Investigations into whether the ground surrounding the St Albans sinkhole is safe have begun.

Engineers are currently digging deep in to the ground in Fontmell Close, where three months ago a giant sinkhole opened up, to find out what lies beneath the road.

Working alongside Hertfordshire County Council and insurance companies, the engineers from contractor Peter Brett Associates are currently using an A-framed light cable percussion rig to drill boreholes up to 20 metres in depth into the road to find out the geological profile of the soil there.

Once they have the soil samples, they will compare the results with those of the microgravity survey they have already carried out, to determine the safety of the ground.

Clive Edmonds, a geotechnical engineer from Peter Brett Associates, said: “Our microgravity survey has indicated that the ground should be fairly stable and reasonably dense – no notable voids or disturb zones within it of low density.

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

“Then we can match the two together so we can gradually build up our understanding of the ground as we go from this position and then move on to other positions around the collapsed feature.”

Hertfordshire County Council has released a short video which demonstrates the process.