Geomechanical Investigation of Post Reclamation Subsidence of Prairie Strip Mine Spoil

The post-reclamation subsidence of cast-back overburden is of interest to mining companies and regulatory agencies. Although subsidence may be of lesser concern than the prevention of soil degradation and the re-establishment of productivity, the amount of subsidence, the time-frame for the subsidence, and the amount of differential settlement have an influence on land use which must be evaluated. The effects of subsidence can be seen on highways constructed over reclaimed spoil, where a very rough surface develops and requires frequent resurfacing. In cultivated reclaimed fields, a corrugated surface develops over some time after reclamation, affecting runoff and causing waterlogged areas. Other behavior may be affected by subsidence; permeability changes associated with destruction of porosity, subsurface soil erosion because of the disturbed condition, and loss of integrity of buildings built on reclaimed spoil because of differential settlement may be important factors worthy of evaluation. Ideally, an ability to predict these factors should be developed Realistically , the processes are
extremely complex, and a complete predictive framework is not a reasonable goal.  This paper will outline the major practical concerns in the study of the subsidence processes and show how we are developing an approach incorporating understanding of processes, laboratory and geophysical measures, and instrumented field sites to quantify the settlement process and achieve some capacity to predict behavior. This study has in large part been funded by the Plains Hydrology and Reclamation Project, and we are grateful for their support and encouragement.