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BAM Ritchies

Case Study: Hatfield Chalk Mine

Client: Welwyn & Hatfield Borough Council

Consultant: Hyder Consulting

Project Manager: Mace

Location: Hertfordshire

Date: 2007 - 2008

BAM Ritchies undertook a chalk mine stabilisation project (funded from English Partnership’s Land Stabilisation Scheme) which demanded innovative and unique drilling and grouting techniques within a residential area of Hatfield, Hertfordshire. Briars Lane comprised houses, gardens, highway and school grounds which were all at serious risk of sudden collapse caused by mining of chalk up to 200 years previously.

Work included drilling over 700 boreholes up to 25m deep in sands and chalk, injection of 1,900m³ of limestone/OPC compaction grout and 2,280m³ of foam concrete, validation of the treatment by over 250 dynamic probes and reinstatement of highways, gardens and school grounds.

The boreholes were drilled using a variety of drilling rigs, including masts attached to long reach excavators. This allowed drilling to be efficient, safe and reduced disruption to the general public, residents and their properties. Boreholes were also drilled at varying lengths and angles in order to treat the mines under existing buildings.

The boreholes were accurately surveyed via sophisticated CCTV cameras to confirm the location of shafts and the size of the mine workings. Once surveyed the boreholes, voids and mines were filled utilising specialist equipment, such as packers and Lutz monitors to accurately target areas, allowing detailed monitoring of grout pressures and volumes at the point of injection.

Once the stabilising works were completed, BAM Ritchies reinstated the gardens and highway and also installed a new surface water drainage network with remote soakaways.

A true partnering relationship between, BAM Ritchies, Mace, Welwyn and Hatfield Borough Council and Hyder Consulting, ensured that this high profile and innovative project was successfully completed and validated.