Dynamic cone penetration test: the Scala penetrometer explained

A dynamic cone penetration test (DCP) measures how hard the ground is to push through by hammering a steel rod with a cone tip into it and counting the blows. In Australia it is usually called a Scala penetrometer test, and the method is Australian Standard AS 1289.6.3.2, the 9 kg dynamic cone penetrometer test. A 9 kg weight drops 510 mm onto an anvil and drives a rod with a 20 mm, 30 degree cone into the soil (Flux 2013). Few blows per 100 mm means soft or loose ground; many blows means firm ground, dense sand or rock close below.

It is the cheapest strength test in geotechnical work. Two people carry it to site, one to lift and drop the weight and one to write down the readings (Nguyen & Mohajerani 2012), and a test takes minutes.

Where did the Scala penetrometer come from?

A. J. Scala published the method in 1956 as a simple way to design flexible road pavements, and by the early 1960s the Country Roads Board of Victoria was using the device, known by then as the Scala penetrometer (Coman-Walton & Fahey 2009). Its original job was roads, not houses: count the blows, convert penetration per blow into an inferred California Bearing Ratio (CBR, the strength figure pavement designers use), and size the pavement layers to suit.

Western Australia has a cousin. The Perth sand penetrometer, developed at the University of Western Australia in the early 1960s from the Victorian device, uses the same 9 kg weight but a blunt 16 mm rod and a 600 mm drop, and has its own standard, AS 1289.6.3.3 (Coman-Walton & Fahey 2009). It is built for checking compaction of clean sand. The two are not interchangeable.

How is a DCP test done and read?

The rod is stood upright on the surface, or in the base of a footing trench or borehole, and the weight is lifted to the stop and dropped, again and again. The operator records the blows needed to drive each 100 mm. The result is a column of numbers down the page, usually plotted as blows per 100 mm against depth.

Reading it is about pattern more than any single number:

• Low, steady counts suggest topsoil, loose sand, soft clay or poorly compacted fill.
• A sharp rise marks a stiffer layer: natural clay under fill, dense sand, or weathered rock.
• Refusal, where the rod stops moving after many blows, can mean rock. It can also mean a cobble, a lump of concrete in fill or a gravel band, so one refusal on its own proves little.

For pavements the engineer converts millimetres per blow into an inferred CBR using Scala's chart. For footings the engineer compares the counts with the founding material the design assumed.

What is a DCP test used for on a house site?

Alongside the boreholes. Soil testers often run a Scala next to each borehole during the site investigation. The borehole shows what the soil is; the Scala shows how firm it is and whether there is a soft or loose layer near footing depth.

Footing and pier inspections. Before concrete is poured, an engineer or inspector checks that the excavation has reached the material the footing design called for. Hand probing along a strip footing or in each pier hole is a quick way to find a soft spot, a pocket of fill or a wet patch the borehole missed. In Perth, an AGS review of residential footings listed checks on fill compaction and an engineer's inspection of the ground exposed after earthworks among the things that need supervision during construction (McInnes 2003).

Fill and soft topsoil. A run of low counts that stops at a clear step is a fair sign of where fill or topsoil ends and natural ground begins. It tells you how deep footings may need to go to get through it, which matters because uncontrolled fill makes a site Class P unless the footings go through it.

Driveways, pavements and subgrades. Its original use: checking the strength of the ground under a driveway, crossover or estate road.

What can a DCP test not tell you?

What the soil is. The rod brings nothing back. A Scala cannot tell clay from silt, and it cannot measure how reactive a clay is. Reactivity needs a sample and a laboratory shrink-swell (Iss) test, so a Scala test alone never gives an AS 2870 site classification. The site investigation decides the class, from logged boreholes and laboratory results.

Strength that holds in every season. Clay is much harder dry than wet. A stiff reading in a dry summer can soften after a wet winter, so the counts describe the ground on the day.

Design values on its own. Flux (2013) tested the same Perth sands with a DCP, two Clegg hammers and the Perth sand penetrometer and got noticeably different inferred CBR values from each, partly because each relies on its own conversion. His advice was not to lean on DCP or Clegg hammer results alone for design without laboratory tests to compare.

Deep ground. It is a shallow tool. Friction on the rod builds with depth and stones stop it, so it is most useful in the top metre or two. Deeper questions need a drill rig or a cone penetration test.

How is a DCP different from a cone penetration test (CPT)?

The names are close but the tests are not.

	Dynamic cone penetration test (DCP, Scala)	Cone penetration test (CPT, CPTu)
How it goes in	Hammered by a 9 kg drop weight	Pushed at a steady rate by a hydraulic rig
What is recorded	Blows per 100 mm	Tip resistance, sleeve friction and, in a CPTu, water pressure, recorded electronically
Typical depth	Top metre or two	Tens of metres in soft ground
Equipment	Hand-carried, two people	Truck or track-mounted rig
Typical use	Footing inspections, fill depth, pavement subgrades	Soft clay, sand, coastal lowland and larger projects
Australian Standard	AS 1289.6.3.2	AS 1289.6.5.1

A CPT gives a near-continuous electronic profile and lets an engineer infer soil type layer by layer. A DCP gives a coarse strength profile near the surface. For most house sites on firm natural ground, boreholes plus Scala tests are the normal investigation. A CPT earns its cost where soft clay or loose sand runs deep.

Common questions

What is a dynamic cone penetration test?

A dynamic cone penetration test drives a cone-tipped steel rod into the ground with a 9 kg weight dropped 510 mm, and records the blows needed for each 100 mm of penetration. In Australia it is called the Scala penetrometer test and follows AS 1289.6.3.2.

What is the difference between a DCP and a CPT?

A DCP is hammered in by hand and records blow counts in the top metre or two. A CPT is pushed in at a steady rate by a hydraulic rig and records tip resistance, sleeve friction and often water pressure electronically, to much greater depths.

Can a Scala penetrometer test give a site classification?

No. A Scala test measures how firm the ground is but brings back no soil, so it cannot measure reactivity. The AS 2870 site classification comes from the site investigation, using logged boreholes and laboratory shrink-swell tests.

How deep can a dynamic cone penetrometer test go?

It is most useful in the top metre or two. Rod friction builds with depth and stones, gravel or rubble in fill stop the rod, so deeper ground is checked with a drill rig or a cone penetration test.

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Before the soil tester arrives, generate a desktop report for the address to see the mapped geology, nearby bore logs and likely fill the Scala readings will be read against, 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

• Flux, S. (2013). A comparison of inferred CBR test results on 'Perth Sands'. Australian Geomechanics, Vol 48 No 2. geomechanics.org.au
• Coman-Walton, A. and Fahey, M. (2009). Effect of depth on results from the Perth sand penetrometer test. Australian Geomechanics, Vol 44 No 2. geomechanics.org.au
• Nguyen, B. and Mohajerani, A. (2012). A new lightweight dynamic cone penetrometer for laboratory and field applications. Australian Geomechanics, Vol 47 No 2. geomechanics.org.au
• McInnes, D. (2003). Residential footings - Perth metropolitan area. Australian Geomechanics, Vol 38 No 3. 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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