Geo-engineering of mining subsidence affecting the highway network at Holywell, North Wales

Abstract

The Holywell district of North Wales is underlain by Lower Carboniferous Limestone that hosts a large number of base-metal (silver-lead-zinc) mineral deposits. A long history of mining has left an unwelcome legacy of mine workings that pose a hazard to the local community. Ground movements caused by gradual deterioration of the abandoned workings result in surface instability that affects the highway network. The incident rate is about one collapse per year. The large number and variability of mining features together with diverse geological conditions combine to preclude the possibility of predicting the collapse events. Each subsidence that damages the carriageway must be treated rapidly as emergency work and be designed to minimize disruption to traffic. Reliance is placed in the first instance on geological predictions based on interpretation of the mining records, stratigraphy, structural history and hydrogeology. These are supplemented by observations and measurements during site investigations. Experience has shown, however, that the most critical geological factors are often difficult to ascertain. The problems and successes are described for five case histories of mine workings that affect the highway network at Holywell and post-remedial inspections are advocated as a prudent strategy of risk management through which to monitor such fragile sites.