This report describes an investigation into the validity of night-time deflection measurements which was undertaken by WDM Limited under contract to the Transport and Road Research Laboratory. Deflection surveys to assess highway strengthening requirements are normally confined to day-time hours because the current Department of Transport design method has been validated fully only for day-time conditions. The investigation involved a large number of deflection surveys at three typical motorway sites, M4 Wiltshire (near Badminton) Eastbound, M5 Gloucestershire (near Dursley) Northbound, and M56 Cheshire (near Chester) Eastbound and one TRRL full-scale experimental site (A30 Hampshire at Nateley Scures) during 1990. It is concluded that deflections obtained from surveys carried out at night can, without adjustment, directly replace day-time values as inputs to the present design method with only a small loss of accuracy, providing the survey is carried out within the temperature limits specified currently. It is also concluded that modifications to the present temperature correction procedures would improve the accuracy of the deflection method for both day-time and night-time surveys. The test equipment operating procedures and analysis method are described in the Department of Transport's Departmental Standard HD/10/83 and accompanying Advice Notes HA/24/83 and HA/25/83, and also described in TRRL Laboratory Reports, LR833, LR834 and LR835.

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