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Underground surveying goes overground

Radar and geophysical surveying is coming of age. New standards and guidelines plus a higher profile all bode well for the future of the sector. Mike Walter reports.

Efforts are being made on a number of fronts to raise the profile of radar and geophysical testing and to give the sector's credibility a much needed boost.

New documents are due giving guidance on carrying out surveys and a number of specialist contractors are organising seminars to promote the techniques they employ.

A working group set up last year to look at the use of ground radar for road monitoring has helped to clarify which pavement conditions can accurately and reliably be identified using the technique. Results from tests carried out by the working group form part of an amendment to the Highways Agency (HA) Design Manual for Roads & Bridges, volume seven, section three, part two. The amendment was published by the Stationery Office in February 2001.

The working group comprises the HA, research body TRL and half a dozen specialist contractors and users. It was set up primarily to improve the quality of surveys and encourage further research and development into the use of ground radar for monitoring the condition of roads.

Tests were carried out on a range of pavements to see if ground radar could accurately and reliably be used to identify different subsurface characteristics. A total of 13 pavement features including cracks, layer thickness and the condition of steel in concrete are listed in the design manual and each feature is given a score. This is to indicate which features can truly be identified using ground radar, and which cannot. It also advises where radar can be used to confirm results gained from another source and where radar results should be treated with caution.

Stuart Wood is group manager of the pavements department at TRL. He says: 'The working group has been instrumental in bringing good practice to the fore. There are many specialist contractors working with subtly different pieces of equipment.

Bringing contractors together means we can agree on a common standard of practice.'

Impulse Geophysics from Bedfordshire is one of the specialist subcontractors on the working group. Owner Bob Milligan says that many in civil engineering have mixed feelings about radar and geophysical testing and the working group can do much to change perceptions for the better. 'Traditionally, very little money has been allocated for site investigation and that money is usually spent on digging trial pits and bore holes. Establishing guidelines and common standards of practice for the use of radar and geophysical surveying actually gives the techniques' potential user base added confidence.'

Another best practice document published this year is British Standard BS10175, titled 'Investigation of potentially contaminated sites - Code of Practice'. The document provides guidance and recommendations for carrying out site investigation and says the standard can be used in conjunction with other codes of practice for combined investigations including geotechnical investigations.

The document gives grounds for optimism, says Dr Kath Watson of STATS Geophysical.

'Although only brief mention has been made of non intrusive testing in BS10175 the fact that it has been included at all is very encouraging, ' she says.

Particular areas of growth in environmental geophysics, she says, include the development of landfill leak detection systems, time lapse monitoring of changes in groundwater saturation and the introduction of fully automated two and three dimensional surveying techniques.

'More and more civil, structural, geotechnical and environmental engineers are coming round to the idea that geophysics is not so bad, ' says Watson. 'People tend to think that geophysical investigation is expensive but if it is applied on a potentially contaminated site it could save expensive to solve problems later on.'

One of a number of specialist contractors hoping to generate increased use of radar and geophysical surveying is Aperio, which is hosting a one day pavement assessment conference in Birmingham in May. Topics will include a look at the use of impulse radar and high speed condition surveys.

Many surveying specialists using radar and geophysical means agree that the use of 'non destructive' techniques does not necessarily remove the need to carry out traditional destructive methods of testing. GB Geotechnics' director George Ballard says: 'Radar is not a replacement for taking cores but sits within a whole suite of investigative techniques. Non destructive methods can give you the greater picture more quickly, but a hole often needs to be dug to confirm what a survey tells you.'

Use of more than one radar or geophysical technique as part of a survey allows results to be verified. Geotec Surveys managing director Nigel Knowles says: 'No one piece of equipment is standalone. Comparing the results of one technique with another allows you to accurately confirm, for instance, the position of utilities beneath the surface, ' he says. 'Radar detects changes in the density of an underground environment but it can't always accurately tell you what it has found. It could have located a cable, pipe or a buried hard hat.'

There are limitations to some techniques. Knowles says that radio frequencies can locate metallic objects, cast iron or steel pipes and power cables within the ground, but cannot, for instance, locate a void.

Aperio managing director Jon Baston-Pitt adds that those involved in radar and geophysical testing must make sure that the wider engineering community can understand and interpret survey results. 'As a sector we have got to provide information which is a lot more auditable, so people can pick up a report and understand where our findings have come from and see information in a context they can understand, ' he says.

One of the major benefits of radar and geophysical methods is that the techniques can be used on heavily congested urban sites without disrupting traffic flow, says Geotec Surveys' Knowles. 'Our surveys are carried out without having to reinstate the ground, they leave no scars on the pavement, and no barriers need to be put up around our survey site.'

Geotec Surveys

Ground probing radar was one of a range of techniques used to carry out a utility mapping survey of a pedestrian precinct in Tunbridge Wells. The precinct is being repaved, trees planted and new lighting columns installed.

Geotec Surveys was commissioned to provide a comprehensive account of the underground environment from which the scheme's consulting engineer based its street design.

Utilities including power cables, water and gas mains, telecom ducting and sewers as well as subsurface hazards needed to be accurately identified and located.

A Gssi Sir2 ground radar device with a 400 Megahertz antenna was pushed the length of the precinct and across its width. The Sir2 detects plastics, fibre optics, clay, asbestos cement and concrete, and conductive and metallic services. Real time results were shown on a portable screen.

Other non destructive techniques were used to find electromagnetic fields. Geotec Survey's managing director Nigel Knowles says: 'Attempting a survey without radar would have meant incomplete data leading to reliance on statutory utility drawings that are known to be less than accurate.'

Stats

Geophysical Buried tanks, old foundations and a gas main were among the obstructions and utilities successfully located on a potentially contaminated site in Bedfordshire last year.

Specialist consultant STATS Geophysical used electro magnetic testing as part of a thorough geoenvironmental investigation of the 1.5ha site which is now home to a large open car park.

The site was formerly used by a gas works and had seen extensive development since Victorian times.

An electromagnetic mapping survey included the collection of data from around 3,000 measurement stations using a Geonics EM31-D Ground Conductivity Meter on a 2m x 2m orthogonal grid. At each measurement station the equipment recorded the bulk electrical conductivity of the ground and the phase difference between a transmitted and induced electromagnetic field.

The illustration alongside shows the in-phase response recorded to a depth of between five and six metres below ground level. The information provided by the geophysical survey was used to target specific areas of concern during a phase of intrusive investigation.

Zetica

Oxfordshire based geophysical testing company Zetica has developed a method of detecting unexploded ordnance in partnership with BAE Systems.

The system, known as SiteSafe, involves the insertion of a downhole magnetometer into the ground to a depth of around 10m using a CPT rig. The magnetometer detects magnetic objects ahead of the probe covering a radius of around 1m.

Piling positions can be cleared by probing appropriate points on a design engineer's layout and site wide clearance can be achieved by probing on a grid system.

SiteSafe generates digital data to allow sensitive remedial planning, verification and clearance. A further survey confirms that adequate clearance has been carried out. The system has been used to confirm safe sub-surface conditions on a development site in the Thames corridor to a depth of 13m.

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