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Ground improvement: Strength in depth

Construction of Bolton’s new waste transfer station is now underway following fast delivery of ground improvement using a new technique for the UK market.

Using a new geotechnical technique is always a challenge, but when you’ve promised the client that you can deliver the work in less than half the time planned in the programme, the pressure is on.

Throw in a rig breakdown and the need to source the part from France and you’d think the site team would be hot under the collar, but that wasn’t the case when it came to one of Vibro Menard’s recent ground improvement contracts.

The project in Bolton is being undertaken by Costain on behalf of Greater Manchester Waste and is part of a wider scheme to develop a number of new waste transfer sites around the city. The site at Bolton will comprise an in-vessel composting facility, amenity waste building and a weighbridge.


C12 to C15 concrete was used to increase the ground density

Vibro Menard’s parent company Bachy Soletanche is working with Costain to provide piled foundation solutions at the other sites. But the geomorphology and ground conditions at the Bolton facility made it better suited to ground improvement.

The project is one of the first in the UK to use controlled modulus columns (CMC) and work was initially scheduled to take seven weeks. But Menard cut the programme to three weeks despite the issue with the rig.

Vibro Menard general manager Gerry Crawford says using ground improvement was more cost effective. “A piled solution would have required spoil disposal and a suspended floor slab that would have had to have been thicker and need more reinforcement,” he says.

The greenfield site, located close to the M61 south west of Bolton, originally sloped away to the west but Costain has used balanced cut and fill processes to level the plot. So in addition to natural ground with clay layers, the western part of the site also has a varying thickness of unconsolidated fill up to 3.5m deep. Lime stabilisation of the fill material was carried out before Vibro moved onto site.

“Costain originally asked us to price a conventional vibro stone column solution for the site but analysis showed that the differential settlement from this approach was too much,” says Vibro Menard senior estimator Gabriel Portillo, who has joined the site team from the French operation of Menard which is experienced in use of the CMC technique.

“We offered the CMC solution to help reduce the settlement by transferring load to the stiffer layers within the underlying geology”

Gabriel Portillo, Vibro Menard

Costain’s design for the warehouse structure was expected to exert 50kPa on the slab and 100kPa on the footings.

“The clay layers that run through the site mean that stone columns wouldn’t actually improve the ground, just create piles which would not offer the support necessary,” says Portillo. “We offered the CMC solution to help reduce the settlement by transferring load to the stiffer layers within the underlying geology,” he adds.

According to Menard, the CMC technique is designed for use with weak soils and works by the auger displacing the soil laterally and increasing the density of the surrounding soil with a low pressure grouted column that increases the bearing capacity. Unlike vibro stone columns or vibro concrete columns, CMC is carried out by augering rather than vibration.

The design for the Bolton site, which was analysed using finite element software, was checked by URS. This showed that settlements using CMC would reach a maximum of 23mm, compared with the 35mm to 40mm expected if conventional stone columns were used. Tolerances offered by CMC were well within the 30mm maximum demanded by the client.

The columns installed at Bolton vary in depth from 3m to 12m, depending on the depth of the fill. They were placed on a grid pattern with closer spacings for the pad foundations. Spacings of 3m x 3m were used in the natural ground and were gradually reduced down to 2.3m x 2.3m in the fill area of the site.

In total, 1,900 columns were installed - 1,850 for the mainbuilding and the remainder for the weighbridge.

The completed slab at the Bolton site will measure 89m by 260m. The 250mm thick concrete slab will be built on a 600mm-thick load transfer platform, formed in two layers of high-quality granular fill with the first 300mm placed ahead of the ground improvement work to act as a working platform for Vibro’s rig.


Menard used a specially designed displacement auger

The rig that was used for the work is an Italian-built Entec E500 machine that has a specially designed displacement auger for CMC work but that can be re-rigged to undertake continuous flight auger piling operations. Vibro brought the rig over from France especially for this contract, although it was due to go on from Bolton to work on the Connaught Tunnel in London.

“Calibration of the technique was carried out next to a previously excavated borehole at the start of the contract,” says Portillo. “This allows us to check the instrumentation on the rig and to give penetration data for the dense areas. The criteria is based on torque, which increases by 5t per metre in the good layer, and drilling speed, which decreases as the ground improves.”

The displacement auger creates a 320mm diameter column which is backfilled with concrete. “Once the column has reached the required depth, the auger is lifted 800mm and then concrete is pumped through the hollow stem until a specified pressure is reached and then the auger is lifted as pumping continues,” says Portillo.

“The bottom of the auger is fitted with a cap to prevent soil entering the hollow stem during driving and the pressure of the concrete blowing this open.

“The concrete used for CMC is not needed for strength but to transfer loads into competent ground. In France we use 8N to 12N concrete, but in the UK we use a C12 to C15 concrete with an S3 slump.”

Concrete used at Bolton was supplied by a nearby Tarmac plant and Vibro ensured that work kept on track by calling the plant each day to estimate demand. delivery of the concrete to the rig was also unusual in that it made use of a trackmounted concrete pump with its own compressor and water tank that could follow the rig around the site, keeping hose lengths to a minimum. “The shorter hose length means that there is less waste and less cleaning is needed at the end of each shift,” says Crawford.

Work on site started in early January. Vibro believed it could easily complete the scheme in five weeks rather than the seven allocated for the work by Costain, but actually delivered the work in three.

“This was the first time the site team had undertaken CMC work so we were training them on the job and we had a rig operator from France working with them in the first week,” explains Crawford. “They picked up the technique very quickly and site conditions meant that even though the auger broke part way through the contract and we had three days of downtime, we were still able to complete CMC installation in three weeks.”

Crawford reports that the site team managed to create more than 1,000m of columns on some days, but he says greater rates of productivity are possible when working on contracts that call for longer columns and when working in weaker soils.

Once the columns were installed, Vibro trimmed the tops to the required profile, leaving Costain to clean and regrade the working platform before making up the thickness ready for slab construction.

The site has now been handed over to Costain and work on the new waste facility is underway .

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