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The terms "bimrocks", "bimsoils" and "soil-rock mixtures" indicate different and very common types of geological units with a block-in-matrix fabric that are also "geotechnically complex formations" and are characterized by an internal heterogeneity, and spatial variability of mechanical parameters and lithological compositions. Due to this internal complexity, the understanding of their geomechanical behavior presents a key challenge in geotechnical engineering. However, the lack of a standardized and clear terminology complicates the discrimination of different types of complex formations and their internal mechanical properties, which leads to inconsistency in the literature and research studies. This inconsistency causes misunderstandings, with possible practical implications for the characterization, analysis, design and construction of engineering works. By a combination of geological and geotechnical observations, we propose a new classification for geotechnically complex formations, with particular attention to those with a block-in-matrix internal fabric. Four properties are at the base of this new classification and have a primary role in controlling the geotechnical behavior of block-in-matrix units (bimunits): (i) the composition (i.e., lithology, degree of lithification/consolidation, nature, and rheology) of blocks and the matrix that affects the water sensitivity, (ii) the degree of internal anisotropy (DA) of the block-in-matrix fabric, (iii) the degree of stratal disruption and mixing, and (iv) the volumetric block proportion (VPB). As a result, we classified bimunits in those with "anisotropic", "isotropic", and "mixed" (i.e., different behavior depending on the DA of the matrix) textures and, each of these types, into block-in-matrix rocks and block-in-matrix soils (bimrocks and bimsoils in the following). According to the water sensitivity of the matrix, bimrocks are also differentiated into "hard" and "soft". The novelty of the classification is that it is not limited to few types of geotechnically complex formations (e.g., flysch) but it can be easily applied to all field-based investigations of the different types of complex formations, regardless of their internal degree of stratal disruption, composition, and mechanical response to water sensitivity. [Display omitted] • Bimrocks and bimsoils are geotechnically complex formations. • Understanding their internal complexity is challenging for geotechnical engineers. • We propose a new practical classification of complex formations to aid comprehension. • Properties presented are composition, anisotropy/disruption degree, and block content. [ABSTRACT FROM AUTHOR]

The geological term "mélange" is used to indicate a vast group of "chaotic rock" units with a block-in-matrix internal arrangement exhumed in orogenic belts. Geotechnically such units are often defined as "complex formations", and are commonly associated with flysch-type formations. Such units range from coherent, bedding-concordant broken-formations to fully disrupted, discordant block-in-matrix rock assemblages. Mélanges derive from the progressive disruption and mixing of flysch-type successions from their earliest phases of development to their complete lithification and exhumation. They are the product of sedimentary (submarine mass transport), tectonic (thrust-related shearing) and mud-diapiric (fluid/gas driven remobilization of mud/shales) processes. Throughout a review of geological data, we show that the different intrinsic anatomical features and relationships with the host rocks shown by these various units allow differential internal anisotropy of the lithological properties, and, depending on their origin, size and distribution, they can variably influence the overall mechanical behavior of the host flysch-type formation. The review of geological data thus provides useful observations to distinguish different types of "complex formations" and block-in-matrix rocks with different geotechnical and mechanical characteristics, and then potentially suggesting the application of different rock or soil mechanics procedures. • Mélanges can be sedimentary, tectonic or diapiric in origin, or a combination thereof • These 3 types of mélanges are characterized by different internal architectures and relationships with the host flysch formation • Such characteristics deeply impact on their overall geotechnical behavior at different scales, for instance in controlling the orientation of the mechanically weak zones [ABSTRACT FROM AUTHOR]