Pile Design and Construction Practice, Fifth edition

Miscellaneous piling problems 457
wing walls. The abutment is only partially restrained from moving forward under the
influence of the retained soil. A small degree of restraint is provided at the top of the wall
by friction or rotation in the bearings supporting the deck structure. At pile cap level higher
restraint is provided by the stiffness of the supporting piles, but the amount of forward
movement should, theoretically, result in earth pressure on the back of the abutment
corresponding to the ‘active’ state. However, heavy compaction of the embankment filling
is required to prevent settlement of the road surface, such that the earth pressure, particularly
near the top of the wall, can be higher than the ‘at rest’ (K 0) condition. The UK
Department of Transport (9.23) specifies that the abutment retaining walls should be designed
to resist a pressure of 1.5K 0 at the Ultimate Limit State.
Raking piles to support abutments should be avoided if at all possible because rigidity at
pile cap level could result in earth pressures at low levels approaching ‘passive’ conditions.
If, because of ground or loading conditions, the use of rakers is unavoidable, the angle of
rake should be varied as shown in Figure 9.16 to prevent a high proportion of load being carried
by a single row of rakers when used in combination with vertical piles on the rearward
side of the foundation.
Bending moments and deflections in rows of vertical piles caused by earth pressure on
the abutment can be calculated by the methods described in Sections 6.3 to 6.5. Where the
abutment is underlain by a weak deformable layer such as soft clay, horizontal and vertical
movements take place in the soft layer under the loading of the embankment. The vertical
movements are restrained if there is a stiff underlying layer, but the only restraint to
Piles at varying angle of rake
Spread of load
Figure 9.16 Bridge abutment supported by raking piles.