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Results of dynamic load tests

Validity must be demonstrated by static

Calculation

Empirical or analytical calculation methods

load tests in comparable situations

Observation

Observed performance of comparable pile foundation

Must be supported by results of site investigation and ground testing

This book does not attempt to provide complete guidance on the design of pile foundations, for which the reader should refer to any well-established text on the subject.1

13.3.1 Design by static load tests

As the table above shows, EN 1997-1 places great emphasis on the use of static load tests, either as the primary design method or in providing validity to designs based on dynamic load tests or calculations.

Pile load tests must be performed when there is no comparable experience of the proposed pile type or installation method; the results of previous tests under comparable soil and loading conditions are not available; theory and experience do not provide sufficient confidence in the design for the anticipated loading; or pile behaviour during installation deviates strongly and unfavourably from that anticipated (and additional ground investigations do not explain this deviation). [en 1997-1 §7.5.1(1)P]

Eurocode 7 distinguishes between static load tests carried out on piles that form part of the permanent works ('working piles') and on piles installed, before the design is finalized, specifically for the purpose of testing ('trial piles' - or what, in the UK, are commonly termed 'preliminary piles').2 Trial piles must be installed in the same manner and founded in the same stratum as the working piles. [EN 1997-1 §7.4.1(3) and 7.6.2.2(2)P]

The load test procedure must allow the pile's deformation behaviour, creep, rebound, and - for trial piles - the ultimate failure load to be determined. The test load applied to working piles must not be smaller than the foundation's design load; piles tested in tension should be loaded to failure to avoid having to extrapolate the load-displacement curve.

[EN 1997-1 §7.5.2.1(1)P, 7.5.2.1(4), and 7.5.2.3(2)P]

A footnote in Eurocode 7 refers to the International Society of Soil Mechanics and Foundation Engineering's (ISSMFE's) suggested method for axial pile load testing,3 which will presumably be replaced by reference to EN ISOs 22477-1 to -34 when published (see Chapter 4). [en 1997-1 §7.5.2.1(1)P footnote 5]

Eurocode 7 does not specify how many piles should be tested, leaving this decision to engineering judgement. For trial piles, this must be based on ground conditions and their variability across the site; the structure's Geotechnical Category (see Chapter 3); documented evidence of relevant pile performance in similar ground conditions; and the total number and types of pile in the foundation design. For working piles, that judgement must be based additionally on the piles' installation records.

In the absence of detailed rules in Eurocode 7 on the number of test piles, we recommend following the guidance summarized in the table below, which is given by the UK's Institution of Civil Engineers5 and Federation of Piling Specialists.6

Risk Essential to test ... piles Number of tests

Preliminary Working High Preliminary and working 1 in 250 1 in 100

Medium Preliminary or working 1 in 500

Low (nothing specified)

Eurocode 7 states that, if only one static load test is performed, it must normally be located where the most adverse ground conditions occur. Failing this, the characteristic compressive resistance must be adjusted accordingly. When two or more tests are performed, one must be located where the most adverse ground conditions occur and the others at locations representative of the pile foundation. In practice, it is often the case that access to the part of the site that has the most adverse conditions is not possible at the time of preliminary testing. [EN 1997-1 §7.5.1 (4)P and 7.5.1(5)P]

Static load tests must not be performed until the pile material has achieved its desired strength and excess pore pressure generated during installation has fully dissipated. In practice, pore pressures are rarely measured during static load tests, so it is difficult to see this latter requirement being met.

13.3.2 Design by calculation

Eurocode 7 gives greater emphasis to determining pile resistance from tests (typically static loads, ground, or dynamic impact tests) than by calculation.

Almost as an aside, the standard states that a pile's characteristic base and shaft resistances (Rbk and Rsk) may be obtained from an 'alternative procedure' involving the equations:

Rbk = Abqbk where Ab = the pile's base area and qbk = its unit base resistance; and:

Rsk = ^ As,iqsk,i i where Asi = the pile's shaft area and qski = its unit shaft resistance in layer 'i'.

In the UK, the majority of pile designs are based on calculations that involve these two equations and Eurocode 7's use of the adjective 'alternative' does not give them sufficient recognition.

13.3.3 Design using dynamic load tests

Eurocode 7 allows the compressive resistance of a pile to be estimated using dynamic load tests, provided the tests are calibrated against static load tests on similar piles, with similar dimensions, installed in similar ground conditions. These requirements limit the applicability of dynamic load tests for design purposes - but they remain useful as an indicator of pile consistency and a detector of weak piles. [en 1997-1 §7.5.3.1(1) and 7.5.3.1(3)]

A footnote in Eurocode 7 refers to the American Society of Testing and Materials' (ASTM's) standard test method for high-strain dynamic pile testing,7 which has no equivalent in the geotechnical investigation and testing standards being prepared by ISO and CEN (see Chapter 4). Draft standards on dynamic load tests and rapid ('Statnamic') testing are expected to be submitted to CEN TC341 in the future. [en 1997-1 §7.5.3.1(1) footnote 6]

13.3.4 Design using pile driving formulae or wave equation analysis

Eurocode 7 allows the compressive resistance of a pile to be estimated using pile driving formulae or wave equation analysis, provided the ground's stratification has been determined and the method's validity demonstrated by static load tests on similar piles, with similar dimensions, installed in similar ground conditions. [EN 1997-1 §7.6.2.5(1)P,(2)P; 7.6.2.6(1)P,(2)P]

The blow count used in pile driving formulae should be obtained from driving records from at least five piles. [en 1997-1 §7.6.2.5(4)]

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