Engineered Mine Design in Soft Strata Environments - Geomechanical Behaviour of Soft Floor Strata in Underground Coal Mines

The work described in this thesis has been concerned with investigating the geomechanical behaviour of soft floor strata in underground coal mines, especially in the context of bord and pillar layouts. Many cases of significant mining problems have been reported from underground coal mines located in soft and weak floor strata environments. These range from local operational problems like bogging of equipment and support systems, floor heave leading to disruption in transportation and ventilation, regional deformation and collapse because of pillar settlement, pillar punching or indirect pillar or roof collapse through to long term unplanned surface subsidence .

This investigative study defined four behavioural modes and distinguished the associated long term deformation trends on the basis of the geomechanical process operating in the field

• Plastic failure of the floor material (due to either ultimate bearing capacity failure under pillars or local yielding in the floor strata) would lead to continued deformations due to plastic flow over time, potentially, forever.
• Consolidation of the floor strata under the pillar load would lead to continued settlement deformation over time until the floor material is drained, after which, no further deformation would take place. The time in which total settlement would occur would be directly proportional to the square of the drainage path.
• Swelling (expansive) clays, when present in the floor strata in moisture deficient and unsaturated state, would exhibit floor heave over time when subjected to an increase in moisture.
• In Awaba tuff environments, creep strains in the floor material under pillar load will lead to continued settlement deformation over time until such time that the material fails following tertiary creep, exhibits secondary creep indefinitely or ceases to exhibit creep, depending on the deviatoric stress levels in the material.

Based on the outcomes of the study, a mine design investigation scheme has been proposed in order to gain an insight into site specific behaviour of soft floor strata. This includes

• Developing a sound geological database
• Conducting strength and deformation tests with special emphasis on post failure material characterisation.
• Conducting creep tests at a range of different confinement levels
• Using the nomograms, developed in this research study, as guidelines to estimate the deformation potential of a soft floor strata environment
• Conducting site specific numerical modelling.
• Consolidation tests

The most significant implication of the material characteristics is that when analysing or designing a mine panel in soft floor environment, it is of paramount importance to accommodate the fact that long term deformation and even failure of strata may set in, due to one or more geomechanical behaviours distinguished in this study, even though the stress amounts may be low enough not to induce peak strength failure in material.