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Testing procedure

Large square section piles at the test sites simulated a section of the track bed for the 270km/hour high-speed trains and were chosen to test pile stiffness and lateral loading generated by the trains braking and accelerating.

The 23 square section test piles, designed by CTRL project manager and designer, Rail Link Engineering, were installed at locations at Rainham and Purfleet and driven through soft marshy ground to toe into the underlying flood plane gravel beds. Piles were driven in two rows of four, spaced at 5m between rows, and capped with a 500mm thick slab.

A similar layout, using 600mm diameter continuous flight augered reinforced concrete piles, installed by Amec, was built adjacent to the precast test piles. A high capacity hydraulic jack was installed between the two pile cap slabs to apply an initial 320kN lateral loading, equivalent to a working load of 40kN/pile, followed by horizontal cyclic loading up to 200kN/pile at 0.005Hz for 10 cycles to represent train braking and accelerating forces.

A separate line of piles was also installed in each location to perform static vertical load tests of up to 2000kN. The Aarsleff/ Amec joint venture also applied a vertical pulse loading to a single pile, which was cyclically tested between 300kN and 900kN at a frequency of 2Hz in sets of 1200 cycles to simulate the effect of train axles passing over the track at high speed.

Some of the precast piles were made with internal steel and PVC tubes for the installation of strain gauges, accelerometers, and inclinometers to monitor pile behaviour during the extensive testing programme. Testing and monitoring of pile behaviour in the soft ground was subcontracted to PMC.

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