St Albans sinkhole: New video footage released showing ground survey near site

Boreholes being dug in the ground at Fontmell Close, next to the St Albans sinkhole

Boreholes being dug in the ground at Fontmell Close, next to the St Albans sinkhole - Credit: Photo courtesy Herts county council

Investigations are underway to determine whether a second sinkhole is lurking beneath the first collapse.

Twenty metre deep plugs of soil are being plucked from the earth near the St Albans sinkhole to determine what lies beneath the ground.

Herts county council has uploaded a video on YouTube showing Clive Edmonds, geotechnical engineer from Peter Brett Associates, investigating ground conditions and checking the safety of the ground at Fontmell Close, the site of a former clay pit.

There are fears that a second, larger, cavity is looming near the existing collapse.

Working on behalf of the authority, along with residents’ insurance companies, Peter Brett Associates has been using an A-framed, light cable percussion rig to drill boreholes below the road surface.

Clive explained that a winch mechanism is used to drive a tool into the ground, cutting the soil away, which is then brought up to the surface.

This plug of subsoil is placed into a bag for examination, “so we know what the geological profile is like and what the ground is made of”.

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He added that in order to keep the borehole open, it is lined with 150mm diameter steel casing “which gradually goes down the full length into the ground, like a tube, as we deepen the borehole.

“We will take the borehole to perhaps 20 metres depth, perhaps more, depending on the results we get.”

Clive said that in one part of the cul-de-sac, an initial microgravity survey carried out after the sinkhole appeared on October 1, “has indicated the ground should be fairly stable, and reasonably dense, [with] no notable voids or disturbed zones of low density within it”.

Thus one borehole was centred on that particular location, as it was believed to be safe for in-depth surveying work.

Clive went on: “We are checking the ground conditions to see whether the profile we get from the borehole matches what we expect from the microgravity survey results.

“Then we can match the two together, so we can gradually build up our understanding of the ground.”

Further boreholes will be created close to the collapse, with work expected to carry on until the New Year.