Site Investigation for Abandoned Lignite Mines in Urban Environment

Abstract.

The existence of shallow coal mining in urban environments poses a great danger to engineering works due to either the creeping subsidence at the surface, the induced deformations at depth, or settlement at the surface in the case of potential roof collapse. The detection of such works requires detailed site investigations in order to acquire as much data as possible on the layout and extent of mined areas and the geotechnical quality and behaviour of the geological formations. In this context, a review of the site investigation methodology in re-developed former mining areas is made. The applicability and efficiency of geophysical methods is discussed. Site-specific information on the extent of lignite mining activity in Peristeri area, in the western outskirt of Athens, was gathered in view of the new proposed Metro extension. A three-dimensional plan view of the Peristeri Lignite Mines was produced, based on existing data, in order to define most accurately the depth of the exploitation works and their possible interaction with the tunnel construction works. During the site investigation that was planned as accurately as possible, the core borings encountered voids and backfill materials confirming the assessed depth of the galleries and allowed a validation of the model. Additional data were provided during construction. The depth of exploitation works is limited to 30–90 m, developed in three main levels in a rather scattered pattern due to the presence of faults. The geology of the site consists of marly deposits possessing soft rock behaviour. The results of this study lead to a prediction of the size of a potential roof collapse and an analysis of the subsequent distribution of the ground deformation.