FUGRO ENGINEERING Services recently completed site investigations for two access road tunnels at Heathrow Airport.
Work had to be carried out between 2am and 5am to avoid interfering with aircraft and support vehicles at airside locations.
Accurate location of the interface between the various geological units, including the underlying clay, was crucial. Fugro used a combination of static cone penetration testing (CPT) and geophysics to contour the interfaces along the line of the proposed tunnels.
CPTs were used first at 5m centres to accurately determine the interface at each test position. This was followed by geophysics, involving ground probing radar, resistivity, electromagnetic and seismic refraction surveys.
Fugro modified the hydraulic cylinders on one of its 20t CPT trucks to increase the thrust capability to get through the tough ground, which includes very dense gravel caused by aircraft moving around the airport.
This required putting extra ballast on to the truck, bringing it up to nearly 26t. Cones were pushed to more than 200% of their standard full scale output - equivalent to 12t on a 36mm diameter cone penetrometer.
A total of 400 CPTs and 14,000m of radar survey lines were performed over the six-week contract.
Fugro has carried out a number of projects at UK airports using CPTs.The firm says the technique is used as a primary investigation tool because of its high rate of production, with a 15m test taking under half an hour. This is particularly useful when only a few hours are available to work on runways and other airside locations.