7.5 Pile load tests
(1)P Pile load tests shall be carried out in the following situations:
- when using a type of pile or installation method for which there is no comparable experience;
- when the piles have not been tested under comparable soil and loading conditions;
- when the piles will be subject to loading for which theory and experience do not provide sufficient confidence in the design. The pile testing procedure shall then provide loading similar to the anticipated loading;
- when observations during the process of installation indicate pile behaviour that deviates strongly and unfavourably from the behaviour anticipated on the basis of the site investigation or experience, and when additional ground investigations do not clarify the reasons for this deviation.
(2) Pile load tests may be used to:
- assess the suitability of the construction method;
- determine the response of a representative pile and the surrounding ground to load, both in terms of settlement and limit load;
- to allow judgement of the overall pile foundation.
(3) Where load tests are not practical due to difficulties in modelling the variation in the load (e.g. cyclic loading) very cautious design values for the material properties should be used.
(4)P If one pile load test is carried out, it shall normally be located where the most adverse ground conditions are believed to occur. If this is not possible, an allowance shall be made when deriving the characteristic value of the compressive resistance.
(5)P If load tests are carried out on two or more test piles, the test locations shall be representative of the site of the pile foundation and one of the test piles shall be located where the most adverse ground conditions are believed to occur.
(6)P Between the installation of the test pile and the beginning of the load test, adequate time shall be allowed to ensure that the required strength of the pile material is achieved and the pore-water pressures have regained their initial values.
(7) In some cases it can be necessary to record the pore-water pressures caused by pile installation and their subsequent dissipation in order to take a proper decision regarding the start of the load test.
7.5.2 Static load tests
220.127.116.11 Loading procedure
(1)P The pile load test procedure5, particularly with respect to the number of loading steps, the duration of these steps and the application of load cycles, shall be such that conclusions can be drawn about the deformation behaviour, creep and rebound of a piled foundation from the measurements on the pile. For trial piles, the loading shall be such that conclusions can also be drawn about the ultimate failure load.
(2) Devices for the determination of loads, stresses or strains and displacements should be calibrated prior to the test.
(3) The direction of the test load applied to compression or tensile piles should coincide with the longitudinal axis of the pile.
(4) Pile load tests for the purpose of designing a tensile pile foundation should be carried out to failure. Extrapolation of the load-displacement graph for tension tests should not be used.
18.104.22.168 Trial piles
(1)P The number of trial piles required to verify the design shall depend on the following:
- the ground conditions and their variability across the site;
- the Geotechnical Category of the structure, if appropriate;
- previous documented evidence of the performance of the same type of pile in similar ground conditions;
- the total number and types of pile in the foundation design.
(2)P The ground conditions at the test site shall be investigated thoroughly. The depth of borings or field tests shall be sufficient to ascertain the nature of the ground both around and beneath the pile tip. All strata likely to contribute significantly to pile behaviour shall be investigated.
(3)P The method used for the installation of the trial piles shall be fully documented in accordance with 7.9.
22.214.171.124 Working piles
(1)P It shall be specified that the number of working pile load tests shall be selected on the basis of the recorded findings during installation.
(2)P The test load applied to working piles shall be at least equal to the design load for the foundation.
7.5.3 Dynamic load tests
(1) Dynamic load tests6 may be used to estimate the compressive resistance provided an adequate site investigation has been carried out and the method has been calibrated against static load tests on the same type of pile, of similar length and cross-section, and in comparable soil conditions, (see 126.96.36.199 to 188.8.131.52).
(2)P If more than one type of dynamic test is used, the results of different types of dynamic test shall always be considered in relation to each other.
(3) Dynamic load tests may also be used as an indicator of the consistency of the piles and to detect weak piles.
7.5.4 Load test report
(1)P It shall be specified that a factual report shall be written for all load tests. Where appropriate, this report shall include:
- a description of the site;
- the ground conditions with reference to ground investigations;
- the pile type;
- description of the pile installation and of any problems encountered during the works;
- a description of the loading and measuring apparatus and the reaction system;
- calibration documents for the load cells, the jacks and the gauges;
- the installation records of the test piles;
- photographic records of the pile and the test site;
- test results in numerical form;
- time-displacement plots for each applied load when a step loading procedure is used;
- the measured load-displacement behaviour;
- reasons for any departures from the above requirements.