## 3 Materials and Methods  
An outcrop located in the surrounding area of Arroyo de la Encomienda (Valladolid, Spain) was selected for describing and sampling the Duerhas Formation. Although its thickness is low, it contains the main features of the facies and it can be sampled easily. Small block fragments were obtained from the slope surface for identification tests, and mineralogic and petrographic descriptions. Undisturbed block samples 30 cm \(\times\) 20 cm \(\times\) 15 cm were recovered for strength and deformability tests by digging two parallel trenches with the aid of a hydraulic excavator and leaving a bench between. The final dimensions of the blocks were shaped using manual spades and a soil saw. Once isolated, they were wrapped in transparent film, placed in wooden boxes, and protected with polyurethane foam injected between the samples and the boxes. The semiquantitative mineralogical content was determined by X-ray diffraction analysis (using the BRUKER D-8 Advance diffractometer equipped with Cu K\(\alpha\) radiation, at the X-ray Diffraction Service of the University of Salamanca) of total rock and the \(<\)0.002 mm fraction (oriented aggregates, OA, and samples solvated with ethylene-glycol, EG). Thin sections were obtained from samples on each of the levels identified in the outcrop and observed under petrographic microscopy (Leica DM2500P optical microscope under transmitted light at the Geodynamics Department of the University of Salamanca), enabling their description and classification considering [23; 24]. Because structure is one of the factors that determines the mechanical response of the soils, the most significant characteristics across the various levels were described. The term structure is used in this article to define fabric and bonding, where the former is the arrangement of the component solid particles, and bonding refers to the interparticle forces which are not of a purely frictional nature [25; 26]. Depending on the scale in which the observations are made, macrostructure is used to refer to features observable to the naked eye, and microstructure for the remainder. Regarding the microstructure, petrographic and electronic microscopy observations were carried out, the latter only for the claye levels at the bottom. The clay levels were also studied through scanning electron microscopy (SEM). The surfaces selected were obtained after fracturing the slope surface sample. They were air-dried, and after the surfaces were peeled, embedded with a thin film of a conductor. The orientation of the planar macrostructure features observed in the outcrop were measured using a geological compass. On the clay levels, the discontinuities can be classified as laminations, fissures, and joints [6]; laminations are characterized by a thin layer of a siltier material and would be equivalent to the bedding; the fissures are considered small fractures within a lithological level, not crossing its boundaries; and, finally, the joints are important fractures that are predominantly vertical. In geotechnical projects, the DC Formation is usually treated as a unique engineering geological unit (EGU), considering that all the materials have uniform physical and lithological properties [20]. Nevertheless, from the geotechnical point of view, and after describing all the levels, this characterization can be improved, because there seems to be rock and soil materials in the facies. The discretization between soils and rocks is a simple and useful criteria to be applied on the DC Formation, although there are different ways to address it. Ref. [20] established that all terrains with uniaxial compression strength higher than 1 MPa should be considered as rocks. By comparison, soil is considered in [27] as natural aggregates of particles separable by low energy mechanical meansor by agitation in water, and this criterion was adopted for identifying the rock and soil levels within the DC Formation.