7.6.2.2 Ultimate compressive resistance from static load tests

(1)P The manner in which load tests are carried out shall be in accordance with 7.5 and shall be specified in the Geotechnical Design Report.

(2)P Trial piles to be tested in advance shall be installed in the same manner as the piles that will form the foundation and shall be founded in the same stratum.

(3) If the diameter of the trial pile differs from that of the working piles, the possible difference in performance of piles of different diameters should be considered in assessing the compressive resistance to be adopted.

(4) In the case of a very large diameter pile, it is often impractical to carry out a load test on a full size trial pile. Load tests on smaller diameter trial piles may be considered provided that:

• the ratio of the trial pile diameter/working pile diameter is not less than 0,5;
• the smaller diameter trial pile is fabricated and installed in the same way as the piles used for the foundation;
• the trial pile is instrumented in such a manner that the base and shaft resistance can be derived separately from the measurements.

This approach should be used with caution for open-ended driven piles because of the influence of the diameter on the mobilisation of the compressive resistance of a soil plug in the pile.

(5)P In the case of a pile foundation subjected to downdrag, the pile resistance at failure, or at a displacement that equals the criterion for the verification of the ultimate limit state determined from the load test results, shall be corrected. The correction shall be achieved by subtracting the measured, or the most unfavourable, positive shaft resistance in the compressible stratum and in the strata above, where negative skin friction develops, from the loads measured at the pile head.

(6) During the load test of a pile subject to downdrag, positive shaft friction will develop along the total length of the pile and should be considered in accordance with 7.3.2.2 (6). The maximum test load applied to the working pile should be in excess of the sum of the design external load plus twice the downdrag force.

(7)P When deriving the ultimate characteristic compressive resistance R_c;k from values R_c;m measured in one or several pile load tests, an allowance shall be made for the variability of the ground and the variability of the effect of pile installation.

(8)P For structures, which do not exhibit capacity to transfer loads from "weak" piles to "strong" piles, as a minimum, the following equation shall be satisfied:

(7.2)

where ξ₁ and ξ₂ are correlation factors related to the number of piles tested and are applied to the mean (R_c;m)_mean and the lowest (R_c;m)_min of R_c;m respectively.

NOTE The values of the correlation factors may be set by the National annex. The recommended values are given in table A.9.

(9) For structures having sufficient stiffness and strength to transfer loads from "weak" to "strong" piles, the values of ξ₁ and ξ₂ may be divided by 1,1, provided that ξ₁ is never less than 1,0.

(10)P The systematic and random components of the variations in the ground shall be recognised in the interpretation of pile load tests.

(11)P The records of the installation of the test pile(s) shall be checked and any deviation from the normal execution conditions shall be accounted for.

(12) The characteristic compressive resistance of the ground, R_c;k, may be derived from the characteristic values of the base resistance, R_b;k, and of the shaft resistance, R_s;k, such that:

(13) These components may be derived directly from static load test results, or estimated on the basis of ground test results or dynamic load tests.

(14)P The design resistance, R_c;d, shall be derived from either:

or

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