Abstract

Effective engineering characterization of granites altered through processes of kaolinization is critical for safe extraction and design of slopes for the china clay industry in south-west England. This case example describes the remediation undertaken to alleviate the significant hazard associated with a 26 m high legacy slope within an operating china clay pit. Remote rock mass characterization of the hazardous slope was undertaken using terrestrial laser scanning from numerous set-up locations to provide geo-spatial data for blast design, kinematic analysis and subsequent instability analyses. The trial pre-split blast resulted in a reduced overall slope angle and sufficient space for design of a suitable rocktrap, thereby reducing the risk to acceptable levels when following a Rockfall Hazard Appraisal System (RHAS). The trial also provided useful guidelines for future use of the technique, particularly for implementation when considering controlled blasting when approaching final pit wall limits.

1 Introduction

Recent improvements in both digital data acquisition and software for subsequent analysis, through use of photogrammetry and both aerial and terrestrial LiDAR (Laser Interferometry Detection And Ranging), have provided improved tools to capture rock mass characteristics, spatial three-dimensional data, slope geometries and digital terrain models for geotechnical evaluation. According to Stead et al. (2000) potential slope instability mechanisms within china clay operations are controlled by the degree of kaolinization or alteration, presence of adversely oriented discontinuities and detrimental effects of groundwater. For example, less altered granite slopes are dominated by the potential for discontinuity-controlled or kinematic instability, whereas highly altered slopes are more susceptible to rotational failure through the rock mass. Rock mass quality also dictates excavatability, particularly where there is any spatial variability in alteration grade for a particular slope.

This case example describes the remediation undertaken to alleviate the significant hazard associated with a 26 m high legacy slope face. Remote rock mass characterization of the hazardous slope was undertaken through use of terrestrial laser scanning from numerous set-up locations to provide sufficient geo-referenced spatial data for blast design, kinematic analysis and subsequent instability analyses. Kinematic analysis highlighted the significant potential for toppling and the risk posed by rockfall. Post-blast laser scanning of the slope was undertaken for validation of previous characterization, evaluation of the blasted face profile and blast-volume determination. Further analysis of the trial blast used Half Cast Factor (HCF), high-speed video analysis of the blast and in-hole Velocity of Detonation (VoD) investigation.

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