On a busy earthworks or road job, a layer may need checking long before a full set of density tests can be completed. The Clegg impact test gives a quick, repeatable indication of how stiff a compacted layer is, in the time it takes to drop a hammer a few times. It is widely used as a rapid control test to spot weak areas and to confirm that compaction is consistent across a layer.
Background and Standard
The equipment and procedure were developed by B. Clegg in the 1970s at the University of Western Australia, and the test is often simply called the Clegg hammer test. It is standardised in ASTM D5874, Standard Test Methods for Determination of the Impact Value (IV) of a Soil, which covers testing either in the field or on soil compacted in a mould in the laboratory.
The standard version of the test uses a 4.5 kg hammer. ASTM D5874 describes it as suitable for evaluating unsaturated compacted fill, in particular pavement materials, soils, and soil-aggregates with a maximum particle size below 37.5 mm.
How the Test Works
- Set up. The guide tube is placed vertically on a flat area of the compacted surface.
- Drop the hammer. The hammer, fitted with an accelerometer, is lifted to a set height inside the tube and released onto the surface.
- Read the peak deceleration. The instrument records how sharply the hammer stops. A stiff layer stops it abruptly; a soft layer lets it sink and decelerate more gently.
- Repeat. The hammer is dropped several times at the same spot, and the value from the specified drop is reported as the Impact Value (IV), also called the Clegg Impact Value (CIV).
ASTM D5874 includes field procedures for measuring the Impact Value alone, or together with water content and dry density, and a laboratory mould procedure.
Where It Is Used
- Compaction control: checking many points across a layer quickly to confirm the rolling has been uniform and to locate soft spots.
- Targeting other tests: directing slower tests, such as sand replacement density or field CBR, to the weakest and most representative locations.
- Comparing materials: tracking how a fill or sub-base responds to different moisture contents or compactive effort.
- Site-specific correlations: with enough paired results, Impact Values can be related to CBR or density for a particular material on a particular project.
Limitations
- Material-specific: an Impact Value means most when compared against other readings on the same material. Correlations to CBR or density should be developed or confirmed for the material in question.
- Moisture sensitive: the same soil gives different readings at different moisture contents, so readings should be taken with the moisture condition in mind.
- Shallow: the test reflects the upper part of the layer. It does not show what is happening deeper down.
- Surface condition: loose surface material, large stones, or an uneven surface under the guide tube can distort the reading.
- Particle size: coarse material beyond the standard's particle-size range is outside the method's scope.
Note: Acceptance values for Impact Value, where used, are set by the project specification or established by site trials. Results should be interpreted by the responsible engineer alongside density and moisture testing.
