Making Light Work of Compaction Control
Our Technical Manager, Malcolm Durie, assesses the role of light weight deflectometers in construction foundation testing and considers the benefits for contractors, specifiers and roads authorities.
Assessing whether materials are compacted effectively without causing construction delays has often presented problems on site for highways projects. Quick and reliable assessments are crucial to ensure that construction can progress on schedule, and of course to prevent any failure in service which could lead to rutting and cracking, leading to costly repairs much earlier than expected. However, with limitations in technology and testing methods, that has not always been easy.
Traditionally, compaction control required lengthy assessments, which took time to complete due to the equipment used and the disruption that was necessary. Some of the traditional testing is required for reference and compliance but the light weight deflectometer (LWD) can provide the close spacing of data to infill between these widely spaced locations.
Thankfully, there is a solution which continues to punch above its weight to transform the way in which engineers can assess compaction of materials. LWDs may be small, but these portable tools for assessing foundation layers offer a series of unique benefits including manoeuvrability, ease of use and instant test results.
Fit for purpose
LWDs such as the industry leading Prima100 from Sweco can be moved from site to site easily and enable the operator to carry out tests extremely quickly, with immediate results on site. Crucially the test load is applied by a captive 10kg load that is released by the operator, ensuring they are suitable for use in trenches or other areas with limited access and as a one-man operated tool, only the operator is required to carry out the testing and produce initial results.
The assessment of foundations is now quicker than ever before, with a graphical user interface on a handheld PDA (Personal Digital Assessment) that enables operators to review results as the data is being collected (or later if required).
The equipment will report on the screen a plot of the load pulse applied, deflection response from the foundation and the resultant calculated surface modulus. It means engineers can determine whether materials are suitable compacted within the space of one or two minutes at each test site and move onto the next location a huge reduction on traditional assessment methods, which could take a couple of days to report.
Of course LWDs are not new and certainly not an untested solution. The Prima100 in particular has been in use in the UK since 2001 on local authority roads, trunk roads, car parks, football pitches and other sports surfaces. It is currently in use by a range of testing organisations on the Forth Replacement Crossing, the M8-M73-M74 DBFO project, A90 Aberdeen Western Peripheral Route DBFO and many other projects around the UK and Ireland.
The key lies in the speed of results. As a society we are becoming more reliant on instant data and availability of information and the highways industry is no different. The rapid results available from leading LWDs enable the user to assess various foundation layers as they are constructed, which means that poor material performance or compaction can be identified as soon as possible to prevent costly mistakes. Data can be assessed by trained personnel as the works are under construction and before the layer is covered making it much cheaper to correct failures at an early stage.
They are also incredibly flexible. Information can be reviewed by an individual drop, by location or as an average over the entire length tested, with the position of the test drops automatically logged by an integral GPS facility into the database along with all of the test information. The surface modulus values expected at various layers can be taken from the Highways England standard IAN 73/06 or for non-standard constructions by the designer, as Sweco did on the A82 Crianlarich Bypass.
For these reasons and more, light weight deflectometers have become the road builders tool – the method for rapid assessment of as constructed foundation layers.
This comment featured in the November 2016 issue of Highways magazine.
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