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.


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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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