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BOMBS AWAY

SITE INVESTIGATION Second World War bombs detected and rendered safe during a major Cambridgeshire site investigation programme averted a potential disaster

The results of the investigation on the site of the new town of Northstowe made for some serious reading. The 425ha area in Cambridgeshire, which will provide 8,000 homes, includes the Defence Estates owned Oakington Barracks, which was an active Second World War airfield before being transferred to the Army and used as a training ground.

Having submitted an outline planning application for the new town in July 2005, town developer Gallagher Estates began a three month site investigation programme in the latter part of 2005.

And it was at an early stage that project consultant WSP Environmental and Gallagher identified the potential threat from unexploded devices as a result of the site's colourful history and commissioned a desktop threat assessment from ordnance detection company Bactec International.

The former military airfield had sustained at least 12 bombing raids by the Luftwaffe, seen emergency landings from aircraft in trouble and included several aircraft crash sites.

The base was also home to numerous air defence areas and had been used for explosive ordnance training, storage and testing.

Bactec produced a zoned map for the development, identifying low, medium and high risk areas for encountering unexploded ordnance during intrusive engineering works. It identified the main areas of threat as unexploded German bombs and British ordnance.

This included aerial dropped ordnance, land services ammunition such as mortars, hand grenades and anti aircraft shells deployed for the defence of the airfield. There was also the possibility of finding both small arms and training ammunition used during the war and afterwards up until the 1990s.

Bactec technical sales manager Kevin Kneebone says: 'There is no legislation in the UK directly linked to considering and dealing with unexploded ordnance risk during site remediation, site investigation or development. But the Construction Design Management (CDM) regulations require contractors to consider all underground risks along with health and safety regulations and the duty of care.

'This is necessary for the development's future, which could have been blighted, and more importantly for the people involved on site.' Bactec, Gallagher and WSP Environmental developed an explosive ordnance risk mitigation strategy to ensure work was done in a safe environment. This included trial pits, archaeological investigations, boreholes, window samples and cone penetrometer testing.

The strategy included non-intrusive and intrusive magnetometer investigations alongside explosive ordnance safety and awareness briefings. The surveys used bespoke geophysical data interpretation software to provide areas free from unexploded ordnance before the investigation.

From the buried anomalies, Bactec classified potential explosive ordnance. The first item was found to be a 500lb (227kg) British bomb. A safety exclusion zone was imposed, local police informed and an RAF bomb disposal team removed the inert bomb from site.

But two days later a second target was identified - another British 500lb bomb and this time live - which was not prepared to go quietly. After evacuating to a 500m radius the RAF performed a controlled explosion, leaving a crater more than 10m wide.

However this was not the end of the discoveries. The RAF then found two further British bombs close to the crater, even bigger at 1,000lb. The evacuation area was extended to 1.4km before the two could be destroyed in another controlled explosion.

Kneebone explained: 'We used tried and tested technology from city construction sites to greenfield and brownfield developments. It is pleasing the client considered the risk at an early stage ensuring no unexploded ordnance was encountered accidentally, which could have had potentially catastrophic consequences.' There is much speculation as to why the bombs were there in the first place, as their depth and angle meant deliberate burial was unlikely. One possible explanation is they were jettisoned by a bomber prior to an emergency landing or crash.

The clearance programme will continue alongside development to ensure it can be certified free from unexploded ordnance.

Kneebone says although the construction industry in general now considers the risk posed by unexploded ordnance, all too often this is overlooked at the initial stages of the project during site investigation and remediation.

He thinks the value of low cost 'on call' threat assessments, desk studies and risk maps should not be underestimated as the first stage of dealing with this potentially explosive problem.

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