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Flight of imagination

A medley of ground investigation techniques is being put into practice on a vast old airfield in the east of England where plans are afoot to transform it into a new sustainable town

A Second World War airfield has challenged site workers to find out its secrets and preserve its historic and environmental features when it morphs into a new town, 10km north west of Cambridge.

Developers English Partnerships and Gallagher Estates instructed consultant WSP Group to carry out a site investigation and it started work in 2005. The company's Geo-solutions team is responsible for assessing the geo-environmental condition of the site and producing a remediation strategy and hydrogeological assessment.

A combination of desk studies (including aerial photography), trial pits, cable percussion boreholes, window sampling and cone penetration testing is being used across the 600ha site.

One of the perils for workers is coping with the possibility of disturbing unexploded ordnance. The site is situated in an area of the UK that was heavily bombed during the war.

"We had to look at the issue of ordnance because of this, but we also knew the site was used by the RAF," says WSP principal consultant Julian Carr. "Bactec International carried out investigation work – scanning for material in trial pits and typically checking down to 4m across much of the site."

Although the team expected to find evidence of ordnance, Carr says there were still some surprises. "Bactec found the first of three Second World War British bombs discovered on site – although some distance away from the barracks area." A 1.6km area surrounding the discovered bombs had to be evacuated and an RAF Bomb Disposal Unit was called in to carry out a controlled detonation.

The site has had multiple owners – English Partnerships acquired much of the land from Defence Estates prior to site investigation getting under way. Gallagher has the option to purchase the remainder from private owners, parts of which are still in use as farmland and a golf course.

As a result, it is not only the discovery of bombs that has proved a challenge. The team is only on site in stints up to four months, which is partly dictated by gaining permission from each of the tenants and landowners, as well as to allow farmers to continue work. For example, schedules have had to accommodate periods to allow cattle to calf in peace.

Excavators, either a tracked 360 machine or a JCB backhoe excavator, have dug over 1000 exploratory trial pits to date. Carr says this means about 1.73M.m3 of earth will be moved about the site during the project.

Before digging, each pit location is scanned with a cable avoidance tool and if any are discovered the location is moved so it is at least 3m away from the utility. Pneumatic hammers help to break up the material if concrete obstructions are found.

Site engineers monitor removal of material in layers of about 200mm to check for these services and samples are logged and taken in glass jars and plastic bags to the laboratory. Any obviously contaminated material – typically found near fuel storage areas on the barracks and airfield – is also sampled.

Cable percussive drilling is being done across the site on a 50m to 100m grid for the runways and then 100m or further for most of the rest of the site. It is typically at tighter spacings of 10m to 50m at more disturbed areas such as the barracks. The rig is sampling to a maximum 40m depth with a second, an Archway Competitor, also working at the old airfield but to a maximum of 15m.

These boreholes provide information on ground conditions, soil and groundwater contamination and allow the team to install gas and groundwater monitoring wells.

Carr says it is important to the team to learn more about groundwater and aquifers on site through the wells and hydrogeological survey so the development causes minimal disturbance to the environment. The information will also help with planning the best drainage strategy for the new town.

The company expects to be on site until at least early 2009 and is prepared to continue beyond that. It is proposed that the new town of Northstowe will be able to house 24,000 residents and includes 9500 residential properties and 136,000m2 of office space. There are plans for the town centre to include six primary schools, a secondary school, leisure facilities and associated infrastructure.

The site is bounded to the east by the disused Cambridge to St Ives railway line that provides the route for the Cambridge Guided Busway, currently under construction.

It is planned that this will provide a link for the new town to Cambridge and Huntingdon. If all goes well for the project, Northstowe will be finished by 2025.

SUMMARY
Geological unit Indicative thickness Aquifer status
River terrace deposits 5-10m Minor aquifer
Gault Clay up to 43m Non-aquifer
Lower Greensand up to 30m Major aquifer
Kimmeridge Clay up to 33m Non-aquifer
Ampthill Clay up to 52m Non-aquifer

SITE GEOLOGY

River terrace deposits underlie parts of the centre and north of the site and cohesive river terrace deposits underlie half of the golf course.

Kimmeridge Clay underlies river terrace deposits beneath the airfield and barracks with Ampthill Clay across the golf course and the north part of the airfield. Marine fossils, selenite crystals and pyrite crystals emerge within the Ampthill and Kimmeridge clays, which are locally fissured.

Ampthill and Kimmeridge Clays underlie most of the site's south portion with Gault Clay across its south east corner. A channel of Lower Greensand is present in some boundary areas.

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