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Reaching and filling in the voids

An severe remedy was needed when it was discovered some Hatfield houses had been built over chalk mines. By Alexandra Wynne

Abandoned and unstable chalk mines beneath Briars Lane in Hatfield have been making their presence known for almost 30 years.

The structural problems became so severe in places that in 1986 four pairs of semi-detached homes were demolished. But now a project is underway to stop future ground collapses.

Hertfordshire County Council and Welwyn Hatfield Borough Council instructed Engineer Hyder Consulting to begin investigation work in 2004.

It drilled 30 boreholes as well as doing microgravity surveys in a six-month project to determine what was causing the problems. The consultant carried out 1,500 dynamic probes at 1.5 m spacings.

This helped the team establish where there were any offshoot tunnels and gave a clear indication of where the perimeter should be for the follow on bulk infilling work. Rig operators worked within circular perimeters around each of the houses in the areas affected.

The results of the investigations showed mines appearing at depths of between 7 and 20 m in low-grade upper chalk.

Site workers used CCTV cameras inside the boreholes that provided 360 degree images and revealed six large voids pointing to a network of mines.

Remedial measures

Following the discovery of the 2-to-4 m high mines, the councils awarded a £4 million contract to contractor Ritchies and Hyder produced a 3D mine model to monitor the work.

Ritchies is using a twofold approach Đ bulk foam infilling and compaction grouting - to stabilise the ground. The voids range in size from about 50 cu m to 1,000 cu m.

As a result of access difficulties Ritchies is using rigs that can stand away from the property while completing the work.

Over 750 holes need to be drilled to complete the stabilisation work. Site workers pre-drill them using flighted rotary augers before inserting by hand 3 m lengths of plastic casing down to depth.

For this part of the job a high cement grout was mixed with a protein-based foaming agent just before being pumped into the ground. This creates a lightweight fill material. Site workers have now completed the bulk infilling.

Work is now continuing on the next portion of the contract - to compact the loose ground that surrounds the voids.

Alexandra Wynne is Deputy Editor of Ground Engineering. A longer version of this article is printed in November's GE.