Light Weight Deflectometer (LWD) Correlation Trial Report

Executive Summary

The Light Weight Deflectometer (LWD) is a portable device that plays an important role in checking and measuring the stiffness of highway foundations. It is also used to control the quality of base installations for sports facilities, such as football pitches and athletic tracks. National Highways and Transport Scotland commissioned WSP, in partnership with AECOM, to assess the performance of LWD devices used in the UK against the recognised standard device, known as the Falling Weight Deflectometer (FWD).

The third annual UK LWD correlation trial took place over a four-day period in the week commencing 11 September 2023. The test site for the correlation trial was located within AECOM’s Pavement Test Facility (PTF) based in Nottingham. Machines and operators from 57 organisations attended the correlation trial. In total, 134 LWD devices were compared against a pre-calibrated FWD.

Trial operatives followed Procedure A, as outlined in Clause 885 (MCHW1). The procedure involved applying three seating drops from the 100 kPa test height of the LWD, followed by three further drops of which the mean is reported as the surface modulus value. The trial involved measuring surface modulus values at 30 measurement points with both the LWD and FWD. 

Summary of findings:

• A total of 134 LWD devices took part in the correlation trial.
• All the LWD devices were deemed to show a sufficient correlation with the pre-calibrated FWD and R² values ranged between 0.863 and 0.997.
• The mean of the ratios of the FWD and LWD values produced adjustment factors that ranged between 0.653 and of 1.446.
• Three machines (2.3%) were considered to have produced an excessive number of outliers. Owners were contacted and alerted to this with a view to reviewing and improving the operation and use of the LWD.

The PDF for the full report may not be fully accessible. If you need to request this information in a more accessible format, please contact Alan.Ferguson@transport.gov.scot

Approved Devices

Approved devices are issued an adjustment factors for use on materials covered by range 1, i.e. surface modulus between 25 MPa and 120 MPa.