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CLASSIC CASE

PILING & FOUNDATIONS

Redevelopment of an Oxford University site surrounded by historic buildings needed sensitive handling. Rupert Moles reports.

Over the past couple of centuries the buildings at 65-67 St Giles, Oxford have housed activities ranging from 'druggist' to 'automobile agents', by way of dealers in 'Indian and oriental goods'.

Presently occupied by the University of Oxford Classics Centre, the buildings are about to be refurbished and extended as part of a project that will link them to a new four-storey Centre for Classical and Byzantine Studies.

The oldest, number 65, dates to the early 18th century while the Gothic style 66 and 67 were built in 1869. The site includes a grade-IIlisted facade and is surrounded by historic structures.

To the south is the Ashmolean Museum, founded in 1683, which claims to be the oldest public museum in Britain and contains the university's art collection. To the west is the museum's Cast Gallery which has one of the largest collections of plaster casts in the UK. To the north is another part of the university, the Blackfriars Priory, run by the English Dominican friars.

These factors posed a logistical challenge for subcontractor Piling Solutions, which has just completed a limited access CFA project at the site in a stratum of 1m of made ground above 1m of firm clay overlying 6m of terrace gravel over hard Oxford Clay.

The main work, by principal contractor Mace Plus, involves the demolition of the rear of the property, refurbishment of the front listed portion and construction of a basement as well as the new fourstorey building.

Given the 2.8m restricted width between the Ashmolean Museum and numbers 65-67, Piling Solutions proposed its Cl¾ Zironi CA40 limited access rig to install the piles.

The rig has a bottom drive unit and very low centre of gravity, allowing it to be mounted on a tracked Komatsu base unit. This makes it particularly suited to dif'cult and limited access work.

Under its £170,000 contract with Mace Plus, the company installed 85, 500mm diameter, 12.5m long contiguous piles for a basement retaining wall that will get a new reinforced concrete front face. As well as these, site workers installed 40, 500mm diameter bearing piles, 9-13.5m long.

The bearing piles, designed to withstand axial loads from 300800kN, will support the new basement structure and include single piles as well as groups under pile caps that sit below the basement slab. Their pile cages will accommodate 400kN tension loads with 8, T20 bars up to 11m long and T8 helical at 250mm pitch.

Because Blackfriars Priory is adjacent to one line of a contiguous wall, the piling speci'cation required minimal ground movement behind it. These piles will be taking a permanent compressive load as columns from the above ground structure will be positioned on top of their cap.

But Piling Solutions believed the fairly stringent settlement and deflection criteria for the ground near the Priory could not be achieved with the original plan of a cantilever wall and altered it to include temporary and permanent propping.

The contiguous wall retains 4.56.4m of ground and needed the 500mm diameter piles at 650mm centres.

With such an historic building adjacent to the piling works, site workers placed a portable protection barrier in front of the drilling rig to prevent concrete or hydraulic oil splashing the stonework.

The rig crew advanced at an average rate of six piles a day, reaching optimum production of nine per day on the contiguous wall piles.

With consistent soil conditions found across the site, a single working pile test was required in conjunction with the testing of all the bearing piles and a limited number of contiguous piles. Engineers chose a 2.5 factor of safety to limit penetration into the hard Oxford Clay.

Piling Solutions began in early August and completed last month.

The main works are scheduled to finish in September next year.

Rupert Moles is estimating manager for Piling Solutions.

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