Pile Design and Construction Practice, Fifth edition

462 Miscellaneous piling problems
(a) (b) Fictitious fill
(c)
a
P z
Surcharge pressure p
Embankment
fill
p Hf Hf P
Hf H H H
a
a
Figure 9.20 Calculation of lateral pressure on vertical piles due to unsymmetrical surcharge loading
(after De Beer and Wallays (9.29) ).
p is the surcharge pressure, and �� is the effective angle of shearing resistance of the soil
applying pressure to the pile.
It should be noted that when ��0.5�� the lateral pressure becomes negligible. De Beer
and Wallays point out that the method is very approximate. It should not be used to obtain
the variation in bending moments along the pile shaft but only to obtain the maximum
moment. They also make the important point that the calculation method cannot be used
if the safety factor for conditions of overall stability of the surcharge load is less than 1.6.
It has the advantage of being based on drained soil conditions.
Driving piles within or close to the toe of clay slopes can result in the development of excess
pore pressure which may cause slipping of the slope. Massarsch and Broms (9.30) have developed
a method of predicting the excess pore pressures induced by the soil displacement.
It is very difficult to avoid relative settlement between a piled bridge abutment and the fill
material forming an embanked approach behind the abutment. Settlement of the fill often
occurs even when well-compacted granular material is used. Relative settlement can be large
where the embankment is placed on a compressible clay. The concept of allowing piles to
yield under load was adopted by Reid and Buchanan (9.31) for the purpose of reducing the relative
settlement of a piled bridge abutment and the approach embankment which was
founded on soft compressible clay. The arrangement of piles is shown in Figure 9.21. The
piles beneath the embankment close to the abutment were at close-spacing and were
designed to carry the whole of the embankment load with a safety factor of 2. After the first
four rows the spacing was increased to a 3 to 4 m grid and the piles were made successively
shorter so that they would yield under a progressively increasing proportion of the embankment
load. The piles had circular caps 1.1 to 1.5 m diameter. Loading from the embankment
was distributed to the pile heads by a flexible membrane consisting of two layers of Terram
plastics fabric reinforced with Paraweb strapping. If piles are used to support a bridge
approach slab, the embankment design and construction and the subsoil conditions will
affect the drag-down load on the piles.
P z
P z