The CLEGG HAMMER or Clegg Impact Soil Tester (CIST) is a falling weight deflectometer that measures the stiffness of soils or pavements to assist engineers to determine the ability of the pavement to support load.

The device measures and records the peak deceleration upon impact after free-fall of the drop-hammer from a prescribed height.

The “harder” or “stiffer” or “stronger” the material, the greater the deceleration upon impact and therefore the greater the output seen on the display.

The output scale is referred to as the Clegg Impact Value scale, or the Impact Value scale. Output units are termed “Clegg units”. Each Clegg unit equals ten gravity units (with the resulting decimal fraction truncated).

The Clegg Hammer was conceived and developed under the guidance of Dr Baden Clegg in the Department of Civil Engineering at the University of Western Australia in the late 1970’s, originally for testing of earthworks and roadworks materials, with its application extending to other areas as well, such as testing of unsealed airstrips and railroad foundations, hardstands, helipads, trench backfill, and may other forms of earthworks.

The CLEGG HAMMER measures the deceleration of the hammer as it strikes the surface of the pavement or soil that is being tested.

The harder the test surface the less distance the “hammer’ travels between impact and arrest. Softer surfaces allow more travel between the strike and the stop. The variation in stopping distance may be very small but the difference in deceleration is substantial. By measuring the deceleration, it is possible to assess the stiffness or hardness of the test surface.

By comparing many test results to known pavement strengths it was possible to propose a formula for converting the Clegg Impact Values to CBR values or mPa values.

The hammer is lifted until it reaches a stop position, which ensures that it is always dropped from the same height. Every drop must be identical, or the results would not be accurate.

The hammer is raised and dropped 4 times and the results are recorded by the instrument.

Deceleration is then converted to Clegg Impact Values (CIV), California Bearing Ratio (CBR) and megaPascals (mPa). These results have value as comparative strengths which can be used by engineers and others to determine whether the tested surface is fit for purpose.

Operating a Clegg Hammer is intentionally simple.

It is portable in one hand without physical effort.

The instrument can be operated accurately and reliably with less than thirty minutes instruction.

No engineering qualifications are required to test surfaces and gather information, this allows stakeholders at any level to inspect and report the work.

This facility is valuable when contractors are required to report compliance to supervising engineers before progressing to the next section of work.

Junior engineers and technicians can accurately report from remote sites to centrally based supervising engineers.

Internationally the Clegg Hammer complies with

ASTM D 5874 - Standard Test Method for Determination of the Impact Value (IV) of a Soil.

1. Scope

1.1 This test method covers the determination of the Impact Value (IV) of a soil either in the field or a test mold.

1.2 The standard test method, using a 4.5 kg (10 lb) hammer, is suitable for, but not limited to, evaluating the strength of an unsaturated compacted fill, in particular pavement materials, soils, and soil-aggregates having maximum particle sizes less than 37.5 mm (1.5 in.).

1.3 By using a lighter 0.5 kg (1.1 lbm) hammer, this test method is applicable for evaluating lower strength soils such as fine grained cohesionless, highly organic, saturated, or highly plastic soils having a maximum particle size less than 9.5 mm (0.375 in.).

1.4 By performing laboratory test correlations for a particular soil using the 4.5 kg (10 lbm) hammer, IV may be correlated with an unsoaked California Bearing Ratio (CBR) or may be used to infer percentage compaction.

1.5 The values stated SI are to be regarded as the standard. The values stated in parentheses are given for information only.

1.6 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

Note 1-The equipment and procedures contained in this test method are similar to those developed by B. Clegg in the 1970s at the University of Western Australia, Nedlands, Australia. Impact Value is also commonly known as Clegg Impact Value (CIV).

The standard price for the 2022 CLEGG HAMMER is USD6450.

Purchases of five or more units may be eligible for a discount.

The price does not include delivery or local taxes.

Each test takes less than 30 seconds and preparation for the test takes a few seconds.

Allowing a minute for each test and the travel time between each test site allows an estimate of a “per test” cost.

The cost applicable to the instrument is negligible since the unit has a life expectancy of more than ten years and is durable enough to perform thousands of tests if used with care.

The total cost “per test” should not exceed US$2 and in many cases will be less than US$1.

By comparison the use of conventional CBR testing or Benkelman Beam testing will exceed US$100 per test.

Test results are delivered within two seconds of every drop of the hammer.

