St Albans sinkhole: experts continue investigations into surrounding area

PUBLISHED: 15:00 26 November 2015 | UPDATED: 10:32 04 December 2015

Surveyors working on the playing field with a machine to dig bore holes in the search for other ground anomalies

Surveyors working on the playing field with a machine to dig bore holes in the search for other ground anomalies

Archant

Experts have returned to the site of St Albans' sinkhole, this time to carry out further checks of the underlying geology of a nearby playing field where an emergency access was created.

Concerns about a ‘significant’ anomaly – weak ground – in the undulating field behind Bridle Close has prompted surveys to be carried out to identify the best route for an improved temporary stone road.

The anomaly was picked up in an initial microgravity survey by Geotechnology Ltd last month, after the road in Fontmell Close dramatically collapsed and left a 603 cubic metre cavity in the early hours of October 1.

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

Geotechnology Ltd’s investigations showed the anomaly appeared to cross the temporary access road, which was created for residents immediately after the sinkhole appeared.

The firm, in its report to Herts county council, said this area was part of another clay pit, and a well was shown in a similar position on a historical map from 1965.

A letter sent recently to residents of Fontmell and Bridle Closes from St Albans district council explains that the route of the temporary road has been changed “in light of the significant ground anomaly identified beneath the playing field.

“The use of this route is now limited to the 4X4 shuttle vehicle and emergency vehicles.”

Residents have been able to use a shuttle service provided daily by the council between the cordoned off cul-de-sacs to their vehicles parked at the nearby former fire station.

The council started providing transport across the field as vehicles were becoming bogged down in the muddy ground.

In its letter to residents, the authority said surveys carried out to find the best route for the improved temporary stone road have resulted in two possible options, which are being investigated further.

Contractors will be asked to tender for the work and a planning application submitted.

Meanwhile, the council has also updated residents on a potentially larger cavity that was discovered to be looming underground, right next to the current sinkhole, by Geotechnology Ltd.

The letter said that a separate microgravity survey has recently been completed at three private properties closest to the sinkhole, and a further microgravity study is planned for two homes near the second possible cavity, described by surveyors as a 3,200 tonne ‘missing mass’.

Work is planned for a topographical survey, ramping down the sides of the partly concrete-plugged sinkhole to improve engineers’ access, and for a borehole in the road near the cavity.

This preparatory work will be followed by intrusive ground investigations, with inclined (at an angle) drilling – the results of which will help determine remediation measures needed.

No date has yet been set for these intrusive investigations.

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