pile causing a small positive pressure to develop. Generally, the measurements on the model
piles showed increases in maximum bending moments over the 125 week (prototype) loading
period of 30% for the rear (furthest from the embankment) and 15% for the front row of piles.
The research studies of Springman and Bolton, and Springman et al.were based on
considerations of the undrained shear strength and undrained shear modulus of the soft clay.
These undrained soil parameters formed the basis, in the earlier work, of a computer
program given the name SIMPLE(9.26)from which the distribution of lateral pressures,
bending moments and deflections of over the full length of piles subjected to similar
loading conditions can be determined. The general pattern of the effects on piles as predicted
by the SIMPLE program was to some extent confirmed by full-scale measurements on piles
supporting the south abutment of the Wiggenhall Overbridge near Kings Lynn in Norfolk(9.27).
Four rows of 11 piles were driven through 10.4 m of weak Marine Alluvium to a depth of
about 5 m into a very stiff Kimmeridge Clay. Horizontal soil movements were recorded by a
tube close to the instrumented pile. Embankment filling was undertaken over a period of
about 5 months after completing pile driving.
The instrumented ground tube showed that flow of the soft clayey alluvium had occurred
between the piles over the period of the embankment placing with lateral movements at pile
cap level of 29 mm in the soil and 19 mm in the pile. The SIMPLE program had over-
predicted both the pile deflections and bending moments. The instrumented pile head had
deflected about 19 mm compared with the prediction of 60 mm. However, it should be
pointed out that the design of the bridge as a spill-through structure with fill on both sides of
the abutment and the multiple rows of supporting piles did not correspond to the arrangements
studied in the laboratory research.
Generally, before commencing detailed design studies into the behaviour of piles
subjected to the loading conditions described above it is desirable to consider the
maximum lateral pressure which can be applied to the piles within the soft layer. This was
shown by Randolph and Houlsby(9.28)to correspond to 9.14cufor a perfectly smooth
pile and 11.94cufor a perfectly rough pile. When the pressure on the leading face of a
pile reaches these values the clay flows past them and cannot exert any higher pressure.
Hence, if it can be shown that the pile section, as designed to resist vertical and horizon-
tal forces on the abutment or bank seat, has an adequate safety factor against failure or
excessive deflection when subjected to additional forces caused by soil movements
applied directly to the supporting pile shafts, then further detailed design work may be
judged unnecessary.
Springman and Bolton(9.24)recommended that the embankment–pile–soil system should
be designed to ensure that the ratio of the mean horizontal soil pressure (pm) to the undrained
shear strength (cu) should lie within the pseudo-elastic zone shown in the interaction dia-
gram (Figure 9.19). In this diagram the ratio pm/cuis plotted as the ordinate with an upper
limit of 10.5. At this stage the clay flows plastically around the pile (it was noted above that
Randolph and Houlsby put this limit between 9.14 and 11.94 depending on the surface
roughness of the pile). The ratio of the embankment surcharge pressure (q) to cuis plotted
as the abscissa, for which the limit is given by q(2
)cuwhich represents the stage of
plastic yielding of soil beneath the embankment. To avoid excessive deformation of the
embankment causing soil to flow between the piles supporting the abutment, there should
be a safety factor of at least 1.5 against base failure. Elastic behaviour of the system is
defined by the limits of h/din Figure 9.19 between 4 and 10. The height h, the pile diameter d,
460 Miscellaneous piling problems