Ed Owen reports
Keeping blacktop in good condition is a tricky business – even the best roads can have unexpected voids beneath them, which can in turn create potholes and unnecessary expense.
This is bad news on a road – potentially a lot worse on an airport runway. New radar techniques can locate these voids, and allow them to be repaired before they become either expensive or dangerous.
Until recently, most Global Positioning Systems (GPS) allowed for a level of accuracy of half a metre at best. The latest Real Time Kinematic (RTK) systems are coordinated to a set of permanent base stations. Once locked into a base station, the measurements are typically good to sub-centimetre accuracy.
Geophysical specialists Fugro Aperio have linked radar the GPS to generate a powerful new tool. "Integrating the RTK systems with the ground penetrating radar caused plenty of headaches but we have solved problems that nobody else in the industry has overcome yet, and are thrilled with the results," says Mark Thomas, transport infrastructure sector manager.
The survey team load intended survey routes onto a laptop. Once on site, they simply drive the survey vehicle along that path – aligning the vehicle with a symbol on the digital map on their screen.
The new technology has been used to good effect at Gatwick airport. BAA's contractors, Dyer and Butler, commissioned precision radar geophysical surveys to map voids in and below pavements at Gatwick.
Results were used to make immediate repairs, and help in long-term planning for the runway. BAA's lead engineer for building and civils at Gatwick Airport Girish Reesaul. He says: "GPR isprimarily used in identifying voids within various pavement layers, detecting the presence of excess water in the pavement structure, and investigating significant delamination between pavement layers. It is also used to check as-built information confirming the thickness of the pavement's layers where limited information is available.
"The results from GPR are then used in conjunction with data from other investigatory methods to identify areas of poor performance, prioritise maintenance activities and validate the data from our predictive pavement deterioration models, so that an efficient maintenance plan for pavements is in place," he says.
"Survey results from a composite apron (asphalt over concrete) were so clear that we could see voids under the slab joints even though the joints were hidden by the asphalt. We could also see where pipes and cables were buried by picking out the line of the backfilled trenches,"
Local authorities are increasingly keen to charge contractors for overrunning on street works. According to a recent report in Which! magazine, 10 councils charged contractors a total of £1.8M in 2006–2007.
A frequent problem for contractors is discovering that underground utility services are not where they are expected to be. The Health and Safety Executive guidance promotes the idea of searching for utility services before excavation in its HSG 47 guidelines.
But, it is sometimes tricky to discover whose pipes belong to which utility. Plans are not necessarily drawn to the same scale, adding to confusion.
David Gunn of mapping experts Groundwise believes his company has the answer. "Over the years, our customers have been asking if we can organise the myriad of utility responses into one document, but this has proved difficult and expensive," he says.
The company has joined with computer-aided design specialists Geotec Surveys to disentangle the spaghetti of pipes, ducts and cables under the blacktop. "They can take the many utility drawings and convert them into one concise document," says Gunn.
The 1:200 scale drawing resizes all the utility drawings onto one sheet, and overlays their position onto an Ordnance Survey background or other topographic survey. Customers can type in their postcode or geographic information system (GIS) location and the system will drag out all the utility owner maps and dump them into the single CAD drawing.
Geotec Surveys sales manager Neil Bristow says the data put into the Groundwise web "puts into motion a rapid utility record search and CAD drawing process, that delivers a pack of records and a clear drawing of the findings in 14 days".
The company also claims that the information will be invaluable to engineers in the newly created role of CDM coordinator.