A Regulation 33 Geotechnical Assessmnet was carried out for Castle Cement Limited, in accordance with the Quarries Regulations 1999, approved Code of Practice (ACoP), at their Grange Top Quarry near Ketton, Rutland, UK in 2008. The assessment identified a total of 10 tips within the quarry for which the operator faced various constraints and could not undertake operations practically and in accordance with a completely ‘low risk’ methodology. In order to reduce the impact of the constraints a methodology was formulated which entailed increasing stockpile height some by 100%, increasing face angles and utilizing rock fill for features such as noise bunds. The proposed methodology was categorized as a medium risk. Once the methodology had been formulated it was proven by a range of numerical models including limit state and finite element and probabilistic analysis relating to material parameters, slope angles, groundwater, precipitation and also impact on third parties. Finally, upon commencing works, a monitoring regime was put in place to ensure that the proven methodology could be observed and any divergences could be highlighted and input back into the probablistic analysis in order to devise a new way forward for that particular constraint.
This paper documents the approach undertaken for Castle Cement Limited in order to identify the factors affecting the safety and stability of stockpiles within their Grange Top quarry facility in the context of a Regulation 33 ‘Geotechnical Assessment’ and formulation of ‘Excavation and Tip Rules’. Both were undertaken in accordance with the UK Quarries Regulations 1999, Approved Code of Practice (ACoP). Particular emphasis is placed on how the identified hazards were assessed and how a pragmatic and economic solution to the ongoing requirements of complying with the Quarries Regulations 1999 was arrived at.
An initial Regulation 33 Assessment appraisal in 2001 summarised the engineering characteristics of the strata within the Quarry. This is presented in Table 1 (below).
(Table in full paper)
The report did not identify the stockpiles within the quarry to be significant hazards in accordance with the Approved Code of Practice (ASoP) and were described as generally small in volume. It was proposed that stockpiles within the quarry area be limited to less than 7.5m in height and graded in all cases to inherently safe angles of less than 22°.
The 2008 Geotechnical Assessment comprised of field reconnaissance, measurements and observations of rock and soil formations, faces, slopes, tips, stockpiles, ramps and lagoons recorded at key areas around and topographical survey. These built upon and developed information from the previous assessment which also included intrusive ground investigations and laboratory testing.
The assessment comprised inspection and evaluation as follows:
Qualitative assessment of the stockpiles and slopes were undertaken, providing a stability risk (potential for failure) and safety risk (consequences of failure) associated with the geotechnical characteristics at each location;
The hazard associated with each feature was evaluated and classified as significant.