Screw Piling Revives Versatile 19th Century Invention

Recent Australian developments in screw piling have revived an engineering invention which dates back to early 19th century England. Its use for ground stabilisation has opened up possibilities never seriously considered before.

Victorian engineer Peter Yttrup is a leading exponent of modern screw piling who has developed the concept of screw piling for remedial ground stabilisation, particularly for pre-failure rehabilitation of roads. He presented details of this work at the recent Australasian Structural Engineering conference in Auckland and the 2nd International Conference on Ground Improvement Techniques in Singapore. Based on his experience with the technique, he believes screw piling is a well-founded 19th century idea made fully viable by late 20th century machinery.

Based in Geelong, Peter Yttrup first applied his ground stabilisation screw piling technique to a roadway landslip in the Otway Ranges of south-west Victoria. Since that first job, almost two years ago, he has successfully applied the same method to two other similar jobs. The first job addressed an unusually serious concern; the landslip threatened both a significant access road and, more importantly, a large water main beneath the road.

"If the landslip was allowed to continue, it would have significantly disrupted the water supply to the Western Districts," Mr Yttrup said. "Quick remedial action was needed, and it was not viable to bring in heavy machinery because of the remote location and time constraints. After doing the survey and test drilling, we designed a remedial solution using screw piles and called in the contractors to do the actual job. It was all finished in less than a week - two days to install the anchors, two days to do the concrete, and the job was done."

The contractor, Piletech Turson, was able to deliver all the equipment to the remote site using only one truck and a support vehicle. The job itself was a simple, three-step operation. Firstly, an excavator fitted with a torque head was used to screw the piles into the side of the embankment; a two-man operation with one on the machine and one at the point where the pile went into the ground. Reinforcing mesh was then attached to the tops of the piles to completely cover the embankment, with drain pipes inserted at suitable intervals. Concrete was then sprayed onto the mesh to form a strongly anchored reinforcement against further slippage.

Piletech supplied the piles to the required specification mostly from stock, although piles are also purpose made where the job presents unusual requirements. Each pile consists of a steel pipe with one or more helixes welded on near the lower end, which is angle-cut for easier installation. A simple coupling fits the hydraulic torque head used for installation and can also be used for attaching extensions.

It is the modern hydraulic torque head and its related equipment which Peter Yttrup sees as the key to the future of screw piling. In researching the technology, he has traced its first commercial applications in the UK in 1835, when Alexander Mitchell used screw piling for small off-shore lighthouses. In those days installation involved cumbersome steam-driven machinery, whereas today's installation equipment can be carried on a medium-sized truck.

Over the intervening century and a half, Peter Yttrup has discovered screw piling has enjoyed many applications around the world. These include extensive colonial installations by the Dutch in Indonesia, anchoring of vast power transmission masts in Russia, and World War II construction of military wharves.

"The problem then was the same as it has always been, and still is now, people think of these slim metal piles as flimsy, so they tend not to see the advantages," Mr Yttrup said. "In those days the piles were made of a wrought iron shaft with cast iron helixes. The arrival of tubular products and better welding techniques this century paved the way for the tubular steel shaft and welded steel helix of today's piles. Regardless of the materials, the performance is basically the same. I believe its credibility is growing in the geotechnical engineering community, and I think we will see increasing usage of screw piling as its economy and versatility are more widely appreciated."

Piletech piles are all made of quality BlueScope Steel Australian steel. Generally they are fabricated from black steel, and corrosion issues are addressed by using the thickest-wall tubing available, or hot dip galvanizing.

It would be fair to say that Peter Yttrup is a great advocate of screw piling technology, but it would be wrong to suggest that this is his only interest. As a structural and geotechnical engineer, most of his work involves traditional projects such as building design and bridge work without any use of screw piles. Major projects include the award-winning Uluru Aboriginal Cultural Centre and the Brambuk Building at Hall's Gap in Victoria.

Even so, there is no doubting Mr Yttrup's belief in the future of screw piles, especially for preventative work on roads threatened by landslip. Even without the remote location and urgency of the original Otways job, he sees it as an extremely cost-effective treatment to stop impending landslip before serious damage is done.

"The potential of the South West Water case study is that it shows what can be done in road rehabilitation before it slides," he said.
"This is an excellent example of active geotechnical engineering intervention, and it does not even require a great geotechnical knowledge to apply it. The cracking can be detected as part of the general inspection regimes of most road authorities and the treatment can be applied quickly and economically".

"Cost-wise, we are talking typically less than 20 percent of the cost of other solutions, and you don't lose the use of the road because you don't allow it to fail before repairing it.With this technology available, it makes no sense to fill stress cracks with bitumen and wait for them to worsen, when you can effect a permanent solution so easily."

Landslip rehabilitation is only one of countless applications for screw piling systems. According to Peter Yttrup, today's hydraulic torque heads provide the convenience of "a big mechanical screwdriver" to make this technology convenient and effective in a vast range of applications. He sees other ground stabilisation uses for freeway cuttings and bridge embankments, in addition to applications for domestic and commercial building foundations, under-pinning old buildings and even small-scale domestic uses such as quick and easy verandah post supports.

"This is a bit like sky hooks for the ground; when you want to attach something firmly to the ground, you just screw it down with these piles," he said. "It gives you access to tight spaces where heavy impact piling would be impossible, and eliminates problems with impact cracking of nearby structures or noise nuisance for neighbouring properties".

"But the possibilities go well beyond this, and some of them are quite exciting in their scope. For example, we have been working recently on a project with a large basement structure which will be subject to substantial buoyancy forces. Using screw anchors to negate the buoyancy forces can save millions of dollars on a job of this magnitude."

Peter Yttrup is the principal of P. J. Yttrup & Associates, consulting engineers.