The strange case of bored piles
Since 1977, Danish standards for foundations have been extremely restrictive with regard to using bored piles as foundations for buildings and other structures. But are bored piles really as inefficient as the standards imply? A new research project is digging into the matter.
In 1971, construction commenced on a 292-metre-long and 13-metre-wide bridge flanked by Furesø lake to the east and Farum lake to the west. The bridge was to carry the Hillerød motorway over Frederiksborgvej, a major trunk road. It was called Fiskebæk bridge, and the new bridge would take the motorway over the swampy ground around the Mølleå stream that meanders from lake to lake from Hettings Mose to the sea.
But on 8 February 1972, the day before the official opening, the bridge collapsed. Without warning, the western motorway flyover fell into the bog. Fortunately, no one was hurt in the accident, but the collapse may have made its mark on later Danish standards.
The contractor, C.T. Winkel, had used used so-called Franki piles (pressure injected footings); a piling technique by which the pile is established by displacing the soil rather than driving the piles down into it. C.T. Winkel subsequently went bankrupt, and since then, Danish standards have not been well disposed towards bored piles.
Makes no sense
As early as the 1970s, Denmark had noted the experience in the UK, where trials showed that bored piles were unlikely to have such a large load-carrying capacity as driven piles.
But that was then. Even though the geology has not changed much over the last few thousand years, piling technology has developed, so that in Denmark today there is a widespread understanding that bored piles can probably carry a lot more than the standards indicate: only 30% of the surface resistance for corresponding driven piles.
Therefore, the Department of Engineering at Aarhus University and COWI have joined forces to quell the myth in a project that will once and for all clarify the issue of bored piles.
"The Danish standard is very restrictive regarding bored piles. It says that a bored pile can only apply 30% of the surface resistance applied for a corresponding driven pile. However, no one in the industry knows where this very restrictive standard text comes from. A look at the measurements and trials conducted in recent years reveals that this doesn’t really make sense," says Professor (Docent) Kenny Kataoka, who is managing the project for the university.
A treasure hunt for data
He is being backed up by PhD student Jannie Knudsen, who is working on the project, and who so far has been buried in detective work to try to find out exactly why it was decided to introduce such strict restrictions on bored piles.
"It's been something of a treasure hunt. Among others, I’ve contacted the Danish Society of Engineers (IDA), Danish Standards and the Danish Geotechnical Society, and I’ve reviewed old memos and minutes of meetings to find clues in the case. So far, I’ve found data from seven different trials with bored piles from the 1970s, but we’re talking about very different methods. Some are made with drilling towers and others are hand-drilled; some are made with linings and others without. However, they’ve all been pigeon-holed as 'bored piles', so it’s hardly surprising that we’ve had problems with the load-carrying capacity of some of them," she says, and she stresses that the way piling work is done means a great deal for load-carrying capacity, particularly with regard to bored piles.
"There’s long been general agreement that this matter needs looking into. It’s time for us in Denmark to get to the heart of the matter," she continues.
Actually, this is something of a Danish thing. A country where we alone are in the habit of using driven piles. It is cheapest, and the piles are mass-produced. This makes them quick and easy to manufacture.
But pile driving may have impacts on the surrounding environment. It makes a noise, vibrations and can lead to displacement problems that can cause damage to surrounding buildings. In fact, other countries hardly ever use driven piles. At least not for structures in major cities, explains Kenny Kataoka.
"It would be unthinkable to drive piles for a new building in the centre of London. The disturbance for the surrounding environment would be just too great. And this is the way things are going. There are increasingly requirements regarding noise and vibration, and therefore, in future, we’ll simply need far more bored piles in Denmark," he says.
Furthermore, the large loads from the tower blocks often demand bored piles as there are physical limits on the dimensions of piles you can drive. But with bored piles, you can sink piles of even very large dimensions.
The project will collect data from test piles already drilled in place, and it will measure several piles to be installed at a site near Randers. On the test field, new piles will be bored into place with embedded fibre optics that will make it possible to test and measure the piles afterwards.
The project will also be monitored by a group of contractors, who will share experiences with different types of bored piles.
"Everyone’s been very helpful and interested in the project, and I think there’s widespread belief that we’ve been missing information on the area for some time," says Jannie Knudsen.
The project is being conducted in cooperation with COWI and Per Aarsleff A/S with funding from Innovation Fund Denmark and the COWI Foundation.
Usually established by drilling a lined hole in the ground, in which the piles are reinforced and cast with concrete in situ. The lining is usually removed at the same time as the casting takes place.
Pre-cast piles struck directly into the ground using a pile driver. Driven piles can be used in both mini-scale and up to major construction projects, but there are noise and vibrations when the machines drive the piles into the soil.
Displacement piles, where the soil is displaced in connection with establishment of the pile. First a closed steel pipe is driven down to the desired depth, after which the concrete plug at the bottom is released to form an enlarged pile foot. Then the pile is reinforced and cast with concrete, while the steel pipe is pulled up.
Title: Soil-pile interaction for bored cast-in-place piles in stiff clays and soft rocks
Budget: 3 mio. DKK
Time frame: 2018-2021
Partners: Aarhus University, COWI, Per Aarsleff A/S, Innovation Fund Denmark, COWIfonden.
Phone: +45 41893201
PhD student, COWI
Phone: +45 23626364