Ground water withdrawal and land subsidence in northeastern Saxony (Germany)

Abstract

Several large open pit lignite mines are located in northeastern Saxony, Germany. To guarantee the production of electric power until the year 2026, a couple of these pits have to be extended. Most of the open pits are, at most, 120–150 meters deep. Nevertheless, a large area is affected by ground water withdrawal. There is a concern that differential subsidence due to mine dewatering might cause vertical stress and building damage. The area of investigation is a 2km2 large building complex built in the early 1970’s. It will be dewatered to a depth of 70 m below the surface, resulting in subsidence that has been predicted by both analytical and empirical methods. From empirical calculations, based on subsidence measurements, a total subsidence of 0.2–0.5 m was predicted. Analytical calculations, on the other hand, using the principles of Terzaghi’s consolidation theory, yielded an average possible subsidence of 1.1–1.6 m by the end of 2026. These differences might be due to the fact that sand and gravel lenses in the tertiary clays have not been fully dewatered, or that some of the less permeable units will subside very slowly, or that the sediments were compacted more by the Pleistocene glaciers than expected. Taking into consideration the calculations as well as the known measurements, a subsidence of at least 0.2 m is highly likely. Due to the geological situation, differential subsidence and the associated damage is possible for part of the area.