Impact of strength properties of timber linings in former workings on the sinkhole formation hazard. The case study is on the Upper Silesian Coal Basin, Poland

In the Upper Silesian Coal Basin (Poland), numerous former workings have been left unprotected after the liquidation of mines in the 19th and the beginning of the 20th century. The workings have been located at low depths. The paper presents the results of strength tests of wood samples acquired from linings in former workings, and the obtained results have been compared to the results achieved in tests of samples of wood intended to be used in a reconstruction of a historic gallery. The tests consisted in determining the bending strength of wood in compliance with the applicable Polish standard. The results showed that the wood from historic mines was characterised by high variability of bending strength – usually much lower than that of the wood intended for construction. Too low bending strength of timber may result in caving in shallow excavation and lead to sinkhole creation on the surface.