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PILING

Inadequate foundations for a new school in Scotland have been boosted by the installation of 700 minipiles.

Seven hundred minipiles have been used to correct excessive settlement on the site of a new school in Glasgow.

Roger Bullivant installed the piles beneath existing pad foundations for the steel frame structure of the Summerhill School in Drumchapel.

Bullivant underpinning manager Paul Doyle says: 'Vibro stone columns installed beneath the pad foundations were unsuccessful mainly due to the underlying silty soils being much weaker than identified by the site investigation, leading to excessive settlement of the steel frame.

'Bullivant was called in to suggest an acceptable method of underpinning the entire structure and we decided to install minipiles.' The ' rm won the £300,000, 10week job from main contractor Kier Scotland. Construction was put on hold while it underpinned all of the 130 pad foundations with between three and six minipiles, depending on load, with each pile designed to accommodate a safe working load of 110kN.

Each minipile has a hollow drill bar, installed to a depth of 10m and flushed with grout to create a stitch pile. The stitch piles installed beneath the steel frame provide a competent means of transferring load from the pad foundations, through ground containing loose silty clay, to a ' m stratum of medium dense sands.

The piles work in compression relying mainly on skin friction, with an element of end bearing on the medium dense sands. They are formed from 2m lengths of 40mm diameter hollow bar, coupled together with screwfixings as the pile descends into the ground.

Bullivant's remedial work began by drilling a 150mm diameter hole for each of the 700 pile positions, through the 600mm deep reinforced concrete pad foundations. A down-the-hole hammer, driven by one of two 901 series mini piling rigs, provided a clean cut through the foundations.

Site workers used the same 4t rigs to install each drill bar through the newly created apertures in the pad foundations. The first 2m length of bar contained a sacrificial steel cross-bit cutting head that bored through the silty clay as the bar was rotated into the ground.

Cementitious grout was continuously pumped through the centre of each drill bar and was injected into the ground through a pair of holes in the cutting head.

This helped to displace soil to aid the bar's descent and provided a structural outer core of grout - the design specified that each stitch pile had to have a body of grout at least 120mm in diameter along the entire length of the bar.

Grout was mixed on site with about 300kg injected into every pile position to create a stitch pile. It also provided a secure bond between the drill bars and the existing concrete bases, effectively turning what were pad foundations into pile caps spanning at least three piles.

'The use of drill bars in this way to underpin a structure is fairly unusual, ' Doyle says. 'Drill bars are typically used to provide a foundation for buildings as opposed to a remedial solution. Also, we do not often get to underpin a building that has not been fully constructed.' Further underpinning on site included stitch pile installation every 1.5m beneath ground beams that linked the pad foundations.

Doyle says underpinning using rotary bored drill bars provides a vibration free, quiet and quick alternative to driving piles. 'We considered installing bottom driven minipiles but that would have taken twice as long, ' he says. 'We were also concerned about obstructions that could have impeded the installation of a driven pile.' Bullivant's work is now finished and Kier Scotland is due to complete for client Glasgow City Council in September.

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