3D characterization of localized shear failure in loess subject to triaxial loading

Abstract

The reduction of shear strength of loess slopes is generally driven by localized failure in loess that lowers the trigger thresholds of major landslide geohazards in these materials. Shear failure is initially manifested as discontinuous microscale fractures that progressively develop into macroscale rupture surfaces. In this study, the development of localized shear failure in Malan loess is investigated through an approach combining laboratory triaxial shear test and X-ray micro-computed tomography (μ-CT). The spatial distribution and morphological evolution of fractures associated with the shear failure plane are observed at different loading stages in a non-destructive manner. It is found that, with ongoing strain, the number and persistence of multiple undulating fracture surfaces gradually increase after peak strength condition are reached until a single, dominant shear failure plane is formed. Most fracture surfaces are oblique to the major principal stress direction forming a shear band of finite thickness. Within this band it is possible to observe zones where varying strain magnitude is accommodated by a network of fracture surfaces. The effect of strain accumulation and local shear failure on the sample's macroporosity was determined by performing calculations on the CT images of each loading stage. Whereas the bulk sample underwent a reduction in macroporosity as a result of the triaxial compression, movement along undulating fracture surfaces results in the opening (and closing) of apertures with ongoing shear strain within a shear band of finite thickness. The overall result is an increase in macroporosity in this shear band with an estimated thickness of approximately 5 mm. Image analysis also allows characterization of changes in the 3D macropore network of the original loess fabric. Macropore network can be disconnected, dislocated and even destroyed when affected by shear failure development. These morphological variations of macropores serve as benchmarks that indicate the early initiation and expanding range of local shear failure and also allow the evaluation of the effect of local shear failure on seepage characteristics. The results of this study could provide important insights into multi-scale modelling of mechanical behaviour of loess.