Effects of Surface Topography on Mining Subsidence Damage to River Channels

Longwall mining has for some time been recognised to cause perceptible horizontal movements well outside the area of active mining. Substantial coal resources have been sterilised in providing barriers to protect surface features that are sensitive to such movements. This project investigated the potential to provide more effective and substantially smaller barriers by better understanding the mechanics of the processes that cause horizontal subsidence movements. The report presents our current understanding of the mechanics of mining induced horizontal ground movements and the opportunities available to reduce the size of protection barriers based on this understanding.

The understanding of mining induced horizontal ground movements has developed in parallel with improvements in monitoring technique. These improvements have gradually allowed the magnitude, direction, and lateral extent of mining induced horizontal ground movements to be measured. Understanding of the mechanics of the processes that cause these movements is still evolving, but with ever better monitoring techniques becoming available, better strategies to control the far-field impacts of longwall mining on sensitive surface features will improve.

Early subsidence monitoring involved measuring only vertical displacement and differential horizontal movements in one dimension so the development of a more general understanding of horizontal movements was hampered by the various assumptions implicit in this technique.

Three dimensional surveying of distributed points was initially used for specialist subsidence monitoring applications in the late 1980's and early 1990's but has now become routine as total station survey instruments have become more readily available. The understanding of the processes that cause horizontal ground movement has improved significantly as a result.

Satellite based differential interferometry using synthetic aperture radar (DInSAR) is expected to provide the platform for monitoring far-field ground movements across large areas. This technology is currently still limited by satellite availability and orbit geometries, but this limitation is expected to be overcome within the next decade. Initial investigations using currently available satellite information show the nature of the current limitations of DInSAR but also point toward the enormous potential of this technique as more satellites and look angles become available.

Within the next three to five years radar data will become more available in multiple bands, for example, X-, C-, S- and L-bands, with weekly or even daily revisits. The constellation of radar satellites will allow multiple image acquisition at the same location with different heading direction, look direction and incidence angles in short periods of time so that the overlap times between each pair will be much higher. By having the much higher overlap time and more image pairs of different viewing geometry, there is no doubt that it will become feasible to generate maps of ground movements in three dimensions with high accuracy. This technique will then become a very powerful tool for monitoring mining induced ground movements.

The horizontal ground movements observed in two sets of data from different satellites above Appin and Tahmoor Collieries analysed as part of this project show that perceptible ground movements are observed to extend to a distance each side of the active subsidence bowl of approximately 1-1.3km over a monitoring period of approximately one month.

Monitoring of subsidence movements indicates that horizontal movements associated with longwall mining can be broadly divided into three components:

· Systematic horizontal movements that occur initially toward the approaching goaf and subsequently in the direction of longwall retreat. These movements are evident in flat terrain, tend to be largest at the start of longwall panels, and typically decrease with distance outside the goaf at a slower rate than vertical subsidence.

· Horizontal movements associated with a release of horizontal stress. These movements typically occur toward the goaf, but may occur in the direction of the major horizontal stress or toward deeply incised valleys. They appear to be characterised by discrete episodes of relatively sudden movement over a large area rather than incrementally with mining activity.

Horizontal movements associated with topographic relief. These movements occur predominantly on slopes that have been mined below but their effects can extend laterally outside the mining area to the bottom of the slope. These movements occur almost universally in a downslope direction and tend to be much greater when overall mining proceeds in a downslope direction.