The "Chaotic Complex" is a mélange largely present in the Northern Apennines (Italy). It resembles both sedimentary, olistostromes, and tectonic mélanges, which geostructural assemblage ranges from "broken formation" to "scaly fabric". The Chaotic Complex (CC) is characterized by a clayey matrix including calcareous rock blocks which size can range from coarse sand to boulders and huge fragments of a formation. In this paper its geomechanical characterization has been faced with these criteria:
Classical geotechnical approach;
In general it results that Mohr-Coulomb data from GSI cover a wider range than those from lab tests, while the Bimrock approach resulted into higher values of friction angle and lower values of cohesion.
The Chaotic Complex (hereafter called as CC) is widely present in the Northern Apennines orogenic chain, Italy. It is a redefinition (Abbate & Sagri 1970) of the classical term "Argille Scagliose" introduced by Salmojraghi (1881) in a paper facing the geological concerns encountered by the "new" Napoli- Foggia rail-line, Southern Apennines, where it is also largely present and known as "Varicoloured Shales". Since that experiences, which led to the introduction of the classical "Italian Method" to drive a tunnel through complex squeezing and swelling rock-mass, many tunnels, large mine and civil excavations were executed into the CC, every time with severe concerns (short-term response in terms of strong swelling and squeezing) regarding both the stability of the excavations and how to face that problems by a correct rock-mass characterization and parameters definition.
In almost all those experiences, the classical geotechnical approach, which implies the definition of the mechanical parameter of the shaly component of the CC on reconstructed samples, was applied.
Recently, the Bimrock (Medley 1994) and GSI (Hoek et al. 1998, Marinos & Hoek 2001) approaches were introduced in order to define the rock-mass parameters to be used in the design for tunnels and large civil and mine excavations.
In this paper, on the basis of published data and personal experience, we compare results from these three different approaches in order to outline the advantages and difficulties in defining the best approach suitable to be used for CC in tunnelling and excavations.