Design by testing

Design by testing involves using the results of static load, dynamic impact, or ground tests to define the total pile resistance. This approach can only work where trial piles are installed and the results of tests on these piles are used to design the working piles. Traditionally, on smaller contracts, pile testing is avoided by using a large factor of safety on the calculated capacity.

Although EN 1997-1 emphasizes design by static load testing, for most contracts this is generally impractical as there is insufficient lead time between the main piling works and the test programme. Preliminary tests are rarely performed on piles with similar diameters and lengths, making it difficult to derive a sensible mean test result. In many cases, the ultimate load from a test is obtained by extrapolation of the load-displacement curve, adding further to the uncertainty in any calculated mean.

Verification of strength for pile foundations is illustrated in Figure 13.8, where the right-hand channel ('Test results') deals with piles designed by testing. Figure 13.13 expands this channel to show the specific correlation factors (^ to Q that are applied to measured or calculated resistances (Rm or Rcal) to determine the characteristic resistance Rk. Factors and are applied to the mean values and §2, and to the minimum values of Rm or Rcal. The characteristic resistance Rk is given by the smaller of the results, i.e.:

mean min

mean min where Lean = L L °r ^5 and Smin = L L °r L 13.9.1 Correlation factors

Correlation factors for calculating pile resistance are specified in Annex A of EN 1997-1, in Tables A.9 for static load tests, A.10 for ground tests, and A.11 for dynamic impact tests. The table on page 467 summarizes their values.

Eurocode 7 allows the correlation factors for static load tests (^ and Q and for ground tests (^ and §4) to be reduced by 10% when designing piles in groups, provided the structure has sufficient stiffness and strength to transfer loads from weak to strong piles (but ^ and St3 must not be taken as less than

The correlation factors for dynamic impact tests (§5 and Q may be reduced by 15% if signal matching is used. When using pile driving formulae, they should be increased by 10% if the quasi-elastic pile head displacement during impact is measured; and by 20% if not. [EN 1997-1 §A.3.3.3]

Characteristic resistance of piles from test results

Test results

Ground test results