Experimental study on shear strength and failure zone evolution mechanism of loose spoil soil considering water sensitive factor
Authors: Jiang Jiwei , Pan Jiajun , Hu Shenggang , Zhu Jiebing , Ren Jiali 
Published: August 2023

Abstract
Loose spoil soil (LSS) is a kind of granular material with unstable structure, for which the deformation and failure should be induced by changing of water conditions. This research was conducted based on an engineering spoil dump, considered the relative density (Dr) and existed stress environment of LSS, 9 groups of large-scale stacked-ring simple shear test were carried out, to study the strength and big deformation characteristics of LSS under different dry/wet states, conclusions were obtained as follows: 1. Under the three selected Dr0 (0.35/0.40/0.45), all tested LSS samples showed strain hardening during whole shear process, shear deformation mechanism could be summarized as big friction deformation of unstable granular materials; 2. The water sensitive effect of LSS was significant, during shear process, the volume strain εv under wet and saturated states showed continuously shrinkage in whole process, while under dry state, εv showed an evolution of contractancy - stable - dilatancy, and based on specific experimental curves, the concept of critical equilibrium normal stress (CENS) was proposed; 3. According to the principles of classical soil mechanics, shear strength of LSS mainly reflected the friction component of total shear strength, which could provide a reference idea for separating various shear strength components of coarse-grained soil in various compaction conditions; 4. The shear strength of LSS could be considered to fully follow a linear relationship, and it’s greatly affected by dry/wet states, in the range of 0.35<Dr0 <0.45, the internal friction angles φ were 33.1 ° ~ 38.0 °, 22.9 ° ~ 27.7 ° and 20.9 ° ~ 27.3 ° respectively under dry, wet and saturated states, compared with dry state, φ deteriorated about 30% under wet state, but there’s only a litter decline when state changes from wet to saturated, it showed that strength deterioration of LSS had been basically released in wetting process; 5. LSS showed characteristics of relatively uniform friction deformation under dry state, so there’s no obvious shear zone, and the section in shear direction kept approximately parallelogram in whole process, but under wet and saturated states, shear zone changed to localization and mostly accompanied with secondary shear zone, also the thickness of shear zone narrowed significantly, and when shear stress level exceeded 0.70, local strain increment of shear zone increased sharply, localized strain should be the main reason in strength attenuation, inducement was the changing of dry and wet states essentially; 6. 3D particle flow method was considered to make a simple shear numerical test of LSS with rapid shear process, the judgment that shear zone of LSS tended to localize when considering lubrication effect of water was verified, and based on the relative motion among all stacked rings, it was found that under lower normal stress level, LSS exhibited semi-fluid characteristics when encountering rapid shear, which could help us to reveal the phase transition process at the moment of LSS instability from a mechanistic perspective.