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Home help New owners inject a new lease of life into an innovative solution to subsidence.

Moving house is commonly cited as one of the most stressful events in life, but for those with the misfortune of experiencing it, a moving house is a far bigger ordeal.

Remedial works on a subsiding house are characteristically drawn out, disruptive, financially unnerving, and physically and emotionally intrusive.

Traditional underpinning methods are likely to involve your lovingly tendered garden being turned into a building site. You watch in horror as your prize roses and marigolds are mercilessly crushed, and your shag pile carpets acquire a new shade of earthy beige.

It is a brave occupant who chooses to stay put while the damage rendered by their Leylandi hedge or leaking drains is put right. But moving out for a couple of months brings its own stresses and anxieties.

It seams that this need not be the way. Geotechnical contractor Van Elle, one of the country's leading domestic underpinning companies, last year acquired the patent and sole UK rights to Hoopsafe, a structural stabilisation system that overcomes many of the drawbacks of conventional underpinning work.

Hoopsafe is in effect a surgical truss for a building. It uses a post- tensioned ring beam to induce lateral compression in the structural walls, thereby stabilising the building by making it behave as a rigid unit.

The system involves casting a reinforced concrete beam cast around the affected structure just below ground level but above existing foundation level. Typically this requires excavation of a small trench, of around 300mm deep by 250mm wide, against the external perimeter wall. Shuttering is set up and reinforcement positioned in the trench. The key component is an unbonded Dyform tendon, which runs centrally down the beam's length, and is securely fastened to the reinforcement cage.

This as Van Elle (Hoopsafe) managing director Victor Handley puts it 'reinstates the integrity of the structural walls.' The structure can still go up and down but differential movements are excluded. While there remains a potential for the whole structure to tilt, ground movements have to be pretty extreme before this is significant and according to Handley 'if we think it will tilt, we can put flexible joints in the service connections.'

Anchorages are placed at beam ends and the concrete cast. Once adequately cured, the beams are post-tensioned to 22.5t in two equal stages, using a small electrically powered hydraulic jack. The tendon is then locked off, cut at the anchorage point and protected from corrosion by a grease filled cap.

Beams can be fitted with single, double or even triple tendons, increasing the spanning and cantilever capabilities of the structure.

A typical project including full reinstatement takes three to four weeks. Unsurprisingly then Hoopsafe is also generally cheaper than traditional underpinning methods. Stabilising a typical modern semi will cost around £12,000 for the pair.

'Hoopsafe needs no heavy machinery, and presents no access problems,' says Handley. 'Only rarely do we decamp people out of their homes during the work.'

A further advantage is that the work does not require a formal submission of building regul-ations, as the District Surveyor's Association considers the tech- nique to be a repair, not a down-ward extension of the property.

Handley says Hoopsafe is suitable for a range of appli- cations including clay shrinkage and heave, mining subsidence, and consolidation of deep fill. 'We are not interested in doing a job when the system is not suitable,' he says 'as we don't want to feed the sceptics.' However he adds Hoopsafe can be used in a hybrid solution; for example, if a building is positioned across the edge of a backfilled quarry it could be used in conjunction with piling.

The system has actually been around since 1980, but, explains Handley, the system's inventor (a structural consulting engineer), only granted short term operating licenses for contractors. 'Consequently no-one was ever in a position to market it fully.'

Now that Hoopsafe is within the Van Elle Group, this situation is set to change. After just four months, the company has picked up £0.5M of work, and Handley says he will soon increase operations from three crews to five.

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