7.6.3.3 Ultimate tensile resistance from ground test results
(1)P Methods for assessing the tensile resistance of a pile foundation from ground test results shall have been established from pile load tests and from comparable experience as defined in 1.5.2.2.
(2) A model factor may be introduced as described in 2.4.1 (9) to ensure that the predicted tensile resistance is sufficiently safe.
(3)P The design value of tensile resistance of a pile, Rt;d, shall be derived from:
where:
NOTE The values of the partial factor may be set by the National annex. The recommended values for persistent and transient situations are given in Tables A.6, A.7 and A.8.
(4)P The characteristic value Rt;k shall either be determined by:
where ξ3 and ξ4 are correlation factors that depend on the number of profiles of tests, n, and are applied respectively to the mean (Rs;cal)mean and to the lowest value (Rs;cal)min of Rs;cal, or by the method given in 7.6.3.3 (6).
NOTE The values of the correlation factors may be set by the National annex. The recommended values are given in table A.10.
(5)P The systematic and random components of the variation in the ground shall be recognised in the interpretation of the calculated tensile resistance.
(6) The characteristic value of tensile resistance may be obtained by calculating:
As;i · qs;i;kwhere qs;i;k are characteristic values of shaft friction in the various strata obtained from values of ground properties.
NOTE If this alternative procedure is applied, the value of the partial factor γs,t recommended from Annex A, may need to be corrected by a model factor larger than 1,0. The value of the model factor may be set by the National annex.
(7)P If Design Approach 3 is used, the characteristic values of ground parameters shall be determined according to 2.4.5; partial factors shall then be applied to these characteristic values to obtain design values of the ground parameters to calculate the design values of the pile resistance.
(8) The assessment of the validity of a model based on ground test results should be in accordance with 7.6.2.3 (10).
7.6.4 Vertical displacements of pile foundations (Serviceability of supported structure)
7.6.4.1 General
(1)P Vertical displacements under serviceability limit state conditions shall be assessed and checked against the requirements given in 2.4.8 and 2.4.9.
(2) When calculating the vertical displacements of a pile foundation, the uncertainties involved in the calculation model and in determining the relevant ground properties should be taken into account. Hence it should not be overlooked that in most cases calculations will provide only an approximate estimate of the displacements of the pile foundation.
NOTE For piles bearing in medium-to-dense soils and for tension piles, the safety requirements for the ultimate limit state design are normally sufficient to prevent a serviceability limit state in the supported structure.
7.6.4.2 Pile foundations in compression
(1)P The occurrence of a serviceability limit state in the supported structure due to pile settlements shall be checked, taking into account downdrag, where probable.
NOTE When the pile toe is placed in a medium-dense or firm layer overlying rock or very hard soil, the partial safety factors for ultimate limit state conditions are normally sufficient to satisfy serviceability limit state conditions.
(2)P Assessment of settlements shall include both the settlement of individual piles and the settlement due to group action.
(3) The settlement analysis should include an estimate of the differential settlements that may occur.
(4) When no load test results are available for an analysis of the interaction of the piled foundation with the superstructure, the load-settlement performance of individual piles should be assessed on empirically established safe assumptions.
7.6.4.3 Pile foundations in tension
(1)P The assessment of upward displacements shall be in accordance with the principles of 7.6.4.2.
NOTE Particular attention should be paid to the elongation of the pile material.
(2)P When very severe criteria are set for the serviceability limit state, a separate check of the upward displacements shall be carried out.
