Pile Load Testing And What It Proves

You load test a pile to find out, on the real ground, how much it carries and how far it settles, before the building relies on it. Some tests prove capacity and some only look for defects, so it pays to know which one you are buying before the rig arrives.

Piles are one part of a building nobody sees again. Once the basement slab goes over them, the test results are the only record of what is really down there.

Why piles get tested

A pile design rests on soil and rock strengths from the [geotechnical investigation](/blog/understanding-geotechnical-investigation-reports), and those strengths come from a handful of boreholes. The pile is then built by a rig, in ground that changes between those holes, with concrete placed where nobody can see it.

Testing closes that gap. It checks the ground assumption and the workmanship together, on a pile built the same way as the production piles.

How testing changes the design

Under AS 2159, the Australian piling standard, the geotechnical strength reduction factor applied to a pile's capacity depends on a risk assessment of the site and on how much testing the job includes. More testing, of a better kind, allows a higher factor.

In plain terms, a well-tested job can carry the same building on less pile. On a project with a lot of piles, that saving in concrete and rig time can outweigh the cost of the tests, which is a conversation worth having at design stage rather than after the piles are priced.

Static load testing

A static test is the most direct answer. The pile is loaded in stages by a hydraulic jack pushing against a reaction, either heavy concrete blocks stacked above it or a beam tied down to anchor piles on each side. Gauges record how far the pile head moves at each load, and how that movement changes while the load is held.

It is slow and it takes up space, so it suits a test pile installed before production piling starts, often loaded past the design load to show where the ground begins to give. Piles that hold a basement down against water pressure get tested in tension in the same way, the case covered in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

Dynamic testing

A dynamic test drops a heavy weight on the pile head and records strain and acceleration as the blow travels down the pile. A signal matching analysis then estimates the capacity along the shaft and at the toe.

Because it is quicker than a static test, more piles can be tested for the same money. It suits driven piles naturally, since they can be tested as they go in, and it can be set up on bored piles with a drop weight. The comparison between pile types is in [piling types and when each is used](/blog/piling-types-and-when-each-is-used). The tradeoff is that the result comes out of an analysis, so it needs an engineer who knows both the method and the ground.

Integrity testing

An integrity test finds defects and does not measure capacity. The common method taps the pile head with a small hammer and reads the echo, which shows a change in section, a neck, a void or a pile shorter than the record says.

Large bored piles can have steel tubes cast in, and sound sent between the tubes checks the concrete across the full depth. These tests cost little per pile, so they can be run across a large share of the job as a check on workmanship.

Rock socket inspection

A lot of Sydney's pile work is bored piers socketed into sandstone, and there the most useful check is a look at the hole before the concrete goes in.

Our engineers confirm that the rock in the socket matches the class the design assumed, and that the base is clean of loose spoil and standing water. A socket bottoming out on a soft seam or a layer of mud doesn't behave like the one in the calculation. The design side is covered in [founding in Sydney sandstone and rock sockets](/blog/founding-in-sydney-sandstone-and-rock-sockets).

When results come back low

A test that misses its target isn't the end of the job. It tells you something about the ground or the method, and the first task is working out which one.

The answer can be longer piles, more piles, a change of rig or method, or a revised design for the piles already in. What matters is that the answer arrives before the production piles are all in the ground, which is the whole reason a test pile goes in first.

How we approach it

We set the testing programme as part of the pile design, because the number and type of tests decide the strength reduction factor we can use. Then we review the results ourselves, alongside the piling contractor's records, before the piles are signed off.

A pile carries the building for its whole life, and the test is the only time anyone gets to check its work.

ACSES provides geotechnical engineering, including pile design, across Sydney. Talk to us about a project.

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