Risk assessment of seismic events by mine flooding

Summary

Abstract

Organized coal mining in the german Ruhr mining district lasted for centuries and ended in 2018. Mining took place at depths down to -1200 m bgl and required severe pumping of the otherwise near surface groundwater. To maintain the permitted water level at around -600 bsl, appr. 70 Mio cbm mine water must be pumped each year. The Ruhr Mining District is divided in 6 “water provinces” which are hydraulically separated from each other. For the easternmost water province, the owner started the permitting process for rising the mine water level by 220 m. It is known that during mining and during the initial mine water rise seismic events with maximum magnitude ML = 3,0 (during mining) and ML = 2,3 (during water rise) occurred. The peak particle velocity during mining was vs,max = 52.09 mm/s and during the initial water rise vs,max = 11.53 mm/s The mining authorities requested a risk assessment for water induced seismic events. This paper deals with the methodology for the requested risk estimate and presents its results. It covers the geological and geomechanical basis for advanced 3D numerical modelling, the stress concentrations around numerous longwall panels in several seams and concludes on the risk of seismic events from failure of the rock mass. Numerous faults (normal faults, thrust faults and strike-slip-faults) are present in area of assessemnt. The paper will also include the risk assessment of fault slip by increased water pressure.