Because buoyancy is a destabilizing factor, EC7 (Clause 2.4.7.4) requires verification of
stability by the UPL criteria as given by the equation:
(6.3a)where
(6.4)and
Vdstd design value of the permanent destabilizing vertical action on the substructure
Gstbd design value of the permanent stabilizing vertical actions
Gdstd design value of the permanent destabilizing actions
Qdstd design values of the variable actions
Rd any additional resistance to uplift
Rd considered as a permanent vertical action.
The partial factors for actions for the ultimate limit state (UPL) verification are set out in
Annex A of EC7 as shown in Table 6.1. For verification of the uplift resistances of the soil
surrounding the block, and of the pull-out resistances of the piles in the group, where
derived by calculations using soil parameters the partial factors shown in Table 6.2 are used.
6.2.3 Piles with base enlargements
When bored piles are constructed in clay soils, base enlargements can be formed to anchor
the piles against uplift. The enlargements are made by the belling tools described in
Vdstd GdstdQdstdVdstd Gstbd Rd312 Piles to resist uplift and lateral loading
Table 6.1Partial factors for actions ( (^) f) for uplift
limit-state (UPL) verifications
Action Symbol Value
Permanent
Unfavourablea Gdst 1.0
Favourableb Gstb 0.9
Variable
Unfavourablea Qdst 1.5
Notes
a denotes destabilizing.
b denotes stabilizing.
Table 6.2Partial factors for soil parameters and
resistances
Soil parameter Symbol Values
Angle of shearing resistancea 1.25
Effective cohesion c 1.25
Undrained shear strength cu 1.40
Tensile pile resistance st 1.40
Anchorage resistance a 1.40
Note
a This value is applied to tan .
The factors in Tables 6.1 and 6.2 may be modified when
the British National Annex is published.