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Straight flush for vibro poker

Concrete plugs in the base of vibro stone columns provided a low-cost and environmentally acceptable foundation solution on an industrial brownfield development in north Kent.

Ground improvement techniques are gaining prominence as more brownfield sites are redeveloped and brought back into use. Ground improvement contractors are rising to the challenge by finding innovative variations on the basic vibro stone column methods, which in turn are increasing the applications for ground improvement techniques.

A good case in point is construction of two large industrial warehouses at Crayford in north Kent, where a novel modification to stone column technology has overcome a difficult technical issue and saved the client 30% on foundation costs.

Holmes Building's site at Kennett Road was once a clay pit that was infilled in the 1950s. It is believed these operations led to contamination with hydrocarbons and other minerals. The ashheavy generally granular fill is up to 5m deep and overlies gravel and then chalk. Because the chalk is an aquifer, there was concern that foundation work could dramatically increase downward migration of contaminants from the fill into the chalk.

The North Kent Coast railway, which runs along one side of the site, provided an additional constraint. Railtrack placed tight restrictions on vibrations which precluded use of driven piles.

Consultant Peter Brett Associates developed and put out to tender a scheme using vibro concrete columns as a means of transferring loads into the chalk. The idea was to use 650 low-slump vibro concrete columns to support both the frame loads and floor.

Below the floor area enlarged head columns placed to demanding tolerances of only 15mm would reduce the need to break down pile heads and excavate into the potentially contaminated ground.

The rationale behind this approach was that no contaminated spoil would be generated, the low slump pumped concrete would provide a good seal between the concrete and soil, and because the columns have a relatively large toe, penetration into the chalk to achieve the required bearing would be less than with other foundation techniques. This solution was designed to take account of the client's constraints, particularly in respect of floor performance and the twin problems of aquifer contamination and the proximity of the railway.

Pennine Vibropiling submitted a conforming bid, but proposed an alternative using concrete plugs at the base of conventional stone columns, formed at 2.6m centres.

From a loadbearing point of view the ground could have been treated entirely with stone columns, explains Pennine's Derek Taylor. Stone columns would not increase the hydraulic connection between the fill, gravel and chalk over that which already exists, and long-term the area will be covered by hardstanding which will in fact reduce the potential risk of contamination.

However during construction Pennine recognised there was an increased risk of downward migration of contaminants. To satisfy the Environment Agency, Pennine opted for a plugged stone column - a technique the company had developed on a geologically very similar contract in Hayes, Middlesex, last year.

In this approach the bottom of each column is sealed with a 1-1.5m concrete plug positioned just below the base of the fill, with a conventional stone column formed above.

To achieve this, Pennine adapted its standard vibro poker to enable a small charge of concrete to be pumped into the base of the column. On-board instrumentation of the installation process determined column penetration and when sufficiently dense ground had been reached.

To avoid causing hard spots below the floor area, it was important to ensure concrete plugs were at least 2m below ground level, and the best way of checking this was simply by dipping the columns with a tape after the concrete had been placed. Stone columns, typically 2-5m in depth and averaging 3m , were formed above. Floor slab design allowed only for 20mm settlement at a design load of 50kN/m 2.In short, Pennine's solution met exactly the technical rationale behind the original scheme, but using stone in place of concrete over the top of each column significantly reduced costs and greatly simplified subsequent floor slab construction, leading to major savings for the client.

The method also satisfied Railtrack's vibration restrictions.

Essentially Railtrack required vibration monitoring throughout any work within 17m of its low embankment that runs beside the site. Railtrack stipulated a maximum peak particle velocity of only 10mm/s, which Pennine says was not exceeded during the work.

Pokers have a very high frequency, says the company, which means vibrations fall off rapidly -much more so than with driven piling. Interestingly, vibration monitoring indicated that inserting the poker generated vibrations comparable with those from a passing train.

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