Investigations into St Albans' sinkhole show 'no evidence' of second cavity
PUBLISHED: 06:00 26 May 2016 | UPDATED: 16:19 26 May 2016
Fears that a second sinkhole is looming near homes in a St Albans street have been quashed after extensive investigations prove there is no evidence a further collapse is likely.
The good news will be welcomed by residents in Fontmell Close whose lives were turned upside down after the road collapsed and left a 603 cubic metre cavity on October 1 last year, forcing them to evacuate their houses.
Experts have now established that the cause of the collapse was man-made and was localised beneath the sinkhole itself.
Ground subsidence investigations have revealed new information that in addition to a deep backfilled historic clay pit beneath the site, a shaft had been dug to create an entrance into a ‘bell’ shaped mine to extract chalk.
A county council spokesperson said this week: “We are pleased that these further investigations have given us no further cause for concern over possible mined voids in this location.”
Among those welcoming news of the all-clear was David Kaloczi, whose blind father, Francis, spoke to media after being evacuated via an emergency exit on a nearby field.
David said: “I’m delighted nothing untoward has been found, and hopefully the road will be re-opened shortly. My father has been bearing up well, but he has been wondering why the road has been closed for so long.”
The county council is currently considering a range of remedial options to both the road and utility services before re-opening Fontmell Close.
Days after the sinkhole appeared, an initial microgravity survey carried out along the surface of both Fontmell and neighbouring Bridle Close, as well as a playing field behind both cul de sacs in Bernards Heath, had highlighted potential voids deep in the ground.
The most worrying of those ‘anomalies’ - weak ground - was a second, possible larger cavity, described by surveyors as a significant 3,200 tonne ‘missing mass’, or up to three times larger than the sinkhole, located to one side of the collapse.
It was not known at that stage whether the centre of the anomaly was under the road in Fontmell Close, or private property adjacent to it.
Further geophysical surveys carried out in privately owned properties partially corroborated those results – leaving residents on tenterhooks.
But Herts county council last week released the official findings of extensive intrusive investigations, which run into hundreds of pages and have put paid to those fears.
Peter Brett Associates (PBA) was commissioned by a range of stakeholders impacted by the ground collapse, including loss adjustors for four homes in Fontmell Close, one at Bridle Close, and the county council.
The firm has reported its findings into deep ground investigations completed below the road and adjacent private properties, to help the authority decide future remedial action.
It confirmed that the site of the collapse, and surrounding properties, coincide with the location of a historic clay pit – a lime kiln and brick kiln were located to the south of the pit.
PBA also examined a depression in the ground, to the rear of one of the homes close to the 12-metre-wide sinkhole, and hand-dug trial pits to inspect the foundations of two other houses in Fontmell Close.
Little water was used while carrying out deep drilling, to minimise the risk of triggering ground movement by the boring process, in case further voids were in the ground.
The firm found that the source of the ground instability was deeper than the backfilled old pits, so it conducted deeper boreholes, by drilling through the 535 cubic metre foam concrete which was pumped into the sinkhole to plug it.
PBA pointed out that chalk mines tended to contain larger volumes of empty space, and were capable of producing ‘rather large holes that can exceed 10m in diameter’.
It concluded that a shaft was sunk in the base of the clay pit, from which miners excavated out to the sides into adjacent chalk walls to extract chalk from an “irregular bellpit style of mine.
“Positioning a shaft at the base of the pit reduced the depth of excavation to reach chalk for mining. This style of extraction is likely to be repeated elsewhere in the wider Bernards Heath area where historical brickmaking has taken place.”
PBA concluded: “Investigations have shown that the major collapse below the highway is deep seated, to about 40m below the surface.”
The shaft dug below the pit was between five to 10 metres deep.