Pile Design and Construction Practice, Fifth edition

160 Resistance of piles to compressive loads
Adhesion factor a
1.2
1.0
0.8
0.6
0.4
0.2
0
Reduced values for driven
piles where L < 10B and till
is overlain by soft clay
Driven and driven and cast-in-place piles
Bored piles
60 80 100 120
Undrained shear strength Cu , kN/m2 140 160 180 200 220
Figure 4.8 Adhesion factors for piles in glacial till (after Weltman and Healy 4.7 ).
length of the driving tube if an enlarged detachable shoe is used to close its base) becomes filled
with concrete. The tube, while being driven, drags down a skin of soft clay or sandy soil for a
few diameters into the stiff clay and it is quite likely that this skin will remain interposed
between the concrete and the soil, i.e. the skin is not entirely pulled out by adhering to the tube.
However, in one important aspect there is a difference between the driven, and the driven and
cast-in-place pile in that water migrates from the unset concrete into the clay and softens it for
a limited radial distance. This aspect is discussed in greater detail in Section 4.2.3. Thus the
adhesion factor for a driven and cast-in-place pile in a stiff clay may be slightly less than that
for a driven pile in corresponding soil conditions. It will probably be greater over the length in
a soft clay, however, since the concrete slumps outwards as the tube is withdrawn, producing an
increase in effective shaft diameter.
The results of a number of loading tests on driven and driven and cast-in-place piles in
glacial till have been reviewed by Weltman and Healy (4.7) . There appeared to be little difference
in the �–c u relationship for either type of pile. They produced the design curves shown
in Figure 4.8 for the two types of driven pile including a curve for piles driven a short penetration
into stiff glacial till overlain by soft clay. Their review also included a study of the
shaft friction on bored piles in glacial till.
Trenter (4.8) recommended using the Weltman and Healy relationships and stated that it is
essential to obtain 100 mm samples of the till suitable for strength tests.
When using the EC7 recommendations to determine the ultimate limit state resistance of
driven and cast-in-place piles, the procedure described in Section 4.2.1 should be followed.
Compliance with the shaft dimension tolerances in Table 4.9 should be observed.
4.2.3 Bored and cast-in-place non-displacement piles
The installation of bored piles using the equipment and methods described in Sections 3.3.1
to 3.3.6 and 3.4.6 causes changes in the properties of the soil on the walls of the pile borehole
which have a significant effect on the frictional resistance of the piles. The effect of
drilling is to cause a relief of lateral pressure on the walls of the hole. This results in swelling
of the clay and there is a migration of pore water towards the exposed clay face. If the borehole
intersects water-filled fissures or pockets of silt the water will trickle down the hole and