”Interlocking strength” in tunnelling – an attempt at classification

The “interlocking strength” (German: Verbandsfestigkeit) as a description of the tunnel face conditions in conventional tun-nelling is known as a common, albeit not uniformly defined term, by some also considered to be antiquated or even redun-dant. The aim of this study is therefore to establish a quantita-tively substantiated relation between the documented interlo-cking strength of a tunnel project and engineering geological and geotechnical rock mass parameters as well as parameters related to construction measures in order to specify the term based on empirical data. Using the statistical method of ordinal logistic regression (OLR) on data of three tunnel sections of va-rying lithologies, it becomes evident that for differing lithostrati-graphic units, changing parameters of both parameter types, rock mass and construction measures, have a significant rela-tion with the interlocking strength. Consequently, the interlo-cking strength is a system parameter and not solely a rock mass parameter.