporting intermediate piers of Jamuna River Bridge, Bangladesh

(after Tappin et al.(4.33, 4.34)).

Min. water level +6 m

8

1

Min. bed level–30 m

Pile length 83

m

Pile toe level–72 m

Max. water level +16 m

Cast-in-situ

concrete pier

Precast concrete

pile cap

3.15 m OD

tubular

steel piles in-situ

concrete filled