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Grave problem

Piling and foundations

An ingenious foundations design is helping to overcome a grave situation in the extension and modernisation of an historic Hertfordshire church.

Independent architect Barry Roberts decided that the only realistic solution to the overcrowding of the church of St Mary the Virgin in Welwyn was to build a sizeable extension into the neighbouring graveyard.

Following an archaeological evaluation with a series of trail trenches it was decided that piling and foundations contractor Abbey Pynford could excavate to the required depth without the risk of disturbing any archaeologically sensitive material.

Before work began the positions of all the graves below the planned footprint of the new church house were carefully recorded and marker stones removed. Most of the 44 displaced headstones will be re-erected elsewhere in the graveyard, says Roberts.

Moments before the excavation for the foundations began, several more grave sites were discovered leading to a lastminute foundation redesign.

In the end a total of 77 steelcased, bottom-driven, 150 mm diameter Grundomat minipiles were needed to support the 250m2 slab.

A gh-frequency neumatic hammer did the actual driving, keeping noise and vibration levels to a minimum.

Installing the casings, placing the reinforcement and filling the casings with concrete took only two weeks, although the operation did hit problems, says Abbey Pynford operations manager Adam Fisher.

'Within the near-surface zone we encountered bands of impenetrable flints and it was impossible to drive a displacement pile through them.

'Bored piles had originally been ruled out because of the risk of disturbing archaeological remains and the cost of disposing of the excavated material off-site.

'We got round the problem by loosening the hard ground with a conventional small diameter rotary auger, but didn't remove any material.

'We were then able to drive the piles through the loosened soil down to a pre-determined depth of about 5m on average.' A 50mm layer of concrete was then placed over the site and services dug. These included pipework for a planned geothermal heating system, using energy abstracted from the ground within the graveyard via vertical boreholes. This cleared the way for the start of the brickwork.

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