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Summer start urged on Hatfield mine stabilisation


A HERTFORDSHIRE school was closed and nearby homes evacuated last month after it was found the ground beneath them was in danger of collapsing.

The Briars Lane area in Hatfi eld has suffered a number of subsidence events since the 1960s. Investigations have revealed the area is on top of a former chalk mine.

A risk assessment by Hyder Consulting, released last month, revealed 11 properties, the main road and parts of New Briars Primary School's car park and playing field were at high risk of falling into mine voids within the next five to 10 years.

Stabilisation work needed to start by June 2006, the report said, or the ground would deteriorate and a much bigger remediation programme would be needed. 'Collapses could occur at any time both spontaneously or influenced by such factors as heavy rainfall or leaking services.' Ground movements due to mine workings would be of the crown hole type, the study found, taking place very quickly and there would be no time for remedial measures to minimise the damage. The impact of major structural damage to buildings posed great danger to occupants, or in the worst case death, it added.

The risk assessment was part of a larger feasibility study to establish the need for land stabilisation. The project is managed by Mace and joint funded by the Office of the Deputy Prime Minister and English Partnerships.

Mace project manager Dan Kola said the company was aiming to submit the study and apply for funding last month. He said although funding for the project was not guaranteed, if successful work was hoped to start in summer 2006.

The project would involve filling the voids and compacting underground collapses.

Work would take a year and cost between £2M to £3M depending on the materials used, according to Hyder Consulting technical director for geotechnics Chris Milne.

'We will be drilling a grid of holes across the site and infilling the voids with a block fill grout. The grout will probably be a sand cement mix.' Milne said underground collapses would be dealt with by drilling into the ground and injecting grout into the collapsed material to form a solid bulb to prevent the loose chalk from moving any further.

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