Mode I fracturing behavior of Malan loess and implications for the toppling failure of loess slopes

Abstract

Loess-covered areas are susceptible to a variety of geohazards, among which toppling is one of the typical slope failures causing deadly and destructive threat on human life and properties. Loess toppling usually involves slab-like or columnar blocks being detached from the loess mass by vertical or sub-vertical cracks that develop in the manner of Mode I fracturing. Thus, understanding the static fracture behavior of loess is the key to deciphering the mechanism of such failures. This study carried out Mode I fracturing tests on pre-cracked specimens of undisturbed Malan loess following the semi-circular bend test method suggested by ISRM. Through digital image correlation and numerical simulation, the fracturing process (including development of surficial strain and internal stress) was examined, and it is found Malan loess experiences a four-stage fracturing, i.e., compaction, linear deformation, yield and post-failure. The fracturing exhibits fragile or ductile behavior depending on the physical properties and loading conditions. Low water content, high dry density, and/or fast loading rate favor the absorption of elastic strain energy, which tends to lead a fragile failure and a relatively high fracture toughness. The opposite conditions induce gradual consumption of plasto-damage strain energy, promoting a progressive propagation of crack and, therefore a ductile failure and a relatively low fracture toughness. Among these three factors, water content was found to have the first priority when influencing the fracture behavior of Malan loess. These findings depict the development of vertical cracks and toppling failure in loess area, and have practical implication in designing disaster risk management plan.