Shallow site investigation of Quaternary sands inside and in the vicinity of a sinkhole in the former lignite mining area in Zielona Góra (western Poland)

The paper presents a brownfield site investigation of the area where lignite was formerly exploited with an underground mining method. The Miocene lignite seams were folded by glaciers and covered with a layer of highly compacted sediments with sands on top. Yet eighty years after the extraction ceased, new sinkholes still develop. The aim of this work is to determine both the mechanical parameters of soil in the area of sinkholes and the changes in the values of these parameters induced by the process of sinkhole formation. The applied methodology involves in-situ investigations. Soil state and strength parameters were examined with the use of CPTU and DPL tests, while stiffness parameters were determined in the SDMT test. The evaluated parameters of soil may provide data for numerical modelling of the process of sinkhole formation and may significantly simplify future in-situ investigations in the area where the soil profile shows high natural changeability of state. The knowledge of values of parameters in a sinkhole and outside it enables easier differentiation between the zones of undisturbed soil and zones where sinkholes formed in the past (and were then backfilled) or where the sinkhole formation process is currently in progress