Supporting data for "Drained Instability Behaviours of Granular Soils under Constant Shear: Controlling Factors and Mechanisms"

<p dir="ltr">Rainfall-induced landslide is considered one of the more devastating geotechnical hazards to our built civilisation, and studies on its underlying mechanisms have always been one of the key issues of geotechnical engineering. Over the previous decades, the understanding of instability in soil mechanics has developed beyond conventional undrained tests to the constant shear drained (CSD) stress path due to its more accurate and rational representation of pore water pressure increase under a sloped condition. Nevertheless, the current engineering practice of slope design still prominently relies on conventional triaxial compression (CTC), due to the complications existing within the CSD knowledge framework regarding numerous key controversies and knowledge gaps regarding some fundamental issues of instability, as well as the potential complex effects of key factors like higher fines content, fabric anisotropy, and soil type. Such research gaps have undermined the development of the CSD stress path in both academia and engineering practice.</p><p dir="ltr">In light of the current situation, this study aspires to make novel contributions by clarifying the influences and potential coupling of (1) drainage boundary, (2) load control methods, (3) higher fines contents, and (4) fabric anisotropy on the instability responses of granular soils under constant shear by a robust and extensive testing programme that consists of 149 constant shear and 80 conventional triaxial tests. Moreover, a test series on real residual soils is also provided for rigorous validations of the conclusions made from laboratory soils and critical discussions on the instability responses of the two soil types.</p><p dir="ltr">The new findings of this study first interpreted a more quantitatively rigorous instability criterion for constant shear tests and utilised it to establish the instability correspondences of CSD and CTC tests. The effects of drainage boundary and load control methods on the instability modes of loose and dense sands, together with the underlying mechanisms of such effects were discussed from the perspectives of critical state soil mechanics. Utilising the beneficial effects of particle segregation under constant shear, the complex interplays between the key roles of fines content, soil fabric and principal stress directions on the instability parameters and stress-strain-volume responses under CSD conditions were also elucidated. Specific discussions and analyses were also given regarding the strain localisations of granular soils and their relationship with the state parameter. The classical band orientation theories were found to be incapable of predicting the band orientations of residual soils formed under complex strain localisation processes. Finally, a state-dependent model that can estimate the instability parameters of both CSD and CTC tests was established based on critical state soil mechanics. The applicability of such a model was further validated with the test results on laboratory soils with other soil fabrics as well as field soils.</p>