Pile Design and Construction Practice, Fifth edition

Piles driven in this manner are regarded as wholly end bearing: friction on the shaft is not
considered to contribute to the support of the pile. The depth of penetration required to reach
virtual refusal depends on the thickness of any weak or heavily broken material overlying
sound rock. If a pile can be driven to near refusal on to a strong intactrock the safe working
load on the pile is governed by the permissible working stress on the material of the pile at
the point of minimum cross-section; i.e. the pile is regarded as a short column supported
against buckling by the surrounding soil. Where piles are driven through water or through
very soft clays and silts of fluid consistency, then buckling as a long strut must be considered
(see Section 7.5).
When steel piles are adopted, working loads based on the permissible working stress on
the steel may result in concentrations of very high loading on the rock beneath the toe of the
pile. The ability of the rock to sustain this loading without yielding depends partly on the
compressive strength of the rock and partly on the frequency and inclination of fissures and
joints in the rock mass, and whether these discontinuities are tightly closed or are open or
filled with weathered material. Very high toe loads can be sustained if the rock is strong,
with closed joints either in a horizontal plane or inclined at only a shallow angle to the
horizontal. If the horizontal or near-horizontal joints are wide there will be some yielding of

Resistance of piles to compressive loads 197

Pile/soil model

for finite element

method

Vertical displacement, z

Vertical force, t

t–z curve

Figure 4.29 t–zcurve for deformation of a pile under vertical axial loading.