Compaction testing of fill: what it involves and what it costs

Compaction testing checks that fill has been packed down hard enough. A technician measures the dry density of the compacted fill on site, usually with a nuclear density gauge or by the sand replacement method, and divides it by the maximum dry density the same soil reaches in a laboratory compaction test. The result is the density ratio, also called relative compaction, and it is compared with the specification. In Australia the work follows AS 3798-2007, Guidelines on earthworks for commercial and residential developments, and it is done by a geotechnical testing authority, usually a NATA-accredited laboratory.

For a house, compaction testing is what turns fill into controlled fill. Without the test records, the same fill is treated as uncontrolled.

Why does compaction testing matter for a slab on fill?

Because AS 2870-2011 classifies a filled site by how the fill went in, not by what it is made of. Under clause 2.5.3:

• Controlled fill up to 0.4 m deep (0.8 m for sand): the site is classified as the natural ground was before filling.
• Controlled fill deeper than that: the site is Class P, although an engineer can reclassify it by calculating movement for the actual profile.
• Uncontrolled fill: the site is Class P unless every footing goes through the fill to natural ground, on piers or deepened beams.

Controlled fill means fill placed in layers, compacted to a specification and tested as it went in, with the records to show it. A pad that was compacted well but never tested, or whose test reports have been lost, is in practice treated like any other uncontrolled fill. The uncontrolled fill guide covers how to spot fill before you buy.

How is compaction testing done?

There are two halves.

In the laboratory, a sample of the fill is compacted in a mould at several moisture contents to find its maximum dry density and the optimum moisture content at which it is reached. The standard compaction test (AS 1289.5.1.1) is the usual reference for house and lot fill; the heavier modified test is used for road bases. Clay samples need curing time of 2 to 7 days before testing (Look 2020).

In the field, the technician tests each compacted layer. A nuclear density gauge gives a reading in minutes. Sand replacement, which digs a small hole and measures its volume with calibrated sand, is slower. Moisture content is measured at the same spot, because specifications usually require the fill to be placed close to its optimum moisture content (Look 2020). On large jobs a rapid field method (the Hilf method) can compare field and laboratory density without waiting days for the full laboratory curve.

The density ratio is field dry density divided by laboratory maximum dry density, as a percentage. Clean sands are usually assessed by density index instead, which places the sand between its loosest and densest possible states.

What density ratio is required for house fill?

The 1996 edition of AS 3798 listed a minimum density ratio of 95 percent of standard maximum dry density for residential fill, with a density index of 65 percent for sand, and higher figures for commercial fill and pavements (Ewers 2006). The project specification sets the figure that applies, and the engineer who designs the footings should confirm it.

Ewers made a point worth knowing: 95 percent is a convention dating from the 1920s, not a number derived from what the fill has to carry. Compaction is meant to squeeze out air and water so the fill is stiffer and settles less, and density is only a stand-in for that.

What is Level 1 and Level 2 compaction testing?

AS 3798 sets out two levels of inspection and testing.

• Level 1: a geotechnical inspection and testing authority is on site full time while fill is placed, watches the work and tests it, and at the end can state that the fill was placed to the specification. Residential estates where lots are filled generally have Level 1 reports, often published stage by stage.
• Level 2: the testing authority visits periodically, samples and tests as asked, and reports the results. It does not certify the whole job, only the tests it did.

For a slab on filled ground, a Level 1 report covering your lot is what most engineers want to see. If you are buying in a new estate, ask the developer for the Level 1 report and the plan showing fill depths across the stage.

How much does compaction testing cost?

It depends on the level and the volume, and there is no fixed price. The things that drive a quote:

• Level 1 or Level 2. Level 1 is billed for a technician's time on site for the whole earthworks period. Level 2 is billed per visit and per test.
• Number of tests. Tests are set per layer and per area of fill, so a deep pad or a large lot means more layers and more tests.
• Laboratory compaction tests. Each different fill material needs its own reference test.
• Travel and reporting. Regional sites cost more to reach, and a signed Level 1 report costs more than a results sheet.

We have not found published prices we could verify, so get two quotes that state the level, the number of field tests per layer, the laboratory tests included and the report you will receive.

Can fill be tested after the house is built?

It can be retested, but the results need care. Look (2020) found that on one major road project 5,619 density tests took 13 days (median) to 22 days (mean) to be reported, and one in ten took 35 days or more. Contractors kept placing layers on top in the meantime, so the tests recorded quality after the event rather than controlling it. On a house pad, ask for results, even verbal ones, before the next layer and before the slab.

Retesting old fill is not clean proof either way. Burman et al. (2008) gathered cases where clay fills tested properly during construction were retested later and failed. At one berm compacted dry of optimum, retests about a year after completion showed the density ratio down by about 8 percent and the fill wetter. Where the same areas could be matched, retests averaged about 15 percent lower than the original results. The authors argued that compacted clay fills do not stay unchanged after placement, so a low retest does not by itself prove the fill was poorly compacted. Records made at the time carry the most weight, which is the case for getting them. The fill and desktop report guide covers what else to look for.

Common questions

What is compaction testing?

Compaction testing measures the dry density of compacted fill on site and compares it with the maximum dry density the same soil reaches in a laboratory test. The result, the density ratio, shows whether the fill meets the specification. In Australia it follows AS 3798-2007.

How much does compaction testing cost?

It depends on whether the job is Level 1 (full-time inspection and testing) or Level 2 (periodic visits), the number of layers and tests, laboratory compaction tests, travel and the report. Ask for quotes that list each of these.

What is a Level 1 compaction report?

A Level 1 report is issued by a geotechnical inspection and testing authority that was on site full time while fill was placed and tested. It states that the fill was placed to the specification, which is what lets an engineer treat it as controlled fill under AS 2870.

Is fill without compaction test records controlled fill?

No. Under AS 2870-2011, fill without records showing it was placed and tested to a specification is treated as uncontrolled, which makes the site Class P unless the footings go through the fill to natural ground.

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Before you buy a lot or price a slab, generate a desktop report for the address to see the historical imagery, elevation and mapped geology that show whether the block was filled, or see a sample report to check what it covers.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria or South Australia.

Sources

• Ewers, B. (2006). Relative compaction: in search of a rational method for specification. Australian Geomechanics, Vol 41 No 3. geomechanics.org.au
• Look, B. G. (2020). Overcoming the current density testing impediment to alternative quality testing in earthworks. Australian Geomechanics, Vol 55 No 1. geomechanics.org.au
• Burman, B. C., Mostyn, G. and Piccolo, D. (2008). Experiences with post-construction retesting of engineered clay fills. Australian Geomechanics, Vol 43 No 4. geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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