Block-in-Matrix Structure and Creeping Slope: Tunneling in Hard Soil and/or Weak Rock

The city bypass tunnel of Waidhofen an der Ybbs (Austria) is situated in an intricate geological-geotechnical complex consisting of soil, hard soil, weak rock and solid rock. Marls and marly limestone (Waidhofener formation) predominate followed by tectonic breccia of the Alpine cliff zone, which is a tectonic melange zone. Materials of this kind universally have isolated interior blocks embedded in a matrix called “block-in-matrix” (see Fig. 1) requiring a particular characterization of the mechanical rock mass properties. The complex geological and morphological situation required the application of various tunneling methods including open cuts, cut-and-cover sections and the New Austrian Tunneling Method. During construction in creeping slopes increased movements were triggered, which required the installation of a sophisticated monitoring system. On the basis of monitoring data and additional ground investigations structural measures were assessed that enabled safe tunneling in the creeping slope.