Assessment from axial load tests

General

The scatter of bearing capacity values predicted by analytical methods has already been discussed. Semi-empirical methods often predict values with a wide range of variation, and it is recognized that the construction procedure has a strong influence on the pile bearing capacity. These facts explain the need for load testing of piles, in order to determine the safety level, and potentially improve the design. There are three main types of load tests: static load (applied to the top or the pile toe), rapid load testing (Bermingham and Janes, 1989), and dynamic load testing. In the following sections, the static and dynamic load tests will be briefly presented.

Conventional static load test (compression)

In a static load test, hydraulic jacks are used to apply load to the pile head, reacting against a frame supported by other piles or anchors, as shown in Figure 7.14. In some cases, kentledge systems may be used, where dead weights on platforms provide the reaction. The load is controlled with load cells, and the pile top settlement is measured with displacement transducers fixed on reference beams.

When the assessment of base and shaft resistance distribution is required, internal instrumentation such as strain gauges or tell tales are installed prior to concreting. By measuring axial strain at several positions along the pile length, the axial force is determined, as shown in Figure 7.15, and its variation, divided by the shaft area between two adjacent sections, yields the average unit shaft resistance. Load tests should be performed on piles with the same diameter and length as the production piles, but, in some cases, where the loads required to test large-diameter piles are very high, this is not possible and the diameter of the test piles can be smaller. It is recommended that the test pile diameter should be at least 50% of the production piles.

Figure 7.15a illustrates the setup of a long pile, instrumented with ten levels of retrievable strain gauges, installed in alluvial soil. The axial force distribution along the pile length

Reaction systems for static load test

Figure 7.14 Reaction systems for static load test: a) reaction frame supported by piles; b) ground-anchor reaction system.

is shown in Figure 7.15b, and it can be observed how the shaft and base resistances were mobilized for each load increment.

The determination of the failure load is performed with one of several possible criteria (Fellenius, 2019). For small- diameter piles (B <600 mm), Davisson (1972) defined failure as the intercept of the load-settlement curve with a line parallel to the elastic deformation line, as shown in Figure 7.16. The settlement at failure sf-is equal to:

Eurocode 7 recommends the definition of pile failure as when the settlement reaches 10% of the pile diameter. In some cases, when that value is not reached, a careful extrapolation of the load-settlement curve may be performed.

Figure 7.16 shows the application of Davisson’s criterion to the results of a static load test on a 800-mm diameter Continuous Flight Auger (CFA) pile.

 
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