A new auger displacement pile system has recently been tested on two contracts in the UK, carried out by Norwich- based geotechnical contractor May Gurney (Technical Services).
Developed by Spanish piling equipment manufacturer Llamada, the system combines the low noise, vibration and penetration rates of continuous flight auger piling with the reduced spoil and improved bearing capacity advantages associated with auger displacement systems.
May Gurney already holds the UK licence for the Belgian- developed Omega auger displacement pile system. The firm says the innovative design of the Llamada system, comprising two contra-rotating augers, overcomes the problems of auger damage or poor production rates experienced in dense or hard material using existing auger displacement systems.
And while CFA is proven in such dense UK soils, the method produces spoil or causes side loading of the auger, followed by loosening once the layer is passed, says May Gurney.
The system is mounted on a twin rotary head Llamada rig. An upper and a lower auger fit together to form a continuous flight but can also rotate independently.
As drilling starts, the upper and lower sections are locked together and the auger acts in 'CFA mode', producing a small amount of spoil at the surface. As drilling continues, the augers are separated and the rotation of the upper one is reversed. Any material on the upper flight is transported down the hole and material on the lower auger moves upwards.
The tapered lead section of the auger initially displaces the ground and further lateral displacement takes place between the upper and lower sections. By joining and rotating the sections in the same direction - CFA mode - the auger can punch through any hard ground, such as dense granular material, encountered during drilling.
After the dense material has been penetrated, the auger is simply split again and reverts to the displacement system. When final depth is reached, the pile is formed with the augers joined and withdrawn as concrete is pumped down the centre of the hollow stem and reinforcement placed after concreting. The system can form 400mm, 500mm and 600mm diameter piles.
Field trials acted as an assessment of the new system by comparing penetration rates and results from maintained load tests with other auger displacement and conventional CFA systems, under the same conditions.
The method was first used commercially in the UK at a site in Halstead, Essex. A relatively short, 400mm diameter test pile achieved 'more than adequate bearing' under loading conditions within dense river terrace gravel, according to May Gurney.
This and the fact that only a small amount of spoil was produced meant that the system was chosen for the entire project. Some 157 piles were installed down to a maximum of 6m, with no need for spoil removal. Auger wear was reduced due to displacement effects taking over a larger section of the auger, keeping time and repair costs to a minimum.
On a contract at the Alexander Army Barracks in Pirbright, Surrey, over 250, 400mm diameter and 4m to 8m long piles were installed through dense sand at a rate of around 60 a day. Again minimal spoil was produced, with faster floor slab construction as pile heads were smoothed to the correct level without the need for pile breakdown.
With the ability to achieve high capacity at shallow depth in granular material coupled to the penetration capability of CFA systems, May Gurney believes that the new system is set to make a big impression on the UK displacement piling market.