Pile Design and Construction Practice, Fifth edition

Two modes of failure are to be examined:

(1) The pull-out of the pile from the ground mass and
(2) Uplift of a block of ground containing the piles.

For condition (1) the risk of pull-out of a cone of soil adhering to the pile is to be considered.
The adverse effects of cyclic loading as described above are to be taken into account.
EC7 permits the ultimate tensile resistance to be determined by pile loading tests. It is
recommended that more than one test should be made, and in the case of a large number of
piles at least 2% should be tested. Correlation factors (see Table 4.7) are applied to the test
results to obtain the characteristic tension resistance Rtk.
Analytical methods as described in Chapter 4 for compression loading are permitted by
EC7 to be used for calculating resistance to tension loading. The correlation factors shown
in Table 4.6 are applied to the results of the calculations to obtain characteristic values (Rstk).
The factors depend on the number of ground test results used to provide the basis for
the calculations. The partial factor for shaft resistance, , is then applied to obtain Rstd. The
partial factors shown in Tables 4.3–4.5 depend on the type of pile. It will be noted that
the factors are generally higher than those for shaft resistance in compression reflecting
the potentially more damaging effects of failure of a foundation in uplift.
The ‘model pile’and the design approaches, as described in Section 4.1.4, are used in
conjunction with analytical methods of determining tension resistance. Determination by
dynamic pile testing appears to be excluded by EC7. Presumably this is because the method
measures shaft resistance in compression requiring an empirical correction to obtain the
tension value and because the direction, speed and duration of loading are such that
predictions could be misleading.
Where vertical piles are arranged in closely spaced groups the uplift resistance of the
complete group may not be equal to the sum of the resistances of the individual piles. This
is because, at ultimate-load conditions, the block of soil enclosed by the pile group is lifted.
The manner in which the load is transferred from the pile to the soil is complex and depends
on the elasticity of the pile, the layering of the soil and the disturbance to the ground caused
by installing the pile. A spread of load of 1 in 4 from the pile to the soil provides a simplified
and conservative estimate of the volume of a coarse soil available to be lifted by the pile
group, as shown in Figure 6.6. For simplicity in calculation, the weight of the pile embedded

st

310 Piles to resist uplift and lateral loading

Block of soil

lifted by piles

1

4

Figure 6.6Uplift of group of closely spaced piles in fine-grained soils.