Bombs away

Contaminated land site investigations and risk assessments may cover a plethora of chemical substances, but few take into account explosive ordnance contamination which can affect both green and brownfield sites. George Anderson of BACTEC International Ltd ­ an Explosive Ordnance Disposal (EOD) and landmine clearance company which specialises in the survey, investigation and clearance of areas contaminated with ordnance, both on land and under water ­ explains.

Unexploded ordnance may be encountered on greenfield sites, during the regeneration and re-development of brownfield dockland areas, commercial ports, former airfield sites, railways and oil and gas installations to name but a few. Information about the locations of unexploded and abandoned bombs is available; the accuracy and quality of the data, however, is often questionable. Many items of unexploded ordnance were not recorded and hence may be encountered without warning. Such cases pose a threat to civil engineering and construction works, ranging from piling and foundation contracts to dredging and reclamation operations.

Detection and identification
The detection and identification of aerially delivered ordnance in London Docklands, Chippenham and Dartford by BACTEC, and the unearthing of a 1,000kg bomb in Reading last year, shows that even though 55 years have passed since the cessation of World War II, the threat from unexploded ordnance still exists.

BACTEC has joined forces with site investigation specialist FUGRO Ltd to pioneer and patent a new intrusive explosive ordnance survey technique for the deployment of Down-hole Caesium Pump Magnetometry (DCM). The survey can identify all buried ferromagnetic anomalies within the zone of influence of the sensor, from small items to those commensurate with a 50kg bomb or larger.

The new system has been specifically developed for use on brownfield sites where surface or near-surface magnetic contamination precludes the use of non-intrusive geophysical methods.

BACTEC and Fugro's patented DCM deployment system is a major advance in the search for buried ordnance, munitions and other metallic objects. The system uses state-of-the-art technology to provide survey results in real time, and can be used to survey either specific locations or deployed on a matrix layout for general area clearance.

Depth of penetration
BACTEC's Down-hole Caesium Pump Magnetometry system uses Fugro's Cone Penetration Testing Technology to push a sensor and non-magnetic rods through the ground strata at a controlled rate whilst reading pressure and the magnetic variation in real time against the depth of penetration. This enables advance warning of the presence of a buried metallic object.

The sensor head is mounted onto the end of the rods and receives the magnetic signal. The data processor receives the magnetic and pressure readings and processes the data. A depth counter is also attached to the processor to record the depth at which the data is received. The data is then interpreted by custom software on a laptop PC to provide details of the mass, depth and relative location of suspect object.

The nature of the ground and contamination will determine the quantity of automatic interpretation that can be achieved by the equipment. However, the data can be modelled further and adjusted to reduce background magnetic 'clutter', should this be necessary. In the event of unexplained magnetic readings at unexpected depths, the phased response is as follows:

  • A threat assessment of the specific area will be carried out to discount the reading as spurious or indicate the presence of a suspect object. Adjacent DCM borehole graphs can be studied to determine further indications of unexplained readings
  • Additional DCM boreholes can be sunk to provide additional data for interpretation
  • BACTEC's EOD site manager may decide that only access and investigation will provide confirmation of the presence of ordnance
Having located a magnetic anomaly, BACTEC can deploy an EOD team to access the target. The object will be exposed to allow a positive identification to be made.

The system has been used on a number of high profile projects in the London Docklands, Salford Quays, Manchester, Barking and Canterbury.



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