First O-cells in Fundex piles
Construction of a new wind farm by Enercon GmbH near Barenburg, Germany brought forward several geotechnical issues. The site is located within an onshore oil field. A number of wind turbines had already been constructed with several more underway, founded on large excavated raft foundations.
However, due to the location of the project within the German oil fields, many of the remaining locations would mean excavation of potentially contaminated materials. The option of a displacement pile foundation solution would be an alternative as this would not produce contaminated spoil.
One pile type considered was the Fundex screw displacement pile, which is a cast-in-situ reinforced concrete pile. A sacrificial cast iron boring tip is sealed to a hollow pipe drilling mandrel, installed by using torque and crowd force from a rotary head on the hydraulic piling rig. By monitoring the hydraulic pressure applied to the rotary table, a relative measure of ground resistance can be obtained. Once the pile tip has penetrated sufficiently into the desired bearing strata and/or the desired depth has been reached, a steel reinforcing cage is lowered into the hollow section mandrel and concrete is placed. The mandrel is then extracted by oscillation to leave the pile tip, concrete and cage in place, creating a full section cast-in-situ concrete displacement pile.
A single 330 mm diameter O-cell, capable of mobilising a total of 3.9 MN in each direction, was attached to the base of the reinforcement cage and lowered into the mandrel. As with the normal construction technique, all instrumentation connections, hoses and pipes required for testing were attached to the inside of the reinforcement cage. Since this technique does not require the use of a tremie pipe, the size of the O-cell used is only limited to the internal diameter of the mandrel.
Traditional top down tests were also to be performed on identical piles in close proximity providing a good opportunity to correlate the results from the two test methods.
