closed piles were 1160 and 1400 kN respectively. Evaluation of the ultimate shaft friction
and base resistances showed that the external shaft friction on the open-end piles was 20%
less than that on the closed-end piles.
Accordingly, it is recommended that where field measurements show that a clay plug
is carried down, the total ultimate bearing capacity should be calculated as the sum of the
external shaft friction (obtained from equation 4.8 and Figure 4.6) multiplied by a factor of
0.8, and the ultimate base resistance, Qb, obtained from equation 4.4 multiplied by a factor
of 0.5 (see Section 4.3.9). Where an internal stiffening ring is provided at the toe of a steel
pile the base resistance should be calculated only on the net cross-sectional area of the steel.
Attempts to clean out the core of soil from within the pile and replace it by a plug of
concrete or cement–sand grout are often ineffective due to the difficulty of removing the
strongly adherent clay skin to provide an effective bond to the pile surface. Also on large
diameter piles the radial shrinkage of the concrete or grout plug can weaken the bond with
the pile. As already noted the majority of the pile tests used to derive the relationships in
Figure 4.6 were made on open-end piles plugged with soil or concrete. Hence, the shaft
friction derived from them already incorporates the effect of the open end.
Plug formation between the flanges and web of an H-section pile is problematical. The
possible plug formation at the toe of an H-pile is shown in Figure 4.7b. The mode of
Resistance of piles to compressive loads 1571.6
(1.0, 0.35)(0.5, 0.8)Undrained shear strength/effectiveoverburden pressure, Cu/s (^9) vo
Embedded length/width ratio of pile, L/B
0.8
Peak adhesion factor
a^
p
Length factor
F
0.4
0.2
1.6
0.8
0.4
20 40 80
(1.0, 50)
(0.7, 120)
160 320
0.2 0.4 0.8 1.6 3.2
(a)
(b)
Figure 4.6Adhesion factors for piles driven to deep penetration into clays (after Semple and
Rigden4.5) (a) Peak adhesion factor versus shear strength/effective overburden pressure
(b) Length factor.