Van Elle recently used its Hoopsafe system to prevent further damage to an old vicarage through persistent ground movements.
The large detached building was built on masonry foundations bearing directly onto shrinkable London Clay. The site slopes gently over the length of the house which is surrounded by numerous mature trees.
Some of the closest trees were removed in the late 1970s after cracks appeared in the walls but droughts in the early 1980s caused cracks to reappear and underpinning was carried out on the majority of the perimeter walls.
However, by 1994 it was discovered that this beam and pad solution had not considered heave and was not working, as movement was still causing cracking in the walls. Van Elle came up with a solution in the form of its Hoopsafe system.
This consists of casting a high quality, post tensioned reinforced concrete beam around and through the building's substructure at or immediately above existing foundation level. In this case, the beam also surrounds the previous underpinning slabs and had to take into account the slope of the site.
Running through the centre of the beam is a 15.2mm diameter, unbonded Dyform PSC tendon with ultimate tensile stress of 300kN. Once in place, the whole arrangement is hydraulically post-tensioned, and then induces a positive lateral compression on the structure, allowing the building fabric to act as a single rigid unit capable of withstanding differential movement.
For the vicarage, the beam was tensioned to 225kN in two equal stages over one and a half hours. The entire operation took just five weeks and cost less than pounds35,000, compared with estimated piling costs of more than pounds100,000.