7.6.2.3 Ultimate compressive resistance from ground test results

(1)P Methods for assessing the compressive 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 compressive resistance is sufficiently safe.

(3) P The design compressive resistance of a pile, R_c;d, shall be derived from:

(4)P For each pile, R_b;d and R_s;d shall be obtained 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.

(5)P The characteristic values R_b;k and R_s;k shall either be determined by:

(7.8)

where ξ₃ and ξ₄ are correlation factors that depend on the number of profiles of tests, n, and are applied respectively:

• to the mean values (R_c;cal)_mean = (R_b;cal + R_s;cal)_mean = (R_b;cal)_mean + (R_s;cal)_mean
• and to the lowest values (R_c;cal)_min = (R_b;cal + R_s;cal)_min,

or by the method given in 7.6.2.3 (8).

NOTE The values of the correlation factors may be set by the National annex. The recommended values are given in table A.10.

(6)P The systematic and random components of the variation in the ground shall be recognised in the interpretation of the ground tests and calculated resistances.

(7) For structures with sufficient stiffness and strength to transfer loads from “weak” to “strong” piles, the factors ξ₃ and ξ₄ may be divided by 1,1, provided that ξ₃ is never less than 1,0.

(8) The characteristic values may be obtained by calculating:

R_b;k = A_b q_b;k and R_s;k =  A_s;i · q_s;i;k

(7.9)

where q_b;k and q_s;i;k are characteristic values of base resistance and shaft friction in the various strata, obtained from values of ground parameters.

NOTE If this alternative procedure is applied, the values of the partial factors γ_b and γ_s recommended in 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.

(9)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 for calculating the design values of the pile resistance.

(10) In assessing the validity of a model based on ground test results, the following items should be considered:

• soil type, including grading, mineralogy, angularity, density, pre-consolidation, compressibility and permeability;
• method of installation of the pile, including method of boring or driving;
• length, diameter, material and shape of the shaft and of the base of the pile (e.g. enlarged base);
• method of ground testing.

7.6.2.4 Ultimate compressive resistance from dynamic impact tests

(1)P Where a dynamic impact (hammer blow) pile test [measurement of strain and acceleration versus time during the impact event (see 7.5.3 (1))] is used to assess the resistance of individual compression piles, the validity of the result shall have been demonstrated by previous evidence of acceptable performance in static load tests on the same pile type of similar length and cross-section and in similar ground conditions.

(2) When using a dynamic impact load test, the driving resistance of the pile should be measured directly on the site in question.

NOTE A load test of this type can also include a process of signal matching to measured stress wave figures. Signal matching enables an approximate evaluation of shaft and base resistance of the pile as well as a simulation of its load-settlement behaviour.

(3)P The impact energy shall be high enough to allow for an appropriate interpretation of the pile capacity at a correspondingly high enough strain level.

(4)P The design value of the compressive resistance of the pile, R_c;d shall be derived from:

with

(7.11)

where ξ₅ and ξ₆ are correlation factors related to the number of piles tested, n, and are applied to the mean (R_c;m)_mean and the lowest (R_c;m)_min value of R_c;m respectively.

NOTE The values of the partial factor and correlation factors may be set by the National annex. The recommended values are given in table A.11.