Problems in pile construction - Piling Engineering - page 255

Figure 7.1 Damage to head of precast concrete pile. (Photograph courtesy of W. A. Dawson Ltd.)
strata, a rock point should be fitted to assist penetration. Problems can be expected
when piles are driven through strata containing boulders or other obstructions. In
such ground concrete piles may be broken and steel H-section piles deformed or split
(Figure 7.3).
It is important that the designed geometry of the pile group is maintained. Positive
control is possible only at the ground surface, and piling frames should be set on stable
foundations. If guide trestles are used they should be of substantial construction, and
positive guides for the piles should be fitted. It is not possible to correct deviations
once pile driving has started. Piles, particularly flexible piles, may wander off line
considerably below ground level. An extreme case of an H-pile turning through 180 ◦
has been reported. However, a greater danger is that piles will collide in a closely
spaced group.
Misaligned piles will not generally reduce the bearing capacity of the group signifi-
cantly. However, O'Neill et al . (1977) reported that the stiffness of experimental pile
groups was reduced by 30% over that of the designed geometry. Very little can be
done to prevent piles wandering, except to choose a stiff of pile section and possibly