Application of the brittle failure criterion to the design of roof support in the soft rocks of coal mines
The bilinear brittle failure criterion that utilizes the unconfined compressive strength (UCS) and a spalling limit of 3.4, together with a tensile strength cut-off can be used to model the height of failure above coal mine roadways. Transverse isotropy can be incorporated into a continuum analysis by using a Young’s Modulus/Independent shear modulus ratio of 15. To predict the height of failure, the two key variables are the roof strength index (UCS/pre-mining vertical stress) and the horizontal to vertical stress ratio. By factoring in the stress concentrations that occur about a longwall excavation the criterion can be used to predict heights of failure on initial roadway development and in the maingate. A support design based on dead-weight suspension of the failed mass can be utilised.