One of the primary issues relating to reinstatements in the Highway is compaction of bound and unbound materials. Many stakeholders apply use of Clegg Hammers to determine surface modulus and provide conversion to CBR% where required. It is good practice to apply such methods to encourage compliant reinstatement, however it should be noted that each individual compacted layer should be subjected to measurement as effective depth of a 50mm diameter Clegg hammer generally ranges between 50mm and 150mm (source: WRAP report MRF106).
This unfortunately only supplies a reliable value up to 150mm from the surface and will not provide evidence of compaction quality of overall reinstatement unless a measurement is taken from each layer as it is completed. It also may be cumbersome for operatives tasked with backfill and reinstatement as they have to stop at each layer to take such a measurement.
An encompassing measurement can now be taken through physical test at each layer or indeed through multiple layers up to 500mm in total. Such measurement can also be directly related to materials as physical resistance is measured rather than reverberation.
A video of the device can be viewed here: https://www.youtube.com/watch?v=96d7HspKDbE
A copy of the WRAP report can be accessed here: http://www.wrap.org.uk/sites/files/wrap/Recycled%20materials%20in%20trench%20reinstatement%20-%20Compaction%20trial1.pdf
A report undertaken by the Transport Road Research facility can be viewed here https://trl.co.uk/reports/TRL587