By comparison the results of conventional tests may take up to ten days and will certainly not take less than an hour at site.

There have been many comparisons of accuracy of Clegg Hammer results to tests conducted by using conventional CBR testing equipment.

Comparison of hundreds of tests indicate a Clegg Hammer accuracy of +/- 2% deviation from the mean of conventional tests. The conventional CBR tests having been carried out by the same team of experienced operators throughout the tests, a degree of accuracy seldom achieved in reality.

Conventional testing is complex and can have a high degree of inaccuracy resulting from individual application of methodology, approximation or interpretation during recording and the psychological effects of tedium.

Clegg Hammer results offer as many results in one hour as may be achieved in one week by a proficient team using conventional methods and equipment.

Recording is automatic and consistent. Accuracy comes from the consistency and the ability to gather a much larger sample on which to base assessment.

Conventional CBR testing requires expertise, training, and experience. It also requires a heavy weight to impose the necessary surcharge on the equipment.

The setting up of the equipment can take up to an hour and the test can take thirty minutes.

Calculation of the results and the manual tabulation of readings is time consuming.

The results may take between one and ten days to be confirmed.

CLEGG HAMMER results are available in two seconds, no engineering experience is required, no interpretation of results is required and there are no calculations made by the operator.

The most important aspect of Clegg testing is the ability to gather many results which allow more reliable assessment of general pavement properties and more specific identification of non-compliant areas.

CLEGG HAMMER is a fraction of the cost of other falling weight deflectometers. It is faster to deploy, faster to operate and delivers instant results with greater accuracy.

Simplicity of operation, speed of deployment, accuracy of results and instant automatic recording are the main advantages of using Clegg testing.

The additional advantage of self-calibration sets CLEGG HAMMER apart from all previous instruments.

CLEGG HAMMER can be used to test natural ground, compacted soils, trench backfill, foundation preparation, compacted infill around structures, pavements of all kinds (except concrete), and many other soil surfaces where strength, density or compaction need comparative assessment.

The portability of the Clegg Hammer allows for testing in confined spaces where other tests would not be possible.

The non-intrusive, non-reactive nature of the test means that it can be performed in areas where other tests may present an unacceptable risk.

Test Data is stored in a “csv” file. This information can be uploaded to Google Maps and each test will plot as a separate location on the map.

Specific instructions are included in the video instruction.

Video

The instructions on selection of options for colour coding the results of tests for display on Google maps are given in the instructional video here.

Depending on your choice of delivery service it may take up to two weeks to deliver by airfreight services to your chosen delivery point.

Express air freight services may be available as an option by contacting Clegg Global through this web site contact facility.

The entire instrument with accessories and carry case weighs 17kgs (38lbs).

The carry case is 102cm x 46cm x 18cm (40”x18”x7”).

Each Clegg Hammer comes with a twelve-month manufacturer’s guarantee. If used with care the instrument should give service for more than twenty years.

Clegg Hammers are still in service after more than thirty years.

The instrument is self-calibrating. Follow the procedure in the built in program and the instrument will make any necessary adjustments and confirm calibration.

There is no need to return the Clegg Hammer for service and the self-calibration can be performed on site if required.

The GPS capability is included in the tablet that is supplied.

GPS coordinates will be applied to each new test site as the instrument is moved and set down.

GPS accuracy is approximately 3m.

Each CLEGG HAMMER comes with a one-year manufacturer’s warranty. Freight costs are owner’s responsibility.

The California Bearing Ratio has been a reliable indicator of pavement strength since 1928/29 when it was developed by the California Highways Department (now Caltrans).

CBR is accepted globally and is applied in nearly all road building standards.

CLEGG HAMMER reports results in CBR, mPa and CIV. Since the CIV can be related directly to deceleration in units of gravity it will always be possible to relate the results of the test to the strength of the pavement and report in any alternative units.

The software can be revised and uploaded in the application stored in the tablet.

The instrument will never be obsolete and the reporting mode can be adjusted to suit any future changes.

The device replaces time consuming and technically difficult testing procedures. Those procedures limit the number of samples that can be taken and restrict the operation to those who have extensive training.

The Clegg Hammer removes the potential for error or misinterpretation because the correct physical operation is straightforward, and the instrument makes all the necessary calculations.

The size of sample is much larger because tests are usually less than a minute and many tests can be completed. An average is based on more samples and any large deviation from standard is more obvious.