Stability assessment of an ancient landslide crossed by two coal mine tunnels

• Stability of an ancient landslide crossed by two coal tunnels was studied.
• Field surveys, drillings, monitoring and numerical simulations were applied.
• This landslide is stable as a whole, and only local shallow landslides might occur.
• Local instability of the landslide has little impact on the shaft safety.
• This study has led to a budget savings of over RMB 40 million for the owner.

In 2005, when two main tunnels were excavated in Faer Coal Mine, Guizhou Province, China, an unknown ancient landslide, subsequently named Dazhai landslide, was encountered. Roof caving, large convergence and severe support damage in the tunnels, as well as several ground subsidence occurred. The two tunnels have been kept stable after an inner supporting treatment in 2008. However, since a heavy rainfall in July 2010, some transverse cracks were observed at the landslide toe, determining significant additional costs over the normal administration of the mine. Invited by the owner, we performed a comprehensive investigation to evaluate the stability of Dazhai landslide crossed by two main tunnels. Firstly, field surveys and mappings were completed to obtain a preliminary delineation of the landslide surface, and a geological drilling along the central landslide axis was accomplished to depict the sliding surface. After that, a monitoring system containing a GPS–RTK network and six observation sections in one tunnel were established and a 12-month monitoring was conducted. Moreover, to obtain an overall comprehension, numerical simulations were carried out by using GeoStudio and FLAC3D software. The results from site drilling, monitoring and simulations indicate that the Dazhai landslide is stable as a whole, and only local shallow landslides might occur. The local instability of Dazhai landslide has limited impact on the safety of the two main tunnels. This conclusion has led to a budget savings of over RMB 40 million.