The importance of strata strength and defect density in estimating the height of the zone of reduced pore pressures associated with connected fractures above longwall panels has been identified by numerous researchers. In this paper we compile conceptual, empirical, physical and numerical data from Australia, America, England and China. Observations from this data on geological impacts on the zone of reduced pore pressures are compared. Based on a series of numerical models that simulate the physical experiments of Whittaker, we identify the height of bridging in the over burden and the height of connected fracturing. The observations and conclusions from this data review emphasise the importance of simulating site specific strata when considering coal mining water impacts.