SCRIM trucks were back on our highways over the past summer and this pavement management system and surveying has become vital to the cost-effective maintenance and modelling of our highway network. By ALAN TITCHALL.
SCRIM is the acronym for quite a mouthful – ‘Sideway-force Coefficient Routine Investigation Machine’.
The distinctive yellow coloured trucks seen riding over our highway system during the summer months are commissioned by the Transport Agency.
They travel our networks at 80 kilometres per hour collecting pavement information – skid resistance; texture; roughness; rutting; geometry (gradient, crossfall and curvature); and centreline GPS coordinates.
In addition to ‘responding’ to found defects, the NZTA uses the data to monitor highway performance; plan future work programmes; analyse trends; and predict how road conditions might change in the future. Information is stored in the agency’s RAMM database and used for many aspects of network modelling.
The two SCRIM vehicles are unique to this country having been designed to deliver data as required by NZTA. They are built and supplied by Bristol-based WDM, a large manufacturer and provider of highway survey and monitoring equipment, and the largest survey contractor in the UK, serving all UK government agencies and 90 percent of local authorities. WDM also carries out work in South Africa and Australia, amongst other overseas markets, and has been surveying our own roading network every year since 1995, with a focus on the Transport Agency and local authorities.
The surveying is said to be very accurate. The trucks are sent back to the UK each year for a complete overhaul of the technical equipment and every nut and bolt on the truck.
Both the NZ SCRIM+ vehicles are then tested against a fleet of other UK SCRIMs to ensure they are reading correctly after the overhaul. They also go through comprehensive validation testing when they get back to New Zealand. This ensures that the machines are reading accurately and that results can be compared from year to year.
The GPS systems on these trucks mean the Transport Agency can match 20-metre segments of road between different years.
While the truck is on the move, a bar with 20 lasers defines the transverse profile over a road width of 3.3 metres. The rut depth in each wheel-path is calculated from this transverse profile using a simulated two-metre straight edge. This data is used to determine the average, maximum and minimum rut depth, and the standard deviation and the distribution of rut depths every 20 metres.
Longitudinal profile (roughness) is measured using two lasers, one in each wheel-path, together with accelerometers fitted on the transverse beam. Vehicle suspension effects are defined by the accelerometers and deducted from the laser output to provide a road profile. The International Roughness Index (IRI) is calculated from the longitudinal profile using the World Bank Quarter Car model and, again, is reported every 20 metres.
Road texture is measured by three 32 kilohertz lasers, one each in the left and right wheel-paths and one between the wheel-paths. Accelerometers remove most of the vehicle motion relative to the road to provide a stable inertial profile from which the mean profile depth (MPD) is calculated. This MPD is also measured and calculated according to ISO 13473-1:1997.
Survey equipment features a geometry measurement system using inclinometers and gyroscopes to sense the vehicle attitude as the truck travels along the road. Inclinometers measure the forward or back tilt of the vehicle for gradient, and the side-to-side tilt of the axles for crossfall. Gyroscopes are used to sense the movement of the vehicle around corners to calculate the horizontal and vertical curvature. Geometry data is smoothed with a 30-metre moving average and reported every 10 metres.
Oxford Technical Solutions GPS equipment samples the Omni-Star satellite to record the differential GPS coordinates of the centreline. Tilt sensors for crossfall and gradient, together with a gyroscope, provide alignment details when out of sight of the satellite. Information is post processed using LINZ base stations
The survey data, including forward facing video is then sent back to WDM’s offices in Wellington where it is processed, fitted to our roading network and verified before being issued to NZTA.