Detailed engineering geological investigation of the Brunner Coal Measures overburden, Webb Block, Stockton Colliery, was carried out to determine the principal geotechnical constraints or 'Hazards' to continued opencast development. The Stockton Opencast Mine currently produces 600,000 tpa with plans to increase production to 1.2 million tpa. Within the overburden two main lithological units are recognised: coarse, massive, variably cemented sandstones (Unit 1); and fine laminated siltstones and mudstones which are variably carbonaceous and micaceous (Unit 2). The tine laminated unit has a pronounced strength anisotropy, whereas the coarse massive unit is essentially homogeneous. All the materials are within the low to medium strength categories (UCS < 75 MPa). The fine laminated unit dominates the lowest 10 m of overburden and the coarse massive unit the upper 10 m. An iso-ratio map is presented at 1 :5000 scale, and shows the percentage thickness distribution of both I ithotypes within the Webb Block. The Webb Block is divided by the major Augustus and Baynes Fault Zones into three natural mining blocks. Bedding concordant discontinuities within the overburden include minor bedding plane shears (1O-20mm thick) developed within monoclines, and more substantial shear zones, persistent over at least tens of metres, located at lithological contacts. The Baynes Fault Zone is a monocline developed over inferred faulting at depth, as is the Augustus Fault Zone. The later is more complex with a wider deformation zone (c.50 m) and has numerous impersistent fault plane exposures. Shearing and/or minor faulting is common throughout the Webb Block but more prevalent in close proximity to the major fault zones. Jointing is characterised by a parent joint system within which three joint sets, which strike N-S, Nand NE, have been recognised. Within each set fractures are widely spaced and highly persistent. Highwall failure at Webb occurs by toppling, complex wedge/collapse or translational block slide. A Geotechnical Hazards Plan is presented which shows all the major stability hazards within the Webb Block. The principal constraint to opencast development is the unfavourable orientation of geological structures relative to proposed highwall alignments. Instability is further exacerbated by surface and groundwater draining across the high walls with a resultant build up of seepage pressures within the rock mass. The presence of abandoned underground mine openings reduces the basal support for sections of highwall. With continued open pit development at Webb by strike cuts and up dip advance, potential highwall instability is best resolved by:-

1. Identification of potential stability hazards by reference to the Geotechnical Hazards Plan coupled with follow up field investigation.
2. Setting the highwalls at safe stand-off distances from the hazards, highwalls can either be located down dip short of the hazard, or up dip by stripping through the hazard zone.