ITALIAN PILING equipment manufacturer Soilmec and its parent company Trevi has just completed a series of tests comparing CFA piling with displacement piling.
The aim was to quantify the advantages of one system versus the other and to study the behaviour of different shaped tools.
The most representative test was undertaken in a job site in Pisa, Italy using a Soilmec R825 rig. The area has soils which are suitable for displacement piling, namely alternating silty clay and clayey sand layers.
Displacement piling consists of rotating and driving a special shaped tool into the ground, displacing the soil laterally or compressing it downward. Concrete is then fed through a hollow stem and flows out through the opening at the auger tip. The auger is kept rotating during extraction and concreting to displace any loosened material.
The set up for displacement piling is very similar to CFA. The piling rig had a 400Bhp Deutz diesel engine, a rotary head developing 250kNm torque and a winch with a maximum 240kNm single line pull and crowd force of 320kN.
Soilmec used 273mm diameter, 6m long drilling rods with hexagonal couplings. Four sizes of tools were tested, ranging from 300mm to 870 mm diameter. Although the average depth of the piles was about 23m, Soilmec used a drill string capable of reaching 25.5m.
Results (see table) showed that while CFA piles are faster to drill, the time needed to remove the larger amounts of spoil associated with the method, and the longer setting up time make pile formation of the two techniques comparable.
And although only preliminary data is available, Soilmec said it could 'confidently maintain the displacement piles had an increased bearing capacity from 7.5% to 15% compared to CFA piles drilled in the same area'.
An obvious added benefit of displacement piling is the minimal amount of spoil, making it 'a valid and recommended option for contaminated sites', the firm said.
Soilmec's engineering department is now working on a new tool to perform displacement piling using smaller (in terms of both torque and weight) rigs, than those commonly used.