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 fourth annual UK LWD correlation trial took place over a four-day period in the week commencing 9 September 2024. The test site for the correlation trial was located within AECOM’s Pavement Test Facility (PTF) based in Nottingham. Machines and operators from 67 organisations attended the correlation trial. In total, 153 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 153 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 0.924 and 0.996.
• The mean of the ratios of the FWD and LWD values produced adjustment factors that ranged between 0.862 and of 1.383.
• Published Date February 2025

Approved Devices