Pile Design and Construction Practice, Fifth edition

swelling forces on the inner sides of beams they should not be left in contact with the clay.
After casting the concrete has been completed, the clay should be cut back as shown in
Figure 7.15. The space between the side of the beam and the cut-back of the excavation is
left empty, or is only loosely backfilled.
Ground beams are provided to act as ties or compression members between adjacent pile
caps, so providing the required restraint against sidesway or buckling of the piles under lat-
eral or eccentric loading (see Section 7.5). Ground beams and pile capping beams may have
to withstand horizontal loading from the soil due to the tendency to movement of vertical
piles under lateral loading. They may also be subjected to bending in a vertical direction due
to differential settlement between adjacent groups of piles.
It may be permissible to allow the passive resistance of the soil against the sides of pile
caps and ground beams to supplement the resistance of the piles to lateral loading. However,
in clay soils the ground will shrink away from the sides of shallow members in dry weather
conditions. Trenching for building services alongside pile caps must also be considered a
possibility. In any case quite appreciable yielding of the soil must take place before its
passive resistance is fully mobilized. This movement may be sufficient to cause the failure
in bending of vertical piles.
The superimposed loading on the ground beams or pile capping beams is transferred to
the piles by bonding the longitudinal reinforcing steel to the beams into the pile caps.
However, it is not good practice to carry the longitudinal steel through holes burned in the
projecting parts of steel piles. It is quite likely that the pile head will have deviated from the
correct position and it may be impossible to bend the beam reinforcing bars over a sufficient
horizontal distance to pass through the holes in the steel pile without causing complications

Structural design of piles and pile groups 395

Reinforcement lapped with capping beam

steel and extended downwards to anchor

into zone of non-swelling clay

Layer of special low-density

foamed plastic or 'Clayboard'

Beam under

cross wall

150 mm void

Precast r.c suspended floor

Dry bed joint

280 mm

Damp-proof course

Cranked vent

Ground level

R.c capping beam

Face of excavation cut back

and space loosely filled or

left void

Bored and cast

in place pile

Figure 7.15Design of pile capping beam for swelling clay soils.