Creep settlement of opencast mine backfill.

The extraction of a significant amount coal in the United Kingdom has over the last four or so decades been made using opencast mining methods. This method involves large-scale excavation to reach the coal seams (with depths often exceeding 100m). Following extraction various forms of restoration have been employed, including backfilling the mine void with material excavated in order to extract the coal. This backfilling was frequently undertaken in an uncontrolled fashion. In recent years it has been more usual to engineer the restoration such that further development of the site can take place. However, settlement remains the major obstacle to development.One mode of settlement commonly encountered on restored sites is creep settlement. Further, on older sites whose restoration was, most probably, undertaken in an uncontrolled way, the potential for creep settlement. is the major obstacle. Creep settlement is understood to be that component of total settlement which, in a coarse granular soil, takes place under conditions of constant stress. It is generally accepted to occur linearly with the decadic logarithm of time. This study investigates the phenomenon from the visualisation of the granular particle mechanics.The technique used for the visualisations is computed tomography (CT), a technique which is common in medical diagnostics but has rarely been exploited in soil mechanics and never at the scale employed in the study. The use of CT supplements a programme of high quality, large scale laboratory testing, which models typical opencast coal mine backfill.The testing programme has revealed that compaction and the diagenesis of the source materials have a significant effect on the creep rate. Further, that the creep rate is defined in the most part by the aggregation of small or minor movements rather being dominated by large or major movements.