CPTs at Heathrow Airport Tunnelling Project

July 2003

Fugro Engineering Services Limited have used the Static Cone Penetration Test (CPT) on a number of tunnelling projects prior to tunnelling. The CPT results accurately and rapidly determine the variation in the soils which the tunnel boring machine will bore through. If accurate soils data is not collected then there is a great risk of the tunnel boring machine getting stuck below ground at the cost of millions.

The Fugro CPT has been used at a number of airport developments including Heathrow Terminal 5, Luton, Gatwick and recently at Edinburgh airport. The CPT is being used as the primary soil investigation tool because of it's high rate of production, one test to 15m in less than 30 minutes, when there is only a few hours of work permitted on the runways.

At Heathrow the Thames ballast is very dense and tough to push through. The Fugro cones were used because they are built to with stand the very high thrusts needed to get through the Thames ballast and into the underlying London clay. The already heavy (20 Tonne) CPT trucks had their hydraulic pushing rams modified to increase the thrust capability which required putting extra ballast onto the truck bringing them up to nearly 26 tonnes. The Fugro cones were pushed with the greatest force ever in Europe, in fact the cones were pushed to over 200% of their standard full scale output equivalent to 12 tonnes on a 36mm diameter cone penetrometer. See Fig 1.

The CPT was used because of it's high rate of production, one test to 5m in 15 minutes. The work had to be carried out at night on the airport between 2am and 5am.

Case History from Heathrow Airport

To allow access from the Heathrow Central Terminal Area to the remote Western Apron Stands, it is proposed to drive two access road tunnels under existing taxiways and hard-standing areas. Of crucial importance to the design of the tunnels is knowing accurately the interface between the Terrace Gravels and the underlying London Clay.

Mott MacDonald acting on behalf of the client British Airports Authority commissioned Fugro to perform a combined geotechnical and geophysical investigation to accurately contour this interface along the line of both tunnels.

A two phase approach was adopted. Static Cone Penetration Testing (CPT’s) was used first to physically probe the ground at 5m centres to accurately determine this interface at each test position. This was followed by a geophysical survey.

Ground probing radar, resistivity, electromagnetic and seismic refraction techniques were trialled. This was followed by an extended ground probing radar survey to assess conditions beneath the concrete hard standing areas.

A total of 400 CPT’s and 14,000m of radar survey lines were performed over a 6 week period during restricted night time possessions and clearly demonstrated the value of these rapid investigation techniques where detailed soil information is required in as short a time as possible