Mainmark – concrete slab relevelling allowed a Currys store to stay open whilst subsidence was treated

Mainmark, a ground engineering specialist with over 30 years’ experience, used its innovative Teretek® geopolymer resin injection technology to treat a subsidence problem at a Currys-PC World store within Newbury Retail Park. The retail area was constructed in 2007 and built on top of an old rubbish tip. In 2019 the stores concrete floor slab began to show the effects of subsidence.

The sinking concrete floor slab issue at the Currys-PC World store had varying impacts on different areas of the building, with subsidence settlement ranging from 25mm to 100mm across the store. This resulted in the development of numerous cracks in the floor at different levels, which had a detrimental effect on the store’s operations and furthermore created a tripping hazard for both staff and shoppers, posing risks and potential accidents within the premises.

Moreover, the uneven floors made it challenging to unload deliveries such that trolleys would roll back out of the warehouse if the brakes were not applied, and potentially causing accidents or damage to the merchandise.

The ongoing subsidence problem had further consequences, including detachment of a stud wall of the mezzanine floor from the ceiling. As the subsidence continued, the gap between the stud wall and the ceiling increased over time. Battens had been used to conceal the growing gap from the customers’ view.

Overall, the subsidence-related problems mentioned above require prompt action to mitigate safety risks, improve operational efficiency, and provide a safe shopping environment for both employees and customers.

What were the challenges that the chosen slab-lifting solution had to overcome?

The challenges faced in trying to find the most suitable slab lifting solution to return the building to its original level were centred around the need to minimise business disruption and therefore reducing the subsequent costs. When the Currys-PC World reported the rather alarming floor movement, Columbia Threadneedle, the property asset managers, visited the site, saw the extent of the problem, and informed their insurers. This set off investigations by loss adjusters and consulting engineers, Waterman Group.

 The concern was that alternative fixes (including digging out the existing slab to re-lay it) would require extensive excavation work, requiring a lengthy and disruptive store closure. This would then entail Currys-PC World having to temporarily relocate to an alternative site assuming a suitable one could be found at such short notice. This would lead to claims for loss of business revenue and recovery of all costs resulting from the enforced move.

The investigation works included a structural survey and showed that . This was laid over a 30mm membrane on top of 80mm of crushed aggregate with the ground below comprising grey sand with stone pieces. A drain survey also took place to check for possible water escape that could weaken the soil. Although no obvious cause of subsidence was found from the surveys, the finger of suspicion pointed at the root cause being related to the site being located on top of an old rubbish tip, where localised areas of poor soil compaction triggered the subsidence events.

 Waterman Group conducted a thorough evaluation of the available information and decided that Mainmark’s cutting edge Teretek® technique as the most appropriate method for treating the weakened underlying soil and lifting the slab back to level.