This case study details how a retrofit pile and beam system utilised Twistfix Helical Piles in conjunction with retrofit masonry crack repair techniques to rectify ground movement issues in the Thames Valley.
Clay ground is susceptible to heave and shrinkage cycles, which are caused by changing ground-water conditions. As sub-soils dry shrinkage often causes cracking around door and window openings. These cracks often have a vertical staggered formation, typically with wider cracking at the bottom of the wall. Heave occurs when wet subsoil expands, creating cracks that are typically wider at the top of the wall.
Cases of subsidence have increased and blame has been variously attached to the digging of rail tunnels and the expansion of sewage networks along with an increasing number of basement conversions.
Newman Building Solutions, a Twistfix-registered contractor, were recently employed by Oxford City Council to undertake a detailed structural survey of a block of low-level flats where long-standing cracking had been noticed. Newman’s first carried out a visual inspection which was enhanced with the company’s in-house drainage and soil investigation team’s extra assessments.
The cause of the cracking at the flats, which house elderly residents, was found to be due to subsidence exaserbated by vibration - the flats are located just 20m from a high-speed rail track. Built on stiff Oxford clay, a common situation for properties in this area, the property was unusual in that it had a non-reinforced ring-beam instead of a concrete slab as a foundation.
Newman Building Solutions utilised the innovative pile and beam repair system from Twistfix to create a solution to the cracked masonry. More than 300m of helical reinforcement bar was installed to create deep and continuous masonry beams around the base of the walls. Shire piles, a helical mini-piling system, were utilised to provide support to retrofitted reinforced brick beams, which supported the brickwork between the minipiles and distributed structural loads to them in strict accordance with BRE guidelines.
All the work was carried out swiftly and without the use of heavy piling machinery while causing minimal disruption to the residents.
Following installation of the screw piles and helical bars the cracked brickwork was structurally reinforced and aesthetically repaired to complete the pile and beam insertion job. The works took just ten days to complete and costs were well within the budget set by Oxford Council.