St Albans sinkhole survey work continues

PUBLISHED: 11:56 04 November 2015 | UPDATED: 12:06 04 November 2015

Aerial view of the St Albans sinkhole.

Aerial view of the St Albans sinkhole.

Image provided by Geotechnology Ltd

Further survey work is being carried out this week following last month's partial plugging of a St Albans sinkhole.

Graphic of the St Albans sinkhole in Fontmell Close. Image provided by Geotechnology LtdGraphic of the St Albans sinkhole in Fontmell Close. Image provided by Geotechnology Ltd

A Herts county council (HCC) spokeswoman has today (Wednesday) confirmed that geotechnical experts have returned to the scene of the mammoth cavity, which opened up on Fontmell Close in the early hours of October 1.

She said: “Once these works are complete we hope to collate the results with existing survey data, to enable us to have a full picture of the land.”

Last week it was revealed that a possible cavity up to three times larger than the St Albans sinkhole could be looming deep within chalk bedrock under the same road, according to an initial survey.

The county council has recently met with residents of Fontmell Close and nearby Bridle Close to discuss the findings of a geophysical survey of both cul-de-sacs.

Image provided by Geotechnology Ltd shows the overlay of a historic map, over Fontmell and Bridle Closes in St AlbansImage provided by Geotechnology Ltd shows the overlay of a historic map, over Fontmell and Bridle Closes in St Albans

The report warns there is a “very large anomaly” - put simply, weak ground - next to the sinkhole.

Alarmingly for residents, the ground beneath their properties is neither the district nor the county council’s responsibility to investigate, as the homes are privately owned.

HCC has confirmed that it is in discussion with homeowners’ insurance companies with regard to any remedial work and a residents group is being established.

LARGER CAVITY

The blue area near the sinkhole, which is circled, is the 'very large anomaly'. Image provided by Geotechnology LtdThe blue area near the sinkhole, which is circled, is the 'very large anomaly'. Image provided by Geotechnology Ltd

Looking along Fontmell Close from its junction with Seymour Road the potentially larger cavity, not visible from the surface, is located immediately to the right of the sinkhole, and “has similar characteristics”.

An initial estimate of the ‘missing mass’ beneath the anomaly, 19 metres deep in chalk bedrock, is “some 3,200 tonnes or 1,500 cubic metres”, but more data is needed to confirm that.

Worryingly, the current sinkhole is not considered by experts to be at the centre of a potentially larger body.

They do not know if the centre underlies the road or the adjacent private ground and have warned that only further gravity data in private ground would be able to locate the centre.

The sinkhole itself was “most likely” caused by historic chalk excavation - limekilns and brickworks nearby would have needed chalk for lime burning.

The report confirmed that “the collapse lies within the area of [a historic] clay pit”.

But on a more positive note, the sinkhole is a localised feature rather than subsidence extending more generally across the whole area underlain by the backfilled clay pit.

Both cul-de-sacs were developed over a pit in the early 1970s.

Fontmell Close had suffered what the experts call ‘a catastrophic collapse’ which occurs when the ground falls into a void.

Such collapses “can occur when acute ground settlement is associated with water leaks or inundation by groundwater”.

Volume-wise, the sinkhole is now known to be 603 cubic metres - the equivalent of more than four double-decker buses.

The survey has highlighted other anomalies of a much smaller scale, which also need further investigation.

CUL-DE-SAC REMAINS CLOSED

One of these is in the nearby field where a temporary point of access between the old fire station and the end of Bridle Close was set up for residents after the incident. This area was part of another clay pit, and remediation work has historically taken place there.

And a ‘modest’ anomaly has been discovered further along Fontmell Close, which might be “interpreted as a void or tunnel within the chalk [15m below ground level], potentially a chalk mineworking”.

Until further tests are completed and repair work undertaken at Fontmell Close to make the area completely safe again, the cul-de-sac will remain closed to vehicles with the temporary access nearby likely to remain in place for up to one year.

How the sinkhole saga has unfolded so far...

In the early hours of October 1, subsidence beneath the road in Fontmell Close resulted in a 12 metre wide sinkhole opening across the full width of the carriageway, and encroaching upon nearby gardens on both sides of the road.

As the collapse presented an immediate threat to residents, they were evacuated by emergency services. The sinkhole also caused utilities to be severed.

Emergency filling work was carried out, with 535 cubic metres of foam concrete poured in to give immediate support to the cavity’s steep sides.

While services have since been restored and residents’ lives are back to near-normal as most - bar those closest to the sinkhole - have returned to their homes, investigations have been carried out to gauge the risk of further sinkholes developing in the area.

Geotechnology Ltd was asked to urgently survey the public highway, from the junction of Fontmell Close and Seymour Road, to cover the whole of the cul-de-sac and Bridle Close which runs off it, from footpath to footpath.

The survey has involved looking at very small changes in gravity across the study area, to search for ground that is liable to subside.

This method has been used to detect a cavity in Hemel Hempstead, and at many locations across the chalk bedrock belt between Herts and East Anglia.

The firm said an 1878 plan shows several clay pits, two brick kilns, two lime kilns and three shafts in the vicinity - but one of the larger pits is located where Fontmell and Bridle Closes were later developed and where the current collapse occurred.

Apart from the extraction of clay, the underlying chalk was also possibly removed, the experts added.

Chalk was possibly worked through underground workings served by shafts to the west of the site, used for either mine entry or as air shafts.

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