Pile Design and Construction Practice, Fifth edition

should not arise if the piles are driven a short distance below rockhead to achieve a
degree of fixity.
The appropriate codes of practice should be consulted to obtain the reduction factors on
the working stresses to allow for the slenderness of the piles.

7.6 Lengthening piles

Precast (including prestressed) concrete piles can be lengthened by cutting away the concrete
to expose the main reinforcement or by splicing bars for a distance of 40 bar diameters. The
reinforcement of the new length is then spliced to the projecting steel, formwork is set up,
and the extension is concreted. It is usual to lengthen a prestressed concrete pile by this tech-
nique in ordinary reinforced concrete. The disadvantage of using the method is the time
required for the new length to gain sufficient strength to allow further driving.
A rapid method of lengthening which can be used where the piles carry compressive loads
or only small bending moments is to place a mild steel sleeve with a length of four times the
pile width over the head of the pile to be extended. The sleeve is made from 10 mm plates
and incorporates a central diaphragm which is bedded down on a 10 to 15 mm layer of earth
dry sand–cement mortar trowelled onto the pile head. After setting the sleeve a similar layer
of mortar is placed on the upper surface of the diaphragm and rammed down by a square
timber. The extension pile with a square end is then dropped down into the sleeve and
driving commences without waiting for the mortar to set. An epoxy-resin–sand mortar can
be used instead of sand–cement mortar. An epoxy-resin joint can take considerable tensile
or bending forces, but the length of time over which the adhesion of the resin to the concrete
is effective is indeterminate. The bond may be of rather short duration in warm damp
conditions.
Another method of lengthening piles is to drill holes into the pile head. Then bars
projecting from the extension piece are grouted into these holes using a cement grout or an
epoxy-resin mortar.
Timber piles are lengthened by splicing as shown in Figures 2.3 and 2.4, and steel piles
are butt-welded to lengthen them (Figure 7.5a and b). Backing plates or rings are provided

Structural design of piles and pile groups 385

Butt weld

Positioning plates

Tack weld

Butt weld

Backing

ring

(a) (b)

Figure 7.5Splicing steel piles (a) Positioning plates for H-pile (b) Backing ring for tubular piles.