Pile Design and Construction Practice, Fifth edition

162 Resistance of piles to compressive loads
Fleming and Sliwinski (4.12) reported no difference in the adhesion factor between bored
piles drilled into clays in bentonite-filled holes and dry holes. In spite of this evidence it
must be pointed out that if the use of a bentonite slurry to support an unlined hole in clay
does not reduce the shaft friction this must mean that the rising column of concrete placed
by tremie pipe beneath the slurry has the effect of sweeping the slurry completely off
the wall of the borehole. It is difficult to conceive that this happens in all cases; therefore
the adhesion factor � recommended for London Clay, or for other clays in Figure 4.8, should
be reduced by 0.8 to allow for the use of bentonite unless a higher value can be demonstrated
conclusively by loading tests.
In clays other than London Clay, where there is no information from loading tests or
publications, the adhesion factors shown in the curve for bored piles in glacial till
(Figure 4.8) can be used as a guide to pile design. The calculated pile capacity should be
confirmed by field loading tests.
The procedure for checking the ULS resistance of bored piles in clay when using the EC7
rules is the same as described in Section 4.2.1 for driven piles. The �b and �s partial factors
in equation 4.14 are used for conventional bored piles and continuous flight auger (CFA) piles
as shown in Tables 4.4 and 4.5 respectively.
The greater part of the resistance of bored piles in clay is provided by shaft friction for
which the component in equation 4.10 becomes . It will be noted that the value of � s
�c uskA s
is unity in the above tables. Hence, the engineer should give careful attention to the quality
of the undisturbed sample and the laboratory testing techniques.
The higher value of 1.25 for � b in bored piles in equations 4.14 and 4.15 compared with
unity for driven piles reflects the influence of the fissured structure of many stiff clays, and
also takes into account possible inadequacies when cleaning out the base of the pile borehole
before placing the concrete. The latter operation also involves the risk in soft clays of
‘waisting’ or necking when placing concrete in uncased boreholes or when extracting temporary
casing. Allowances for possible reductions in pile diameter due to these causes are
shown in Table 4.9.
When enlarged bases are provided on bored piles in a fissured clay there may be a loss of
adhesion over part of the pile shaft in cases where appreciable settlements of the pile base
are allowed to occur. The effect of such movements is to open a gap between the conical
surface of the base and the overlying clay. The latter then slumps downwards to close the
gap and this causes a ‘drag-down’ on the pile shaft. Arching prevents slumping of the full
thickness of clay from the ground surface to the pile base. It is regarded as over-cautious to
add the possible drag-down force to the working load on the pile, but nevertheless it may be
prudent to disregard the supporting action on the pile of shaft friction over a height of two
shaft diameters above the pile base, as shown in Figure 4.9.
Disregarding shaft friction over a height of two shaft diameters and taking an adhesion
factor of 0.3 for the friction on the remaining length may make a pile with an enlarged base
an unattractive proposition in many cases when compared with one with a straight shaft.
However, the enlarged-base pile is economical if the presence of a very stiff or hard stratum
permits the whole of the working load to be carried in end bearing. These piles can also be
advantageous where the concept of yielding or ‘ductile’ piles is adopted for the purpose of
achieving load distribution between piles as discussed in Sections 5.2.1 and 5.10. Enlarged
bases may also be a necessity to avoid drilling down to or through a water-bearing layer in
an otherwise impervious clay.