Pile Design and Construction Practice, Fifth edition

horizontal movement is shear resistance between the soft clay and the underside of the pile
cap and at the interface between the soft clay and the stiff layer. As the embankment loading
increases plastic deformation occurs in the soft clay which flows horizontally away from the
abutment. In effect, the clay layer is extruded between the piles accompanied by horizontal
pressure on the ‘upstream’face of the piles and an upward pressure on the underside of the
pile cap. The horizontal pressure is low at pile cap level because the pile and soil are moving
together. It is also low at the interface with the stiff layer because the pile movement at this
level is relatively small and the stiff layer is also moving forward as a result of shear stress
on it from the soft clay.
Springman and Bolton(9.24)undertook research on behalf of the Department of Transport
into the behaviour of a single vertical free-head pile subjected to one-sided surcharge pressure
caused by placing fill on a weak deformable layer underlain by a stiffer but yielding stratum.
They used finite element analyses confirmed by centrifuge modelling to reproduce the pres-
sure distribution on the pile and the resulting deflections. These are shown diagrammatically
in Figure 9.17. The research was subsequently extended by Springman et al.(9.25)to deal with
the case of a full-height bridge abutment supported by two rows of vertical piles driven
through a soft clay layer into a dense sand stratum (Figure 9.18). Two loading cases were
considered: (a) rapid placing of the embankment fill at a rate equivalent to a one-day
construction period in the prototype, and (b) slow placing of the embankment over a 21-day
period. Vertical drains beneath the embankment were also incorporated in the model. These
extended from a drainage layer beneath the embankment into the sand stratum.

458 Miscellaneous piling problems

Figure 9.17The effect of asymmetrical surcharge loading on a vertical pile with a free-head pile driven
through soft deformable soil on to a stiff layer (a) Lateral pressure distribution (b) Relative
soil/pile movement (after Springman and Bolton(9.24)). Crown copyright 1995. Reproduced
by permission of HM Stationery Office.

Equivalent surcharge pressure q

h

hu

Free head

pile

Mean

pressure

Group

Pile

(b)(a)

pm

d

Soil movement

Soil line

Pile deflection

p (^9) m=1.5×pm