Cone penetration test (CPT): what the readings tell an engineer

A cone penetration test (CPT) pushes an instrumented steel cone into the ground at a steady rate and records, every few centimetres, how hard the tip is to push and how much the soil grips the sleeve behind it. A piezocone test (CPTu) also records the water pressure the cone sets up as it moves. The standard cone is 35.6 mm across, with a 10 square centimetre tip, pushed at 20 mm per second by a hydraulic rig on a truck or tracks. In Australia the method is covered by AS 1289.6.5.1. The output is a near-continuous profile of the ground, often tens of metres deep, from which an engineer reads soil layers, soil type and strength.

It is not the same test as the hand-held dynamic cone penetrometer, or Scala, which is hammered in and only reaches a metre or two. A CPT is the tool for soft clay, loose sand and deep, layered ground.

Where is a CPT used?

Mostly where the ground is soft, sandy or deep and the cost of getting it wrong is large: road embankments on floodplains, ports and reclaimed land, subdivisions on estuarine clay, and anywhere settlement or liquefaction is the question.

The Ballina Bypass on the NSW north coast crosses about 6 km of Richmond River floodplain over deep compressible soil. Before mid-2007 the ground there had been investigated with 142 CPTs alongside 295 boreholes, 169 test pits and two trial embankments, and the alliance that built it added 51 more CPTs and 70 boreholes (Kelly et al. 2020). Nearby, at the national soft soil test site set up as a legacy of the project, researchers pushed 26 piezocones inside an area of about 91 by 86 m just to measure how much the soil varied over that short distance (Li et al. 2014).

On a single house block a CPT is uncommon. It earns its place where soft estuarine clay, deep fill or loose sand sits under the lot, the conditions that put a site into Class P.

What do the CPT readings tell an engineer?

A CPTu sounding produces three traces against depth, with readings typically every 50 to 100 mm (Kelly et al. 2013).

• Tip resistance is the force on the cone tip divided by its area. High values mean dense sand, gravel or stiff clay. Low values mean soft clay or loose sand. A sudden jump marks the top of a stiffer layer.
• Sleeve friction is the grip of the soil on the sleeve just above the tip. Divided by the tip resistance it gives the friction ratio. Sands tend to show high tip resistance with a low friction ratio; clays show the opposite.
• Pore water pressure, measured just behind the cone in a CPTu, rises sharply in soft clay as the cone squeezes water that cannot escape quickly, and stays near the groundwater pressure in clean sand, where water drains easily. That makes it very good at picking thin sand lenses inside a clay layer.

Engineers plot normalised tip resistance against friction ratio on a soil behaviour type chart, which sorts each reading into zones from sensitive clay and organic soil through silt mixtures to dense sand and gravel (Mayne 2016). From the same data they estimate the undrained strength of clay, how much it has been loaded in the past, and how stiff it is, using correlations checked against laboratory tests.

Dissipation tests add one more measurement. The rig stops pushing and the pore pressure is logged as it falls away. How quickly it falls indicates how fast the clay will drain and settle under a new load. At the Wiggins Island Coal Export Terminal in Gladstone, engineers estimated the consolidation behaviour of soft alluvial clay three ways: laboratory oedometer tests, CPTu dissipation tests, and back-analysis of a monitored trial embankment, then compared them (Kemp et al. 2013).

What are the limits of a CPT?

No sample. A CPT infers soil type from how the soil behaves around the cone. It never brings soil to the surface, so it is paired with boreholes to confirm what the layers are and to recover samples for laboratory testing. On a reactive clay site that matters: shrink-swell (Iss) testing needs a sample.

Correlations are empirical. The conversion from cone readings to strength or stiffness rests on correlations built from other sites. Good practice calibrates them against laboratory tests from the same site.

Equipment and corrections matter. At Ballina, CPTs run in 2006 used 8-bit data loggers that could not resolve soft clay finely enough, giving a blocky trace from which past loading was hard to read (Kelly et al. 2020). Water pressure acting on the cone also has to be corrected for; in the Ballina clay the corrected tip readings were about a third or more higher than the raw values (Kelly et al. 2013).

Hard layers stop it. Gravel, cobbles, cemented layers, rock and rubble in fill can halt the cone before the target depth, and the rig needs access and a firm footing for its own weight.

How does a CPT compare with other tests?

	CPT / CPTu	Dynamic cone (Scala)	Standard penetration test (SPT)
How it goes in	Pushed hydraulically at a steady rate	Hammered by a 9 kg drop weight	Hammered at the base of a borehole
Record	Near-continuous electronic traces	Blows per 100 mm	Blows per 300 mm at set depths
Depth	Tens of metres in soft ground	Top metre or two	As deep as the borehole
Sample	None	None	Disturbed sample in the split tube
Best suited to	Soft clay, sand, layered ground	Footing inspections, fill depth, subgrades	Firmer soils, with a borehole

Common questions

What is a cone penetration test?

A cone penetration test pushes an instrumented cone into the ground at a steady 20 mm per second and records tip resistance and sleeve friction, and in a CPTu the water pressure, almost continuously with depth. Engineers use the traces to identify soil layers and estimate soil type and strength.

What is the difference between CPT and CPTu?

A CPTu, or piezocone test, is a CPT that also measures water pressure just behind the cone. That reading helps separate clay from sand, picks up thin sand layers, and allows dissipation tests that show how fast a clay will drain and settle.

Do I need a cone penetration test for a house?

Usually not on firm natural ground, where boreholes and Scala tests are the normal investigation. A CPT is worth considering where soft clay, loose sand or deep fill sits under the lot, which a desktop study of the geology and nearby bore logs can flag before anyone mobilises a rig.

Does a CPT replace boreholes?

No. A CPT recovers no soil, so boreholes are still needed to confirm the soil type and to take samples for laboratory tests, including the shrink-swell tests used to classify a reactive site.

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To see whether the mapped geology and nearby bore logs point to soft or deep ground before you scope the investigation, generate a desktop report for the address or see a sample report.

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

• Kelly, R. B., Drechsler, M. and Goldsmith, R. (2020). Connecting geotechnical investigations with project risk. Australian Geomechanics, Vol 55 No 1. geomechanics.org.au
• Kelly, R., Mayne, P. W. and Pineda, J. (2013). In situ and laboratory testing of soft clays. Australian Geomechanics, Vol 48 No 3. geomechanics.org.au
• Li, J. H., Huang, J., Cassidy, M. J. and Kelly, R. (2014). Spacial variability of the soil at the Ballina National Field Test Facility. Australian Geomechanics, Vol 49 No 4. geomechanics.org.au
• Mayne, P. W. (2016). Evaluating effective stress parameters and undrained shear strengths of soft-firm clays from CPTu and DMT. Australian Geomechanics, Vol 51 No 4. geomechanics.org.au
• Kemp, A., Kok, B. W.-C., Davis, J. and Menton, P. (2013). The consolidation behaviour of alluvial soft clay in Gladstone, Central Queensland. Australian Geomechanics, Vol 48 No 1. 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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