Damage to the rock mass can be described by defining the extent (local or mine scale), location of the damage (pillar, floor or back or larger scale), mode of failure (tension, shear or coupled), and rock mass response (brittle or ductile). Rock mass behaviour mainly depends on mining geometry and the scale of operations. Manifestation of rock mass response can be visible on a “local” or excavation scale (indicating rock damage), “level” scale ground response, and “mine” scale (indicating rock mass damage).
Mining induced stress depends on mining geometry which is commonly defined by: extraction ratio, pillar shape and geometry of excavations. The extraction ratio, defined as mined out area / (mined out area + pillar area), describes the open unsupported span between pillars. Pillar shape is described as width: height ratio of a pillar. Although higher extraction ratio is often associated with lower pillar width: height ratio, both parameters are independent. Excavation geometry includes shape of the excavation and open span, interaction between neighbouring excavations, sequencing of extraction and ground control measures such as backfill or ground support, etc.
The effect of mining geometry on rock mass response is discussed on cases of pillar behaviour. For squat pillars with width: height ratio less than 2 and extraction ratio larger than 80%, the predominant mode of failure was locally manifested as spalling and vertical fracturing of the pillars between openings. Such mining geometry is encountered when a deposit is extracted by room and pillar mining, leaving small shaft pillars and small pillars between crosscut (e.g. sublevel caving method).
For regular pillars with width: height ratio to greater than 2 but less than 4 and extraction ratio above 40%, the rock mass instability changed from predominantly local scale to a level scale. Such mining geometry is encountered when a deposit is partially extracted.
Mine instability takes place for pillars with width: height ratio more than 4 (squat pillars) and low extraction ratio. For such a ratio pillars start failing on the periphery in a distinct circular fracturing pattern and progresses to the core. In the surrounding rock mass, long distance cracks are formed in shear (through the shoulder of the drive). Stress-strain relationship for squat pillars confirmed that mining geometry had an effect on post-failure rock mass behaviour.