Tunnel block collapse instability characteristics and optimized design of targeted support based on multi-scale modeling method

The uncertainty in the development patterns and combinations of structural surfaces makes it difficult for the effective prevention and control of disasters such as landslides and block falls during the excavation of jointed rock tunnels. It is crucial to consider the impact of weak-scale joints on the mechanical properties of the surrounding rock，as well as the controlling effect of large-scale structural planes on local stability，in order to provide reasonable support. To this end，based on the morphological distribution law of two-dimensional trace lines in sequential tunnel face images，the rapid extension and prediction of dominant structural planes in three-dimensional space have been achieved. Using the embedding coefficient method in Sadovsky's theory，the theoretical values for different structural plane scale divisions were determined. By employing the secondary development function of 3DEC，a multi-scale DFN-DEM equivalent modeling method for jointed rock masses was proposed，constructing an equivalent rock mass model that retains only the dominant structural planes and large-scale joint sets. This method was validated and analyzed through a series of tests conducted in the Erlangshan Tunnel project of Linzi-Linyi Expressway in Shandong Province. The results indicate that the multi-scale equivalent modeling method for jointed rock masses，which incorporates the dominant structural plane extension method，more effectively reveals the collapse evolution laws of the local surrounding rock in tunnels，especially the morphological distribution characteristics of key block groups. The rose diagram of hazardous rock groups provide a theoretical basis for understanding the spatial distribution of overbreak caused by dangerous rockfall in tunnels from a probability and statistics perspective. By integrating these methods，targeted support schemes can be provided for structure-controlled surrounding rock，offering both a theoretical basis and field guidance for the prevention and control of local collapse instability in jointed rock mass tunnel engineering.