Pile Design and Construction Practice, Fifth edition

386 Structural design of piles and pile groups
to position the two parts of the pile while the butt weld is made but the backing plates for
the H-piles (Figure 7.5a) may not be needed if both sides of the pile are accessible to the
welder. The backing ring for the tubular pile shown in Figure 7.5b is deliberately made thin
so that it can be ‘sprung’ against the inside face of the pile. When lengthening piles in
marine structures, the position of the weld should be predetermined so that, if possible, it
will be situated below sea-bed level, and thus be less suceptible to corrosion than it would
if located at a higher elevation.
The specification adopted for making welded splices in steel piles should take into account
the conditions of loading and driving. For example, piles carrying only compressive loading
and driven in easy to moderate conditions would not require a stringent specification with
non-destructive testing for welding below the soil line. However, piles carrying substantial
bending moments in marine structures would require a specification similar to that used for
welding boilers or pressure vessels. Advice on specifications suitable for given conditions of
loading and driving should be sought from the manufacturers of the piles.
7.7 Bonding piles with caps and ground beams
Where simple compressive loads without bending or without alternate compressive and
uplift loading are carried by precast or cast in-situ concrete piles it is satisfactory to trim off
the pile square so that the head without any projecting reinforcement is set some 75 to
100 mm into the cap (Figure 7.6a). Some uplift (but not bending) can be carried if the sides
of the pile are roughened over a distance of about 300 mm and cast into the cap
(Figure 7.6b). Where bending moments are to be transferred from the cap to the piles (or
vice versa) the concrete must be cut away to expose the reinforcing steel or prestressing
tendons, which are then bonded into the cap (Figure 7.6c). It is sometimes the practice to
provide special mild steel splicing bars in the heads of prestressed concrete piles, which are
exposed by cutting away the concrete after driving is complete (Figure 2.6). Alternatively,
couplers can be set flush with the pile head to which further tendons or bars are attached for
bonding into the cap. Splicing bars or couplers are satisfactory if the depth of penetration of
75–100 mm
(a)
Reinforcement
in pile cap
(b) (c)
Roughening 300 mm
Concrete in
pile cut away
Figure 7.6 Bonding reinforced concrete piles into pile caps (a) Compressive loading only on piles
(b) Compressive loading alternating with light to moderate uplift loading on piles
(c) Bending moments or heavy uplift loads on piles.