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Size was the biggest challenge on a stone column installation project in Glasgow, while at a Liverpool business park speed was of the essence.

In one of Pennine's biggest ground improvement projects in Scotland, the Lancashire-based firm has installed more than 3,000 stone columns at Glasgow's King George V Dock.

The four-week, £155,000 contract was preparing the way for lead contractor Molplant of Dumfries to build a large extension to a cattle feed warehouse for Clyde Port Operations.

Pennine's contracts manager Richard McKenna says: 'This was a very demanding job for us - not least because of the scale involved.

The warehouse remained operational throughout the project, so special arrangements were made to overcome access difficulties.

'We used dry bottom feed vibro stone columns because there were loose granular soils and a high water table, thereby avoiding the risk of borehole collapse, ' he says.

'With dry bottom feed the flot - or poker - still penetrated the ground in the usual way, but it remained in place at design depth and stone was poured into the bore and compacted in stages as the flot was withdrawn.' Pennine adapted the rigs to suit the job conditions. This allowed the operator to know the exact location of the tip of the flot so that columns were installed accurately. The monitoring device measured hydraulic back pressure and showed this on a graphic display in the cab so the operator could adjust the pressure as necessary.

'Monitoring was key to ensuring the ground was not overworked, so we had computers in the cabins of our two specially adapted Terrafima rigs to ensure accuracy, ' McKenna says.

The subcontractor inserted about 18,000 linear metres of stone column to increase the load-bearing capacity of the soft silty ground up to 150kN/m 2. Depth of the columns varied from 4.5m to 9m.

While size was a feature of the Glasgow contract, speed was of the essence for Pennine on a job at Liverpool International Business Park for Gladman Developments.

Normally the project to install 10,034 vibro stone columns for a major new distribution centre, over a treatment area of more than 55,000m 2, would have taken about 10 weeks.

But main contractor Gladman needed a ground improvement solution in half that time to meet construction schedules. Thanks to a quick turnaround with its design the Pennine team came up with a plan that shaved another four days off the deadline.

Working with two vibro rigs and a pre-auger rig, Pennine installed 32,500m of vibro stone column to the main foundations and slabs to accommodate loadings of up to 150kN/m 2. Depth of the columns varied from 1.5m to 7m across the site, with an average depth of 3.5m.

The stratum typically comprised fine sandy clays and fine brown clayey sands. Site workers carried out a cut and fill operation to bring the ground up to working level and the fill material was lime stabilised which had the advantage of improving the quality of the working platform.

In many places the ground would have proved hard to penetrate with conventional equipment, but Pennine claims the pre-auger rig broke though the stiff upper crust to maintain high production levels.

The team was awarded the contract at the beginning of July, started on site on 18 July and completed the project four days ahead of the 19 August deadline.

Gladman is scheduled to complete on site early January.

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