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Site investigation spotlight: Pharmaceuticals plant, North East

Drilling and investigation specialist Bam Ritchies has carried out emergency stabilisation work at a pharmaceutical plant in the North East.

Under a three-year ground investigation, drilling and grouting term contract with client the Coal Authority, the firm investigated suspected mineworking subsidence at the plant.

“The client asked us to investigate the site to help prove liability for the subsidence that was affecting the building,” says Bam Ritchies project manager Sarah Sherratt. “We devised a method of investigating the area and reporting back to the client.”

The site team used borehole investigation techniques to probe to depths of up to 30m. It found coal, voids and broken ground at depths between 26.7m and 30m and concluded that a mineworks failure beneath part of the building was the most likely cause of the collapse.

Emergency drilling

Bam Ritchies remained on site to carry out the emergency drilling and grouting stabilisation work. It installed a series of grouted perimeter boreholes and primary, secondary and validation holes to a total drilling meterage of more than 11.5km.

This work included drilling holes using Deltabase 100 and 120 rotary percussive rigs equipped with a T38 drill steels for the infill holes and an Atlas Copco A65 rotary rig with a down-the-hole hammer for the perimeter holes.

For the next stage, grout was mixed at a blend ratio of 6:1 pulversised fuel ash/ordinary Portland cement was used for the perimeter holes and at 10:1 across the infill holes and pumped through 63mm diameter tremmie pipes. More than 4,200t of material was used.

“The majority of our work is concerned with shaft collapse and subsidence associated with old mine workings. Typically, if there is a collapse we will investigate and probe around the outside of the mine shaft first, then look at the shaft itself,” says Sherratt.

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