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Piling Spotlight: Piling on pounds

Abbey Pynford came up with an innovative underpinning scheme when trees damaged the foundations of five blocks of flats in north London. NCE reports.

Abbey Pynford was recently asked to underpin two five-storey blocks of flats at Highbury Quadrant in north London after they had suffered tree induced differential foundation movement.

It is unusual for entire multi-storey buildings to need underpinning. These buildings are normally provided with foundations that will cope with normal clay shrinkage, bad ground or drain failure type problems which can effect domestic properties.

Clay strata

The blocks are on an estate of local authority flats, which is currently being refurbished. They are located on London clay strata with substantial trees growing among them. Consequently two of the T-shaped blocks have suffered differential foundation movement, which resulted in part of the building coming away from the rest, where the connection between the two parts of the building is only staircases and walkways.

“The challenge was to remove the existing reinforced concrete floor slab without removing support to the main external walls”

Abbey Pynford was asked to design and construct an underpinning scheme. Due to the depth to which the clay had been adversely affected by the trees, and the dangers of sending workers down deep underpinning excavations, a bored pile scheme was developed.

Mini-piles constructed internally were used to support a new raft slab and only ground floor tenants had to move out during the work. The foundations of the existing buildings comprised four disconnected parallel strip foundations under the two end walls and two main load bearing internal walls supporting sleeper walls. These supported a reinforced concrete ground floor slab with various downstand edge details required to cope with the varying ground level around the building’s perimeters.

The challenge was to remove the existing reinforced concrete floor slab without removing support to the main external walls, to make way for the piling work.

The solution was to carry out temporary excavated underpinning between the ends of the existing strip foundations to provide direct support beneath the perimeter walls.

Diamond sawing

The ground floor slab was then removed by diamond sawing around the perimeter and breaking out with remote controlled hydraulic equipment.

The original ground floor storey height combined with the construction thickness of the new ground floor slab and the anti-heave void below created sufficient headroom to allow the 16m long 300mm diameter augered piles to be installed with mini-rigs.

Temporary props were used to support the structure while the walls were broken out to allow installation of the new reinforced concrete raft slab and anti-heave precautions beneath.

The temporary propping system then had to be removed after the raft slab had reached its design strength, in order that a clear uninterrupted anti-heave void could be provided under the entire plan area of the building. The temporary props had to be removed to ensure that there was no path for heave induced forces after the slab had been cast.

The raft slab was constructed in three sections, to minimise the amount of the building which had to be supported on temporary props at any one time. The propping system comprised groups of purpose made threaded screw jacks encased in polystyrene sleeves. This was to isolate them from the subsequently poured concrete, and that there was sufficient working space around them to ensure that they could be dismantled and removed in sections.

Resulting pockets in the raft slab were back filled with concrete.



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