SINKHOLE STREET FAMILIES TOLD OF DANGERS LINKED TO REPAIRS
Experts say problem could become worse
WORK to repair a sinkhole in a city street could potentially result in the problem getting worse, an independent report suggests.
Stoke-on-trent City Council commissioned Wardell Armstrong to carry out a ground investigation in Boatman Drive, Etruria, which has been afflicted with serious subsidence issues since 2018.
The investigation, which involved drilling four boreholes, found the problem was probably due to ‘adverse soil/water interaction’ within the backfill deposits to the former marl pit, on which the Redrow Lakeside estate is built.
Subsidence could have caused ‘significant defects’ to underground water and drainage pipes.
The report proposes a repair involving a sheet piled wall to create a temporary support, so existing storm and foul water pipes can be removed.
The marl pit backfill would be excavated, with piled slab installed and bored piles constructed into the underlying bedrock. The excavation would then be reinstated, along with the pipework, before the final restoration of the road surface.
But the report says this ‘is not without risk’ as the way the water and soil interact underground is ‘not fully understood’.
It states: “It is therefore possible that any ongoing subsidence will continue to affect property outside the footprint of the piled slab repair to the highway, and/or that the intervention within Boatman Drive could divert or focus any flow of groundwater elsewhere, thus exacerbating any subsidence risk outside the footprint of the remediation works.
“Whilst it is envisaged that all appropriate precautions would be undertaken during construction and that working methods would be carefully considered in an attempt to avoid or control perceived risks, there remains some risk that the construction activity, in itself, could trigger an acceleration or expansion of the subsidence that has already been observed over a wider area of private property, extending beyond the limits of the public highway.”
The report says the council will need to ‘take ownership’ of these risks’, which are unlikely to be eliminated.
To fully remediate the problems with the marl pit backfill, work would have to be carried out across the full extent of the former marl pit, which lies beneath most of the Lakeside estate.
The report says this would be ‘highly problematic’, and this approach is not recommended to the council.
The council does not expect to decide over the repairs until early next year. But it says a road closure order remains in effect, and is urging residents not to drive on the affected section of Boatman Drive.
A council spokesperson said it had made neighbours aware of the report’s contents.
He added:“we want to ensure the highway is safe for vehicles, and we intend to monitor the road for a further six weeks. We hope this will provide us with the data to judge if, and how, the road may be formally reopened.
“The road closure will remain in place and we remind residents that any motorists contravening the formal road closure regulation order, may be liable to legal enforcement action, and more importantly, they are placing themselves and their passengers at an unknown risk.”
The spokesperson said tests indicated ‘the made ground was very soft/loose to firm at variable depths.
He said: “The local geology is made up of soft mudstone used in the ceramics industry. With that in mind, at this stage, we are moving with caution, as the subsidence could be a product of soil erosion within the backfill pit material. We thank residents for their continued patience and remind them if they are encountering any issues with ground settlement, to speak with their property insurers.”