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 | | ABBEY PYNFORD PROVIDE UNDERPINNING SOLUTION FOR LONDON BOROUGH OF ISLINGTON | | | Abbey Pynford | | 10/10/2006 | | |
Underpinning specialist Abbey Pynford has been called to the assistance of three separate seven-storey blocks of flats in London N1, maintained by the London Borough of Islington.
The blocks of flats had suffered subsidence damage from substantial trees surrounding the blocks on the underlying clay substrata. This movement had caused the 'leg' of the T-shaped blocks to separate from the main part and it was decided by the council's Consulting Engineer that underpinning was required.
Various possible underpinning schemes were considered, however, the depth of the underlying clay adversely affected, Health & Safety considerations with respect to deep underpinning excavations and the substantial weight of the buildings meant an excavated underpinning solution was not considered to be a viable option.
The decision was therefore made to install a scheme based on piles. The scheme was both designed and installed by Abbey Pynford with the design work taking place in its in-house department.
Paul Kiss, Managing Director of Abbey Pynford, said: 'The preliminary enabling phase of the works at ground floor level, carried out by the main contractor, involved removal of all fixtures, fittings and services from the property. Following this, the existing suspended ground floor slab was removed and sufficient reduced level excavation was completed to provide a reasonably flat surface for Abbey Pynford's mini piling rig to operate on.
The first part of Abbey Pynford's work was the installation of 25 No. 300mm diameter augured cast in situ piles within the existing building to a maximum depth of 16m. Recent advances in mini rig pile design enable it to pass through a domestic doorway and still be capable of installing 300mm diameter piles to 20m in stiff clay.
The piles incorporated anti heave sleeving and full depth reinforcement as a necessary part of providing comprehensive anti heave precautions. Heave is the process whereby clay that has become desiccated and shrunk, causing the original subsidence problem, subsequently re-hydrates and expands, sometimes with sufficient force to lift or otherwise damage buildings.
Following completion of the piling works it was then necessary to install a reinforced concrete raft to transfer the load of the building on to the piles. To achieve this it was necessary to temporarily support the existing walls to allow the raft slab and anti heave precautions beneath to be installed.
Paul added: 'In order to maintain lateral stability the raft slab was cast in two halves. The installation of temporary props involved breaking out a suitable size hole in the existing wall, installing and tightening into place the purpose made three-legged screw props with reinforced concrete padstones top and bottom, repeating the process for the remaining stools and when all the stools were in place, breaking out the brickwork between the stools.
'This then left a slot in the wall in to which the anti heave precautions, reinforcement and concrete can be placed.'
The anti heave precautions beneath the raft slab comprised 160mm of Clayboard which, when wetted creates a 150mm clear void beneath the new foundation slab into which the underlying clay could expand in the event of rehydration without adversely affecting the structure.
During the course of the structural works, tenants in the floors above ground floor remained in residence in conditions, which while not being ideal, were considered to be acceptable.
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