7.6.3 Ground tensile resistance
7.6.3.1 General
(1)P The design of piles in tension shall be consistent with the design rules given in 7.6.2, where applicable. Design rules that are specific for foundations involving piles in tension are presented below.
(2)P To verify that the foundation will support the design load with adequate safety against a failure in tension, the following inequality shall be satisfied for all ultimate limit state load cases and load combinations:
(3)P For tension piles, two failure mechanisms shall be considered:
- pull-out of the piles from the ground mass;
- uplift of the block of ground containing the piles.
(4)P Verification against uplift failure of the block of ground containing the piles (see Figure 7.1), shall be carried out in accordance with 2.4.7.4.
(5) For isolated tensile piles or a group of tensile piles, the failure mechanism may be governed by the pull-out resistance of a cone of ground, especially for piles with an enlarged base or rock socket.
(6) When considering the uplift of the block of ground containing the piles the shear resistance Td along the sides of the block may be added to the resisting forces shown in figure 7.1.
(7) Normally the block effect will govern the design tensile resistance if the distance between the piles is equal to or less than the square root of the product of the pile diameter and the pile penetration into the main resisting stratum.
(8)P The group effect, which may reduce the effective vertical stresses in the soil and hence the shaft resistances of individual piles in the group, shall be considered when assessing the tensile resistance of a group of piles.
(9)P The severe adverse effect of cyclic loading and reversals of load on the tensile resistance shall be considered.
(10) Comparable experience based on pile load tests should be applied to appraise this effect.
7.6.3.2 Ultimate tensile resistance from pile load tests
(1)P Pile load tests to determine the ultimate tensile resistance of an isolated pile, Rt, shall be carried out in accordance with 7.5.1, 7.5.2 and 7.5.4, and with regard to 7.6.2.2.
(2)P The design tensile resistance, Rt;d, shall be derived from:
NOTE The values of the partial factors 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.
- 1 ground surface
- 2 ground-water level
- 3 side of the 'block', where resistance Td develops
(3) Normally when piles are to be loaded in tension, it should be specified that more than one pile should be tested. In the case of a large number of tension piles, at least 2 % should be tested.
(4)P The records of the installation of the test pile(s) shall be checked and any deviation from the normal construction conditions shall be accounted for in the interpretation of the pile load test results.
(5)P The characteristic value of the pile tensile resistance shall be determined by:
where ξ1 and ξ2 are correlation factors related to the number of piles tested, n, and are applied respectively to the mean (Rt;m)mean and the lowest (Rt;m)min value of the measured tensile resistances.
NOTE The values of the correlation factors may be set by the National annex. The recommended values are given in table A.9.
