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Scientific breakthrough

PILING & DEEP FOUNDATIONS

The project to form foundations for a world-leading Oxford University chemistry laboratory has already notched up a couple of firsts.David Hayward reports.

A new type of plunge column guide frame was tested on an Oxford University site last month in a bid to achieve faster operation with greater accuracy.

The frames, which boast adjustable rams and rollers, are an innovation of geotechnical contractor Bachy Soletanche. Appropriately, the scene of their debut is the site of a state of the art chemistry research laboratory.

Their conventional predecessors - double section guide frames manhandled into final position over just-poured concrete piles - are 'somewhat outdated' says Bachy Soletanche site engineer Thierry Jeanmaire.The steel columns to be plunged would then be lowered down within the frame, guided by preset rollers with only manually operated screw adjustment.

The new frames are positioned using external rams with inserted columns then aligned to 10mm accuracy by internal adjustable rollers - 'A simple but clever innovation that takes most of the hassle out of column positioning, ' says Jeanmaire.

This claim is backed by experience, as he has just sunk the 86th and final column in his company's £1.5M contract to form the new laboratory in the university's science park. The £44M facility, described by main contractor Laing Construction as one of the world's most advanced, will have two of its five floors below ground level.

Since May, Bachy Soletanche has been surrounding the 60m square site with a 15.5m deep diaphragm wall, and forming a grid of plunge columns within it. This allows Laing to start topdown construction of basement floors while simultaneously erecting the building's above-ground steel and concrete frame.

Forming the 256m perimeter diaphragm wall was itself novel. It saw the first UK use in diaphragm wall construction of a polymer based solution, replacing bentonite slurry as a temporary panel support medium (see box). Not to be outdone, three of the new style guide frames arrived on site to speed plunge column construction.

Mostly formed using 14m long steel H-beams inserted in the upper section of 35m deep concrete bored piles, these columns will be progressively exposed by Laing's deepening excavation within the diaphragm walled area. They will then act as column supports for both the basement floors and the suspended ground level slab above.

Standard two section guide frames have been in use for more than a decade. A circular positioning plate is placed at ground level over the top of the pile's exposed steel casing.Then, through a hole in the centre, the main guide frame would be inserted with its lower end stopping just short of freshly poured pile concrete. Through this frame the steel column would be lowered into the concrete between internal rollers preset to the exact size of the beam.

Later adjustment of the rollers is impractical, so accuracy depends entirely on locating the main 1t frame correctly over its positioning plate. Hammers and steel wedges are the main tools employed to achieve final positioning tolerances.

Bachy Soletanche's version adopts the same basic principles and, at first glance, is a similar looking frame. The most visible difference is that it incorporates the previously separate positioning plate into the top of the main 7.5m long unit.

The 'simple but clever' bit is the addition of two sets of fully adjustable, hydraulically operated, external rams and internal rollers. An upper group is positioned close to the top of the square section frame with the second set near its bottom end.

As the frame is lowered down the pile casing, its outside rams - acting on the casing wall - centre it within the hole.The steel column is then guided down within the frame between two sets of internal rollers.

Aided by an inclinometer, plus ground level setting out, engineers use a simple mobile control panel to manoeuvre the hydraulic rams and rollers to achieve a column positioning accuracy of 10mm and verticality of 1 in 400. Generally, the upper set of rams and rollers fine-tune column location; the lower set corrects verticality.

At Oxford, the whole positioning operation took, on average, 40 minutes - considerably faster than with the previous style frame. The team claimed a real positioning accuracy of 5mm.

The programme to install three columns a day was met. Jeanmaire maintains that accuracy could have been even better if required. 'But it would have been a longer operation, ' he says.

Early teething problems included cold hydraulic fluid in the mobile power pack needing a chance to warm up in the mornings before rollers would function. Site dust invaded several times and temporarily clogged up the hydraulics network.

Following this successful trial, Bachy Soletanche plans to develop further new style frames catering for a greater range of pile diameters and column sizes. Future modification could involve a quick release valve on the hydraulics for the rams. These occasionally got stuck in the clay ground, exposed in the gap between pile concrete and upper casing.

But overall, the Oxford team feels the new frame graduated with first class honours.

The right chemistry Another UK debut at the Oxford site was the use of a polymer-based solution to temporarily support diaphragm wall panels, replacing the more commonly adopted bentonite slurry.

It is cheaper, easier to use, needs less surface plant and, above all, can be disposed of down a conventional drain. These advantages helped persuade Bachy Soletanche to attempt what was, appropriately, a bit of a chemistry experiment.

Supplied by Texas based KB Technology, the Slurrypro CDP vinyl polymer already has a track record as a temporary support medium in the oil and gas industry. Its success depends largely on tight control of viscosity and alkalinity.

Accurate preparation of the polymer on site was crucial. Water and a pH regulator Protek 100 were added to the chemically neutral fine powder.

This created a highly alkaline solution with a viscosity rating above 50s.

The mixture then helped hold open panels in the 15.5m deep perimeter diaphragm wall in exactly the same way as bentonite slurry. But care was needed to protect the solution's somewhat fragile chemical chain, so more gentle suction pumps replaced conventional bladed models.

Although the raw powder costs £10/kg - nearly 100 times the price of an equivalent bag of bentonite granules - only a fraction of the polymer is required to form the same volume of slurry. Site settling plant for reusing the solution was considerably smaller, though the polymer's real plus was its easy disposal after the walls were complete.

The polymer was neutralised by adding hydrochloric acid and its chemical chain then broken down with calcium hypochloride. Exhaustive tests with local water authority Thames Water confirmed that the resultant solution was safe enough to be simply be flushed down the nearest foul sewer and forgotten about.

Had bentonite slurry been used, sealed wagons would have been needed to ferry this 'chemically contaminated spoil' to a licensed tip, commanding a penalty of up to £50/m 3in disposal charges.

'Use of the polymer instead of bentonite offered a not insignificant saving in overall diaphragm wall costs, ' Jeanmaire says. 'But, more importantly, it had the advantage of removing the need for some 50 lorry movements through this historic town.

And, on larger jobs, I reckon the cost savings and environmental benefits could be even greater.'

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