A building in the heart of York had been suffering from subsidence for decades but a recent acceleration left engineers at City of York Council scratching their heads for a solution.
The two-storey brick building was originally a police station of massive construction. York stone slabs 230mm thick formed the first floor and part of the ground floor. In the 1930s the building was converted into an electricity substation, containing very heavy transformers and electrical switchgear. Although this has now been decommissioned and the equipment removed, the weight was possibly an exacerbating factor increasing the settlement, says Mark Whitelock, senior engineer for York Consultancy, engineers for the city council.
'The building was only on brick spread footings, which were relatively shallow, and these were resting on soft ground. The original ground level is approximately 5m-6m below the current ground level, and the intervening ground is Roman and medieval fill which can be extremely soft organic material.
'Subsidence has been occurring for at least 15-20 years, but in recent years has accelerated. So we were faced with the problem of having to underpin and stabilise the building, ' explains Whitelock.
In reviewing the various options, major consideration was given to the building's location in the city centre, surrounded on three sides by shops and offices, with three sides being party walls.
York Consultancy chose Uretek's Power Pile because, says Whitelock: 'The alternatives were just too problematic, using mini piles, needles, and lintels under the walls. Uretek's process is quieter, and it's easier on site - there are no excavations, so there isn't lots and lots of spoil coming to the surface.
Plus it's quicker. This work will take about four weeks, but with traditional methods it would be more like three to four months with a lot more disruption, a lot more dirt, a lot more noise, and a lot more cost. Everything was stacked against putting in a traditional system.' Ground mprovement contractor Uretek installed its new Power Pile system, which injects expanding resins into geotextile tubes to improve bearing pressure and strengthen the ground beneath subsided foundations.
The system was chosen in part due to archaeological considerations, which made it vital to minimise disturbance to the ground under the building.
Uretek anaging rector Chris Davies says he recommended the use of the Power Pile system as it would constrict the spread of resin. At the request of York Archaeological Trust, two test piles were installed before the go-ahead was given for the project.
Whitelock is impressed with the results. 'When we excavated the test piles, I was expecting to be able to push my finger into the resins, but they were like concrete. It was remarkable.
Being constricted by the tubes gives the resins a greater density.' The system was found to be ideal for the occasion because it was able to precisely locate the expanding polymer resin using the geotextile tubes in the weak ground.
Davies claims it can produce an increase of up to 600% in ground bearing pressure without excavations.
The piles are currently being installed under all the internal walls of the building, with the project due for completion at the end of May.