An investigation into the potential level of disruption from the controversial Hampstead Heath dam project has begun.

Contractor BAM Nuttall is preparing a survey – due to take place at the end of this month – which could determine how many HGV lorries will drive onto the Heath if the £15million flood defence works goes ahead.

The contractor wants to use as much soil from the Heath as possible to minimise the number of heavy vehicles driving to and from the open space to import soil to the site.

Engineers will take samples of soil from the ponds, the dams and the surrounding areas to identify its chemical properties.

The City of London Corporation, which runs the Heath, say the project to raise the dams by as much as 18ft will stop them failing in an extreme storm.

The works are fiercely opposed by members of the Dam Nonsense campaign.

Jeremy Simons, chairman of the City of London Corporation’s Hampstead Heath Management Committee, said: “These important works will feed into the design process for the ponds project, which will make the ponds safer – as well as improving their water quality and biodiversity.”