Auckland entrepreneur Daniel Coats set up Helix Flight Machine Manufacturing to overcome that problem.

A typical modern screw-pile consists of a steel tube with one or more single-turn helixes (or flights) welded on to the tube. Like a large disk, the helix spreads axial loads from the tubular pile to the surrounding earth. This means screw piles can resist both compression and tension loads, as well as overturning moments and side loads. They are a popular choice for structures like electri-cal transmission towers and wind turbine towers, and they work well in marine applications. According to Mr Coats,

THE TURN OF THE SCREW

screw pile foundations in Christchurch out-performed other types during the 2010/2011 earthquakes and because they can be driven in tight, awkward spaces without traditional brute force pile-driving, they are an attractive option for underpinning existing structures.

Screw-piles are driven into the ground using a hydraulic drive system not unlike a scaled-up battery screwdriver: A hydraulic motor forces the pile to rotate, while a hydraulic ram applies a carefully calibrated vertical force. This drive system is usually mounted on a digger. Often, the system can drive the piles at any angle between horizontal and vertical.

SCREW PILES CAN SOLVE MANY INTERESTING PROBLEMS. BUT HIGH QUALITY IS IMPORTANT, AND IN SOME PARTS OF THE WORLD, HIGH-

QUALITY SCREW PILES CAN BE HARD TO GET.

Writer Kevin Cudby

This means screw piles can be used for bracing and for retaining walls, as well as for ordinary foundations.

The quality of a screw pile foundation depends on the quality of the helical flight. A good screw pile, correctly driven, will advance exactly one pitch per turn. This minimises ground disturbance: the helix can only disturb the ground along its own path. If the pile has multiple helixes, each helix precisely follows the path of the previous one. To achieve this, the pitch angle must be uniform, all radial lines across the helix must be per-pendicular to the shaft and the inside and outside of the helix must be circular. If the

CIVIL STRUCTURAL

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helix is badly shaped it may weaken the ground, or perhaps even tear it up, like a bolt stripping the thread out of a nut.

Screw piles are not usually available off-the-shelf. They tend to be designed and built for specific project requirements. Screw pile manufacturers tend to concentrate on local markets, sometimes offering products designed with local conditions in mind. For example, some North American manufacturers specialise in piles for frozen ground. Manufacturers tend to use home-built machinery for cutting and forming flat steel plate into helical flights, which resemble large outside diameter radial split washers.

Mr Coats says the trick is to make a perfect helix. Helix quality around the world varies quite significantly. Screw piles in New Zealand are excellent, but in other parts of the world, helix quality varies from reasonable to pretty bad, invalidating many screw pile designs.

Mr Coats has been involved in the screw pile market for more than 15 years. In 1998 he established a company called Piletech to produce screw piles for the New Zealand market. While developing Piletech, he regularly communicated with

The new machine would be precise and sophisticated, but no one would ever call it dainty. The biggest version can form helixes up to four metres in diameter from steel plate up to 40 millimetres thick. However, the biggest chunk of the development budget was spent on the computer control system. That required some pretty complex software, Mr Coats says.

Research and development funding from Callaghan Innovation helped bring the project to fruition. Callaghan Innovation provided 50 per cent of the $10,000 dollars budgeted for the initial feasibility study, followed by another grant covering 40 per cent of a six-figure development programme. A lot of companies are doing good R&D, but they arent getting Government funding, Mr Coats says, adding that Callaghan Innovations application process is time consuming. Once you jump through the bureaucratic hoops, you can get access to R&D money.

The result of all that research and development was the Ultra Adjustable Helix Forming Machine, for light, medium and heavy-duty applications. The light-

Opposite: Manufacturing screw piles. Photo: Piletech.

Left: Installing screw piles on site at Hutt Hospital in Wellington. Photo: Piletech.

overseas screw pile manufacturers. After Piletech was sold to Fletchers, Mr Coats was able to think about international opportunities.

In 2012 he set up Helix Flight Manufacturing Machines Limited (HFMML) to develop and manufacture the ultimate helix cut predicting software and forming machine technology. This was not as simple as it sounds. Most screw piles are tailor-made. Engineers adjust parameters such as the size and pitch of the helix to suit specific ground conditions and structural requirements. Sometimes, a project may need several different types of screw pile to suit variable ground conditions, or because different parts of the structure put different loads on the piles. The HFMML machine had to be easily and quickly programmable to make custom-designed helixes with very precise geometry.

SCREW PILES IN NEW ZEALAND ARE EXCEL-LENT, BUT IN OTHER

PARTS OF THE WORLD, HELIX QUALITY VARIES FROM REASONABLE

TO PRETTY BAD.

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Below: Demonstration of the loads working on the upper helix surface during installation. Image: Helix Flight Manufacturing Machines.

duty and medium-duty versions are most in demand, Mr Coats says.

The company began shipping com-plete machines earlier this year and all the machines they have made so far have been sold to overseas customers, including in Japan and North America. They are also developing a simplified, less adjustable machine for high through-put production of standardised piles. Alongside its machines, the company provides software for designing the blank ready for forming.

The adjustable machines deliver more than just superior helix quality. Compared with existing technology, Mr Coats says they are easier and faster to set up, which dramatically improves manufacturing efficiency. In addition, because they pro-duce helixes with very consistent pitch form, it is easy to automate the welding process, eliminating a potential source of human error.

Screw pile manufacturers are not the only people checking out the machines. According to Mr Coats, the technology is popular with firms that make sectional helixes for a range of products such as augers and industrial conveyors.

Mr Coats is embracing the challenge of earning a return on the R&D investment. He anticipates strong demand for the machines because of growing interest in screw pile foundations. Although screw piles with cast-iron tips and wooden shafts were popular in the 19th century, they fell out of favour in the early 20th century. The development of hydraulic

machinery has revolutionised the field, Mr Coats says.

The modern tube and plate screw pile performs well in many situations where ordinary piles fall short. The large-diameter helix makes them ideal for use in weak soil such as marine sedi-ment and their ability to resist vertical forces has made them popular in high latitudes, where freezing groundwater tends to heave conventional piles out of the ground. Driving a screw pile is quick and relatively quiet, and involves minimal vibration. There is no need for

concrete and no spoil to get rid of, which represents a considerable cost saving in remote locations.

Screw piles can be backed out of the ground just like undoing a woodscrew, which makes them suitable for temporary installations. They can also be used in tight spots where there simply isnt room to drive a conventional pile. Long piles designed to be installed in confined places can be assembled by joining multiple sections together. According to Mr Coats, Globally, its a multi-billion dollar market.

COMPARED WITH EXISTING TECHNOLOGY, THEY ARE EASIER AND

FASTER TO SET UP, WHICH DRAMATICALLY IMPROVES MANUFAC-TURING EFFICIENCY.

S H A F T S K I N F R I C T I O N

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28 Engineering Insight Volume 15/6

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