Fugro Aperio is exploiting new developments in ground investigation radar technology to widen its application. Simon Brightwell reports.
Powerful, portable and versatile, ground penetrating radar is a prime tool in unravelling the arrangement of the shallow subsurface, whether a wall, floor, abutment, highway pavement or track ballast.
The latest piece of kit is a palm-sized radar antenna, comprising a transducer and an encoder wheel for positional control. Producing crisp, high quality imaging, this 2GHz antenna is small enough to reach structural elements that would be out of bounds to previous radar hardware.
The compact size is of great value for investigating buildings where access may be restricted to gaps in a false ceiling or below a raised floor. The small footprint is also useful for concrete structures such as bridges, dramatically improving the investigation of slender elements such as columns and beams.
The Fugro Aperio structures team is using the new antenna at a major London museum where it is investigating the integrity of masonry walls.
“We are scanning walls up to 5m high that are partly obstructed by priceless artefacts”
Mark King, Fugro Aperio
“We are scanning walls up to 5m high that are partly obstructed by priceless artefacts and we need to work discreetly without disturbing visitors, ” says project manager, Mark King.
“By mounting the new antenna on a lightweight telescopic pole we can cover the full extent of a gallery wall in just a few minutes without the need for scaffolding or ladders.”
Limited penetration ranges have restricted the use of radar in many civil applications.
Loss of performance in most bound materials beyond depths of about 1m has restricted use on thicker structures. Classic examples are retaining walls, tunnel headwalls and pad foundations - all critical structures where thickness may be variable, unpredictable and unrecorded.
Fugro Aperio’s structures team has borrowed from geophysical colleagues involved in seismic investigations, applying several of their data acquisition and processing principles to enable radar to “see” deeper and more accurately.
A recent development project for a rail client clearly resolved the back of concrete tunnel headwalls at depths in excess of 1m, measuring layer thickness to an impressive tolerance of plus or minus 2%. The approach involves taking a large number of readings at each measurement point to improve signal to noise ratio. The transmitter and receiver units are also separated to enable calculation of a radio wave velocity without the
requirement for calibration coring or exposure.
Road network survey
Fugro is also involved in the surveying of a road network eight times bigger than the trunk road network of England and Wales.
It has a three-year contract to report pavement thickness and material type to the California Department for Transportation, based on surveys of a 80,000 lane kilometre network. It involves using Fugro Automatic Road Analyzer (ARAN) vehicles to gather data.
This major investment comes as governments face considerable financial pressure to cut non-essential spending.
“Engineers at Caltrans believe that, to keep the network in good shape, and to get the most from every maintenance dollar spent, they need to know how thick their roads are, what materials are below the surface and where the major changes in construction occur,” says Fugro Aperio head of transport Mark Thomas. Data being collected and analysed will contribute to cost-effective, long-term prioritisation of public resources.
- Simon Brightwell is a director and head of structures at civil and structural investigation specialist, Fugro Aperio.
Image ref: Caltrans data example
Description: ‘Radar data and interpreted cross sections from the Caltrans survey covering over 80,000 lane km of the Californian highway network.’
Note: Vertical green lines on bottom plot indicate construction changes.