The development of mining-induced horizontal displacement in relation to vertical subsidence

The subject of this paper is horizontal displacement in flat to gently undulating surface terrains directly above longwall extraction within generally horizontal sedimentary sequences. The findings reported in this paper are based on studies of 3D survey data from 14 study sites involving 18 longwalls in the Hunter, Newcastle, Western and Southern Coalfields of NSW. Horizontal displacement, as described above, is one of the core subjects of conventional subsidence engineering, due to its significance in causing subsidence impacts on many different kinds of surface features. The theories and prediction models for this displacement have been well-established in conventional subsidence engineering since the early 1950s and are accepted world-wide to date. The characteristics of the horizontal displacement as observed in NSW Coalfields differ in a number of ways from those documented in the conventional subsidence engineering literature. These observed differences have led to the identification of a component of horizontal displacement that has no association with the differential vertical subsidence. The magnitude of this component of horizontal displacement is estimated to be 20% to 40% of the recorded total horizontal displacement. Mechanisms for this component of horizontal displacement, which is not considered in conventional subsidence engineering, are discussed in this paper taking into consideration typical mining conditions in eastern Australia. This paper further discusses the implications of the authors' findings for the current practices of subsidence engineering / risk management. Importantly, the authors suggest that a number of fundamental aspects, such as distribution, magnitude, mechanisms and principal affecting factors related to mining-induced horizontal displacement, as documented in the conventional subsidence engineering literature, will need to be revised to suit Australian mining conditions.