A large engineering firm based at the Gilchrist Industrial Estate, Blaenavon, proposed to extend their existing premises onto an adjacent site to accommodate new plant.

Terra Firma were retained to undertake a comprehensive ground investigation including a desk study, coal mining risk assessment, laboratory chemical testing of contaminated land, human health risk assessments and foundation design.

During the coal mining risk assessment Terra Firma  discovered a backfilled former mine shaft within the foot print of the proposed development area. Despite being previously backfilled, the shaft required stabilisation for development to proceed.

The stabilisation process required pressure grouting using a mixture of Pulverised Fly Ash (PFA) and Ordinary Portland Cement. To complicate matters an active drainage and ventilation adit from the nearby Big Pit Museum passed beneath the base of the shaft. The integrity of the passage needed to be maintained during the treatment of the shaft.

Terra Firma  undertook the grout treatment and, in addition, coordinated a full time watching brief in conjunction with the Mines Rescue Service, including setting up an underground communication system to ensure the grouting works did not affect the passage beneath the shaft.

The treatment was performed successfully without affecting the passage beneath the shaft. The extension has now been successfully constructed.