Pile Design and Construction Practice, Fifth edition

in the ground is assumed to be equal to that of the volume of soil it displaces. If the weight
of the block of soil is calculated by using a diagram of the type shown in Figure 6.6, then
the safety factor against uplift can be taken as unity, since frictional resistance around the
periphery of the group is ignored in the calculation. The submerged weight of the soil should
be taken below groundwater level.
In the case of fine-grained soils the uplift resistance of the block of soil in undrained shear
enclosed by the pile group in Figure 6.7 is given by the equation:

(6.3)

where Quis the total uplift resistance of the pile group, Land B, are the overall length and
width of the group, respectively, His the depth of the block of soil below pile cap level,
is the value of average undisturbed undrained shear strength of the soil around the sides of
the group, and Wis the combined weight of the block of soil enclosed by the pile group plus
the weight of the piles and pile cap. Submerged densities are used for the soil and portion
of the structure below groundwater level when calculating W.
A safety factor of 2 should be used with equation 6.3 to allow for the possible weakening
of the soil around the pile group caused by the method of installation. For long-term
sustained loading a safety factor of 2.5 to 3 would be appropriate.
If either of the above two methods is used to calculate the combined uplift resistance of a pile
group, the allowable resistance must not be greater than that provided by the sum of the skin-
frictional resistance of the individual piles in the group divided by the appropriate safety factor.
EC7 (Clause 7.6.3.1) recommends calculating the uplift resistance of a block of soil sur-
rounding the pile group in a manner similar to that described above. The design value of the
uplift load combined with the uplift force from buoyancy on the underside of the soil block,
Vdstd, are resisted by the design values of the friction on the vertical outer surfaces of the
block, Td, and the stabilizing forces, Gstbd, of the mass of soil composing the block, the pile
cap, or other substructures supported by the piles, and the weight of any soil overburden
above these structures. The resistances of the piles to pull-out are not included in the stabi-
lizing forces but are considered separately since they provide no resistance if failure is by
lifting of the mass of soil.

cu

Qu (2LH 2 BH)cuW

Piles to resist uplift and lateral loading 311

Block of soil

lifted by piles

B×L

H

Figure 6.7Uplift of group of piles in coarse-grained soils.