New measures planned to cap road sinkhole in St Albans

Sky News aerial footage of the sinkhole in Fontmel Close (Image: Archant)

However, a report to the authority warns that careful consideration needs to be given to controlling surface water drainage.

Following months of in-depth investigation of the soil in and around the sinkhole in Fontmell Close, Peter Brett Associates (PBA) have suggested, given the presence of chalk below the surface, that better controls are put in place.

It said improving drainage would help “minimise the potential for triggering water inundation of disturbed weakened backfills in the vicinity of the collapse”. It has now been established the cavity occurred above a former deep clay pit, which had a shaft sunk at its base to extract chalk.

PBA examined the subsoil around a handful of houses most affected by the October 1 collapse, and found no evidence of ground instability deep below those properties.

However, its experts said ground depressions to the rear of two nearby homes were believed to have resulted from water leaking into the ground from drains.

And while chunks of one garden and a neighbouring driveway appear to have dramatically collapsed into the sinkhole, the experts found “no evidence of disturbance due to past chalk mining activities”.

PBA said the concrete plug used to stop the sinkhole collapsing further was ‘beneficial’ in providing some support to the front gardens of the properties, but final reinstatement of the road and utility services had to ensure those gardens continued to be protected from further damage.

While utilities and the highway could be reinstated, leaving the concrete plug in place, careful consideration would be needed for the level of the sewer, or, alternatively, some utilities “might be able to re-route services to avoid crossing the plug”.

After further backfilling above the plug is carried out, a flexible highway surface could then be reinstated over the collapse, taking into account the potential for future settlement, the report added.

“Once the services are installed and the new road and pavement level is established it will be possible to reinstate the front gardens of the insured properties.”

PBA has advised the council that, “it would be desirable to check where soakaways are located in the front of gardens, to ensure they are not contributing flows of water into the ground that are draining towards the sinkhole.” They should be re-located ‘as a precaution’.

Monitoring of the foam concrete plug used to fill St Albans’ sinkhole to within a metre of the ground surface has shown that it should ‘settle’ about 50mm over the next three decades.

POTENTIAL HAZARDS

No evidence of significant ground surface movement has been recorded along the public highways and footpaths near to St Albans’ sinkhole, a report has found.

Herts county council asked Opus, a firm of consulting engineers, to collate information from a wider area, including along Fontmell and Bridle closes, beyond the site of the large cavity.

The aim was to establish whether any geological or man-made features were likely to be present beneath the road elsewhere that may result in a similar large sinkhole occurring in the future.

In its report, Opus said: “It is not anticipated that significant surface subsidence would occur along the public highway and footpath areas of Fontmell and Bridle closes.”

But it listed main potential geotechnical hazards as including underground chalk mine workings and nearby abandoned shafts, and potential voids within the backfilled clay pit in the area.

Opus said that where small voids were found during exploratory work near to the surface - in boreholes used for investigatory drilling - these were backfilled with a type of cement grout.

Three of those voids were encountered in chalk within 25 metres of the ground surface, with a fourth found nearly 50m deep, believed to have been formed by dissolution of chalk, as the ground water table fluctuates.