The Pico de Navas slump (Burgos, Spain): a large rocky landslide caused by underlying clayey sand fluidification

Abstract

The Pico de Navas landslide was a large-magnitude rotational movement, affecting 50x10 6 m 3 of hard to soft rocks. The objectives of this study were: (1) to characterize the landslide in terms of geology, geomorphological features and geotechnical parameters; and (2) to obtain an adequate geomechanical model to comprehensively explain its rupture, considering topographic, hydro-geological and geomechanical conditions.The rupture surface crossed, from top to bottom: (a) more than 200 m of limestone and clay units of the Upper Cretaceous, affected by faults; and (b) the Albian unit of Utrillas facies composed of silty sand with clay (Kaolinite) of the Lower Cretaceous.This sand played an important role in the basal failure of the slide due to the influence of fine particles (silt and clay), which comprised on average more than 70% of the sand, and the high content presence of kaolinite (>40%) in some beds. Its microstructure consists of accumulations of kaolinite crystals stuck to terrigenous grains, making clayey peds. We hypothesize that the presence of these aggregates was the internal cause of fluidification of this layer once wet. Besides the faulted structure of the massif, was an important factor for the occurred landslide. Other conditioning factors of the movement were: the large load of the upper limestone layers; high water table levels; high water pore pressure; and the loss of strength due to wet conditions. The numerical simulation of the stability conditions concurs with our hypothesis. The landslide occurred in the Recent or Middle Holocene, certainly before at least 500 BC and possibly during a wet climate period. Today, it appears to be inactive.Due to mineralogical features of involved material, facies Utrillas, in the landslide, the study helps to understand the frequent slope instabilities all along the Iberian Range where this facies is present.Keywords: deslizamiento rotacional, Cordillera Ibérica, facies Utrillas, Cretácico, simulación numérica Resumen El deslizamiento de Pico de Navas fue un movimiento rotacional de gran magnitud, que involucró 50x10 6 m 3 de rocas. Los objetivos de este trabajo han sido: (1) caracterizar el deslizamiento a partir de la Geología, los elementos geomorfológicos y los parámetros geotécnicos; y (2) obtener un modelo geomecánico que permita explicar adecuadamente la rotura, teniendo en cuenta la topografía, previa y posterior al movimiento, y las condiciones hidrogeológicas y geomecánicas.La superficie de rotura atravesó, de arriba abajo: a) 200 m de calizas y arcillas del Cretácico superior, afectadas a su vez por fallas, y b) la unidad del Albense de facies Utrillas, compuesta de arenas limosas con arcilla (caolinita) del Cretácico Inferior. Las facies Utrillas tuvieron un papel importante en la rotura basal del deslizamiento debido a la influencia de las partículas finas (limos y arcillas) que alcanzan el 70% de media y el alto contenido en caolinita, >40% en algunos niveles. Tienen una microestructura particular: al microscopio electrónico se ven p...

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