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A test of stability

Cantilevered beams designed to 'float' within the ground are being installed beside an old property in Kent to offer structural stability and overcome the threat of clay heave.

Residents of a farmhouse which is suffering the effects of subsidence have continued to live in their property during corrective underpinning works, thanks to installation of a cantilevered beam system.

Underpinning is necessary because the property is dwarfed by a towering horse chestnut tree that has drawn moisture from clay heavy soil and undermined the structure in one corner.

A total of 12 beams, each supported by a pair of foundation piles, are being constructed to the front and along one side of the 400-year-old property near Gillingham, Kent, by Roger Bullivant (RBL).

Use of a cantilevered system to support the two-storey building was preferred to a more conventional internal piled raft, which would have required removal of the floor inside the house and meant the owners leaving home.

Each reinforced concrete beam is supported by a pile adjacent to the property which takes the load, and a secondary pile that anchors the beam into the ground. The beams have been designed to 'float' within the ground, above a 150mm deep void. This vacant space is needed to accommodate any potential clay heave should the tree die and release moisture back into the ground.

Voids are created by placing sacrificial cardboard supports wrapped in polythene on a firm strata of ground before the beams are cast on top. Once the beams have cured, the polythene wrapping is pierced to allow ingress of water to aid the decomposition of the sacrificial supports. Backfill will then be compacted around the beams.RBL contracts supervisor Dave Clarke is not surprised the tree has undermined the building, but says that damage has been limited by the age of the house. “The tree has a deep network of roots taking water from the ground, but the house is built using lime mortar. This has allowed the structure to move a little more sympathetically with the ground. The more rigid a house, the greater the problems tend to be.”

Work on site began in May with the creation of 24 augered piles to a depth of up to 15m. Limited headroom beneath the tree meant that a 3.5t mini piling rig with a mast height of just 2.8m had to be used. Each foundation pile measures 250mm in diameter and was created by first installing a leading 1m section of flighted auger into the ground. A further 14 sections of flight were connected to the previous one.

After reaching full depth, concrete was pumped through the central hollow stem as the augers were removed. A three bar reinforcing cage was placed into the top of each pile before
a single rebar was inserted to full depth.

Trenches were then excavated from in front of and to the side of the property, which revealed the building was founded on wooden 'sole plate' footings, each around 0.5m in length. These footings were carefully removed and replaced by a series of temporary jacks. RBL then installed a ring beam of reinforcement concrete beneath the perimeter walls to take the load for transfer to each cantilevered beam. Temporary jacks were then removed to complete the work.

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