Pile Design and Construction Practice, Fifth edition

the gap is removed when excavating for the pile cap. The gap may be significant for
relatively short piles with shallow capping beams for house foundations where these are
required as a precaution against the effects of soil swelling and shrinkage caused by vegetation
(Section 7.9). However, house foundation piles are usually bored and cast-in-place types
where a gap is not formed during installation.
Recent research by Bond and Jardine(4.3)on extensively instrumented piles jacked into stiff
London Clay confirmed the findings on the nature of the soil disturbance very close to the pile.
Negative pore pressures were induced in the clay close to the pile wall and positive pressures
further away from the pile. Equalization of pore pressures after installation was very rapid
occurring in a period of about 48 hours. There was no change in shaft friction capacity after the
equalization period as observed by periodic first-time loading tests over a 3^1 ⁄ 2 -month period.
Earlier research, mainly in the field of pile design for offshore structures, has shown that
the mobilization of shaft friction is influenced principally by two factors. These are
the over-consolidation ratio of the clay and the slenderness (or aspect) ratio of the pile. The
over-consolidation ratio is defined as the ratio of the maximum previous vertical effective
overburden pressure, , to the existing vertical effective overburn pressure,. For
the purposes of pile design, Randolph and Wroth(4.4)have shown that it is convenient to
represent the over-consolidation ratio by the simpler ratio of the undrained shear strength to the
existing effective overburden pressure, cu/. Randolph and Wroth showed that the cu/
ratio could be correlated with the adhesion factor,. A relationship between these two has
been established by Semple and Rigden(4.5)from a review of a very large number of pile
loading tests, the majority of them being on open-end piles either plugged with soil or
concrete. This is shown in Figure 4.6a for the case of a rigid pile, and where the shaft friction

vo vo

vc vo

Resistance of piles to compressive loads 155

Load on pile head

199 kN

Gap

Weakly

adhering

clay

Strongly

adhering

clay

Clay probably

moving down

with pile

168 mm O.D.

steel tubular

pile

Possible

tension crock

Load

on pile

shaft

1

2

3

Depth below ground level in m^4

5

5.6

Figure 4.5Load transfer from pile to stiff clay at Stanmore(4.2).