Influence of Grading on the Undrained Flow Instability of Saturated Loess Soil

Loess slope failure caused by static liquefaction can result in a great number of fatalities. However, the influence of grading on the mechanical responses of saturated loess, especially its critical states and liquefaction potential, remains unclear. Loess collected from a typical silty loess zone on the northwestern part of the Chinese Loess Plateau where loess flowslides have widely occurred was studied. Detailed experimental research using undrained standard triaxial tests was carried out on artificial and natural loess soils with various gradings under saturated states. The results reveal that during undrained shearing, the critical state lines moved downward and rotated synchronously from the natural grading position as the fines content decreased, until reversed shift occurred at a fines content of around 25%. For a similar global void ratio and lower confining stresses, the potential for flow liquefaction of loess soils showed a reverse pattern with increased fines content. While they became stronger monotonously with the addition of fines under higher stresses. Additionally, the applicability of the intergranular and interfine state variables in analyzing the undrained fragility of the loess soils was discussed. The patterns obtained from these laboratory tests provide some new insights into the influence of grading on the static liquefaction of loess soils, as well as new information for risk mitigation of loess flowslides.