Pile Design and Construction Practice, Fifth edition

158 Resistance of piles to compressive loads
formation of a dragged-down soft clay or sand skin has not been studied. A gap has been
observed around all flange and web surfaces of H-piles driven into stiff glacial till. An H-pile
is not a good type to select if it is desired to develop shaft friction and end-bearing resistance
in a stiff clay. The authors recommend calculating the shaft friction on the outer flange
surfaces only, but plugging can be allowed for by calculating the end-bearing resistance on
the gross cross-sectional area of the pile. Because of the conservative assumptions of shaft
friction and the relatively low proportion of the load carried in end bearing the calculated
resistance need not be reduced by the factor of 0.5 as recommended for tubular piles.
Applying the recommendations of EC7, for calculating the ultimate resistance of piles
driven into clay, the permissible stress equation 4.1 becomes
R cd � R bd � R sd
(4.9)
where R cd, R bd, and R sb are the design values of the total, base and shaft resistances respectively.
The first step is to calculate the latter two components using the arithmetic average of the
undrained shear strength c u for each borehole or in-situ test profile. Thus:
and
(a) (b)
L p
Clay plug
carried down
with pile
Open-ended
pile
Shaft friction acts
only on
outer flange surfaces
Shaft friction on
gross perimeter
only on this length
Rc cal mean � 9 � cub mean � Ab � F � �p � cus
mean � As Rc cal min � 9 � cub min � Ab � F � �p � cus
min � As Sheared clay
lumps partly
carried down
Flanges
End bearing on
gross area
Figure 4.7 Formation of soil plug at toe of small displacement piles (a) Open-ended tube (b) H-section.
(4.10)
(4.11)
Rc cal mean is calculated from equation 4.10 for each borehole or test profile using cub
mean as
the average value from tests in the region of the pile base, and cus mean as the average for each
Web