St Albans sinkhole survey shows possible larger cavity

The partially filled in sinkhole on Fontmell Close which has had over 500 cubic metres of foamed con

The partially filled in sinkhole on Fontmell Close which has had over 500 cubic metres of foamed concrete poured into it - Credit: Archant

Weakened ground has been discovered next to the St Albans sinkhole, raising fears that a second, potentially larger hole, is lurking beneath the surface.

The results of an initial survey of the roadway and footpath at the site of the cavity, which appeared in Fontmell Close in the early hours of Thursday October 1, were released at a press conference held by Herts county council today (Monday).

A number of ‘anomalies’ have been identified, one of which appears to have characteristics similar to that which resulted in the sinkhole.

According to the council, “initial modelling suggests this could be larger than the existing collapse, possibly by up to three times”.

The report itself says that interpretation of initial data shows: “There is a significant missing mass on one side of the existing collapse, but not on the other.

“Unfortunately, the data is limited to the public highway … interpretation would be improved by additional data obtained within the private property adjacent.”

The report suggests that if there is a second void, it would be positioned at a depth of 19 metres, and “would therefore lie within chalk bedrock”. But: “Unfortunately it is not known whether the centre of the anomaly underlies the road or the private ground adjacent to the ground.”

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It adds that following the initial collapse of the sinkhole, “a number of tension cracks were observed in the surrounding ground.

“Accounts from staff on site during the infilling of the collapse with concrete suggest that some of these cracks have become visibly wider and more extensive.

“Both during and after completion of the infilling of the sinkhole, loose material from around the edges of the collapse has been seen to fall into the hole.”

Because of concerns about the stability of the ground near the sinkhole, further monitoring will be carried out and residents of the homes closest to it remain evacuated.

With further surveys being carried out from this week, and given the uncertainty about the road, Fontmell Close will remain closed until further in-depth information is available.

More details in this week’s Herts Advertiser.