In this note, results of triaxial laboratory tests on very weak sedimentary limestone from the construction of the "Geusselt A2" tunnel in Maastricht in the Netherlands are presented. The main purpose of the triaxial tests was to evaluate the strength of this rock. Particularly interesting was that the strength parameters obtained in the laboratory, were much lower than what was expected after preliminary visual inspections. The two most popular models in soil and rock mechanics, the Mohr-Coulomb and Hoek-Brown failure criteria, were used to estimate the strength parameters and both did not give satisfying results. Still the Mohr-Coulomb model is the best model to use.
The highway A2 crosses the middle of the city of Maastricht in the south Netherlands. This is why a shallow tunnel is constructed here. For purposes of the building pit’s design, the strength of the local rock, which is part of the Maastricht Formation, had to be properly determined. Therefore, triaxial laboratory tests on this soft rock were conducted.
The Maastricht Formation is a geological formation in the Dutch Limburg province, Belgian Limburg and adjacent areas in Germany. The rock belonging to Maastricht Formation, locally known as "mergel", is an extremely weak, porous rock consisting of soft, sandy, shallow marine, weathered carboniferous limestone, well as chalk and calcarenite. The tests were conducted on a very young and shallow rock layer, so the material was not much compacted and cemented. It was therefore expected that the strength parameters will be low. To obtain the samples for laboratory tests, a visitation and sampling on site at the building pit Geusselt A2-Maastricht was done by the University of Luxembourg, with the help of E. van Herk and B. Vink from the contracting combination Avenue2 (Strukton and Ballast Nedam). Figure 1 presents the site from where the rock samples were obtained on 5 September 2013.