The UK Deflectograph has been used for over thirty years to monitor the structural strength of trunk roads in the UK. The Traffic Speed Deflectometer (TSD) is being developed into a research tool for monitoring the structural strength of roads at traffic speed.

The Deflectograph

The Deflectograph originated in France in the late 1950's and was introduced by TRL to the UK in the late 1960's. By the mid-1970's the UK Deflectograph was being routinely used by Highway Authorities to monitor the structural strength of their networks.

The Deflectograph measurement principle is based on two pivoted beams, one in each wheelpath, supported by a T-shaped frame resting on the road surface. The vertical movement of the tip of each beam is measured continuously. The whole assembly operates between the front and rear axles of a rigid lorry. During the measurement cycle the assembly is stationary on the ground, recording the deflection of the road surface as the dual rear wheels approach and pass the point of measurement. Once the maximum deflection has been achieved, the assembly is pulled forward at twice the speed of the lorry to the next measuring point. The measurements are spaced at between 3 and 4m along the road and the vehicle moves at up to 2.5 km/h.  

Measurements of deflection response are normally made under standardised conditions and at defined periods of the year to assess the structure under the weakest likely conditions. Corrections are made for the effect of pavement temperature and axle load.

Methods of interpreting these deflection methods have been developed to enable them to be used, in conjunction with construction and traffic information, to predict the residual life of the road and any need for strengthening.

The TRL Deflectograph is used:

• To provide data to support the development of deflection measurement and interpretation standards;
• As a reference device during annual correlation trials of the Deflectograph fleet.
• To support research into the structural strength of new pavement designs.

The TSD

The first prototype of the Traffic Speed Deflectometer (TSD) was initiated by Greenwood Engineering A/S and the Danish Road Institute in Denmark in 2000 as a potential method for measuring the deflection response of roads at traffic speed.

On the Highways Agency's behalf, TRL have brought to the UK a second prototype of the TSD in 2005. We are developing this into a research tool for the measurement of deflection response with the aim of defining a specification on the basis of which routine network surveys can be commissioned from 2009.

The TSD uses non-contact laser sensors to measure the vertical velocity of the road pavement surface at a range of positions just ahead of the rear nearside wheel. A further sensor, mounted well in front of the loaded wheels enables corrections to be made for any unwanted component of the movement of the vehicle. Other motion sensors enable further corrections for unwanted movement and rotation of the sensor mounting frame.

It is anticipated that the operating conditions for the vehicle will affect the measurements made. Part of the aim of TRL's research will be to define which operating conditions can be standardised and how others can be corrected for.

The ultimate aim of the research is to discover the fundamental relationship between the deflection response, as measured by this machine, and the structural condition of the road pavement. However, in the shorter term, we shall be looking for a robust relationship between the measurements from this device and the maximum pavement deflections measured by the Deflectograph. This could enable a relatively rapid deployment, of similar commercially operational devices on the Highways Agency's trunk road network.

The Traffic Speed Deflectometer is being used:

• To provide data to enable the development of methods of measuring road deflection response at traffic speed
• To support research into the interpretation of such measurements for determining the residual life and strengthening requirements of roads

For more information contact:

Brian Ferne
Telephone: +44 (0)1344 770668
Email: bferne@trl.co.uk