Pile Design and Construction Practice, Fifth edition

Effective
length
Shaft friction not
allowed over
this length
2B
B
Resistance of piles to compressive loads 163
Clay moves down
to close up
incipient gap
Figure 4.9 Effective shaft length for calculating friction on shaft of under-reamed pile.
Piles for marine structures are sometimes installed by driving a steel tube to a limited
penetration below sea bed, followed by drilling-out the soil plug then continuing the drilled
hole without further support by the pile tube. A bentonite slurry is sometimes used to support
the borehole. On reaching the design penetration depth a smaller diameter steel tube insert
pile is lowered to the bottom of the borehole and a cement–sand grout is pumped-in to fill
the annulus around the insert pile. The grout is injected either through a small-diameter pipe
or is pumped directly down the insert pile.
Kraft and Lyons (4.13) have shown that the adhesion factor used to calculate the shaft friction
on the grout/clay interface is of the same order as that used for the design of conventional
bored and cast-in-place concrete piles. Where bentonite is used as the drilling fluid a reduction
factor should be adopted as discussed above. A considerable increase in the adhesion
factor can be obtained if grout is injected under pressure at the pile–soil interface after a
waiting period of 24 hours or more. Jones and Turner (4.14) report a two- to threefold increase in
adhesion factor when post-grouting was undertaken around the shafts of 150 mm diameter
micropiles in London clay. However, the feasibility of achieving such increases should be
checked by loading tests before using them for design purposes particularly if there are
doubts about the ability of the grouting process to achieve full coverage of the shaft area.
The post-grouting technique is used as a first step around the shafts of bored piles where
base grouting is used as described in Section 3.3.9.
4.2.4 The effects of time on pile resistance in clays
Because the methods of installing piles of all types have such an important effect on the
shaft friction it must be expected that with time after installation there will be further
changes in the state of the clay around the pile, leading to an increase or reduction in the
friction. The considerable increase in resistance of piles driven into soft sensitive clays due
to the effects of re-consolidation have already been noted in 4.2.1.