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Food Store Foundation

Vibro stone columns and ground mixing are helping keep a tight schedule for a new food superstore in Cheltenham.

As on most projects, the ground is the first challenge at a new supermarket in Cheltenham for one of the big retail chains.

The ground is nearly always the first challenge for a major project in Cheltenham for one of the big supermarket chains.

Around 1.8m of fill underlies the 21,000m2 site, sitting over the fairly stiff Lias clays common to Gloucestershire.

Furthermore, the site is on a slight incline which needs evening out.

“The made ground is also a bit disturbed and loosened because the previous use of the land was for a small factory - and it has been remediated recently,” explains Rob Herring, general manager at the project for Keller Foundations.

He is on site at present for a soil stabilisation and vibro stone column installation programme which will strengthen the ground before the superstore’s steel frame structure starts to go up and car park and service areas are created.

The two-part solution has been chosen by design consultant GDP from Eynsford in Kent in preference to piling the site, or removing and replacing the top layers which would be time-consuming, expensive on fuel, would produce carbon emissions and be locally disruptive.

“Piles always take a long time because the concrete has to cure for around seven days before you can break back the tops and make the pilecaps,” says Keller marketing manager Derek Taylor. “This may not sound long, but it holds things up on the kind of tight programme you have for stores.”

Stone columns can take the loads and can be ready for casting work above almost immediately he adds.

“Loads are not huge on the floor, but the tolerances are reasonably tight. There must be no danger of trolleys wanting to roll off to one side, or shelves to topple over.”

The columns are being formed in clusters for loadbearing points under the structure.

They will have concrete pads cast above them “almost like conventional basic foundations”, says Herring.

A grid of the columns is also going across the rest of the main shopping floor slab area.

“Loads are not huge on the floor, but the tolerances are reasonably tight. There must be no danger of trolleys wanting to roll off to one side, or shelves to topple over.”

Before work began on installing the columns it was necessary to make a working platform for the machines and for this, soil stabilisation has come in handy.

The soil mixing method has been chosen for the rather larger areas of carpark around the store and rather than remove a normal 450mm of soft ground and replace it with granular material, it has been extended across the main store area as well.

The move helps ensure a cut and fill balance for the small amount of earthworks that are included in Keller’s £500,000 package, in line with the client’s requirement.

By stabilising the ground it was possible to make do with 200m of added granular material for the pile mat instead of taking away soil and importing a thick granular layer.

Subcontractor Geofirma, which has worked on a number of jobs previously with Keller, has been doing the soil stabilisation work for the car parks and the piling area.

It brought in a big Wirtgen 2500 soil mixing unit for the job, which is essentially a modified version of the German plant company’s road planer, using a different set of tool picks on the cutting wheel to act as a rotovating and churning unit.

The ground is mixed after an overlying layer of stabilising additive is spread.

It is then compacted directly after the big rotovator has passed.

For the store area, which was treated first so the vibro units could get on with their job, a pile mat was created by adding a granular material in a 100mm layer over the soil, explains Geofirma technical manager John Boyle.

“This was rotovated into the underlying soil together with PFA and CEM II to produce a 200mm thick stabilised layer.

On completion of this, which we call a Firmabase layer, there was another 100mm of granular subbase placed over the top which provides an allowable bearing pressure of 125kN/m2.”

The site overall was treated to an approximate depth of 1.5m, tackling the weak soil “more or less just into the underlying Lias”, says Boyle.

For the work, the soil was stripped out from a section of the site and stockpiled and then replaced in 350mm thick layers which were treated and compacted to dry out moisture and bind the soil.

“Normally we would use a lime stabiliser, but we were aware that the Lias has a tendency to a high sulphate content which ground investigations confirmed,” says Boyle.

“Lime and sulphate can react in the presence of water causing heat and heave in the ground so we did not want to use that.”

Instead, an admixture of Cem II was used for the bulk of the ground, some 2% for the bulk, where a CBR value of 5% was needed, and up to 4% mixed into the top layer which achieves a 30% CBR for the direct bearing capacity under car parks.

The achieved CBRs are tested using test cylinders at seven days in a portable drop hammer testing machine.

“In the wetter areas where lime would have a drying effect usually, we add PFA into the mixture,” says Boyle.

“For the concrete pad areas the top 600mm of ground and columns is removed before casting”

The additive was spread in advance of the rotovating, using a spreader trailer.

The achieved thickness was checked with a “tray test” which involves simply passing the machines over a tray to see what thickness it is spreading.

Meanwhile, with the store area prepared, Keller has been able to get on with the stone column formation.

Some 1,850 columns are being installed, mostly in a square grid with 2.4m centres underneath the floor areas and in concentrated clusters for the concrete bearing pads for the steel frame structure.

Keller has two machines on site, little “Minicats” which it has been building in its own workshop in Clay Cross since around 2000 using vibrator units imported from Germany mounted on an excavator body.

The small machines have a mast of up to 9m high and weigh around 25t which is useful, says Taylor, because they are not big enough to require police escort on the roads.

“And it reduces the need for a massive working platform as well, which can be almost as expensive to install as the columns themselves sometimes.”

The little units have a side hopper on the vibrator mast which helps feed the stone into the vibrated hole as the machine operates.

A side tipping bucket on a JCB is used to charge the hopper as work proceeds.

“We use a top down method,” explains Herring.

The vibrator is used to compact the ground with an amount of stone added as it is withdrawn.

It is reintroduced to compact the stones and then more is added. “It takes about three passes to produce the 2.5m long columns we are doing here,” he adds. “They key into the Lias by about 500mm essentially to refusal.”

Columns are notionally 600mm in diameter “but that varies with the strength of the ground and the amount it consolidates as the vibration is done”, says Herring. “They can sometimes be just 450mm, sometimes over 600mm.”

Tests are done to ensure the strength of the columns and the client will select a random half dozen columns at the end of the job for deflection bearing tests using a hydraulic jack and the Minicat as a reaction load.

“Finally the area will need trimming, because there is always a little heave and regrading is needed,” says Herring. “For the concrete pad areas the top 600mm of ground and columns is removed before casting. It is a function of vibro work that you do not use the top 600m or so,” he adds.

Fortunately for Keller and its partner, Geofirma, the weather has been good for the four-week job, despite some nervousness in the light of the Gloucestershire floods three years ago.

By mid-April they were due to leave site allowing the main contractor to get on with the building work.

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