Regional-scale high-plasticity clay-bearing formation as controlling factor on landslides in Southeast Spain

Abstract

Complex landslides in clay-bearing sediments are investigated in two moderate-relief regions of Southeast Spain. Both regions, more than 100 km apart show landslides affecting the same Flysch formation, which outcrops widely in the central and western Betic Cordillera along the contact between the External (South Iberian Domain) and Internal (Alborán Domain) zones. Intense rainfall episodes can be considered as the main triggering factor for slope failures in these two areas. We have chosen two landslides (Diezma and Riogordo landslides), one from each area of study, to investigate their morphological and geotechnical features in order to establish the relative importance of the different controlling factors. From a kinematic point of view, the two features studied in detail can be referred as to rotational failures, evolving downhill to slow earthflows. The movement was concentrated on several surfaces developed on a clay-rich layer mostly constituted by smectite. This clay mineral is of critical relevance to the mechanical behaviour of soils and Flysch-like formations, being very consistent at dry conditions, but rapidly losing its strength at wet conditions. Thus, softened smectite-rich clay layers with high water contents can approach the properties of a lubricant, which, in turn, can be critical for slope stability. In addition to their high plasticity, these clays have a high swelling potential, which can induce significant vertical overpressure, thus reducing even more the strength properties of the Flysch formation. In Southeast Spain, a region with a Mediterranean rainfall regime, slope stability can be seriously influenced by the presence of these smectite-rich clay layers in a formation of regional extent, as is the case of the Flysch formation. Therefore, this lithologically-controlled factor should be taken into account when evaluating landslide hazard in the Betic Cordillera.

Application of attenuated total reflectance Fourier transform infrared spectroscopy in the mineralogical study of a landslide area, Hungary

“…Consequently, smectite plays the dominant role in slope failures as recently demonstrated by Azañón et al (2010).…”

Slope stability in slightly fissured claystones and marls

“…He found an increase in smectite content when moving from the top to the toe of the slide. Azañón et al (2010) report the presence of bands of clay rich layers in the position of sliding surfaces of two landslides in shales and flysch in the south of Granada province, Spain. Smectite is the dominant mineral in these clay-rich layers.…”

“…In the slides described by Azañón et al (2010) carbonated rocks located at a higher elevation provide a continuous supply of calcified water which explains the abundance of Ca ++ and CO 3 H -ions. Direct infiltration from rain also increases after instability because of the heterogeneous nature of the involved soils: a mixture of clay and blocks of different lithologies (marls, gypsum, limestone, sandstones).…”

Unconventional pseudostatic stability analysis of the Diezma landslide (Granada, Spain) based on a high-resolution engineering-geological model

“…Numidoide Formation outcrops throughout this area and is characterized by an unconformable contact with the deposits of the Maláguide and Dorsal domains (Azañón et al, 2006(Azañón et al, , 2010Fig. 4).…”

“…Several authors (e.g., Azañón et al, 2010) The complex structural setting of the Diezma landslide was reconstructed by direct field observations and by correlations between borehole stratigraphic logs (Fig. 4).…”