14 / 2025-04-24 11:27:17

Multidirectional cyclic shear of gap-graded soils considering suffusion-induced fines loss: A DEM perspective

Earthquakes and traffic loads subject sand to multi-directional cyclic shear, potentially causing excessive plastic deformation. Concurrently, increasing extreme weather events have highlighted the role of suffusion in geotechnical structure failures during disasters like heavy rain and floods. This study employs the discrete element method (DEM) to investigate the effects of suffusion-induced fine particle loss on the liquefaction behavior of medium-dense sand under multidirectional cyclic shear.
A novel particle removal algorithm is proposed, incorporating particle stress, contact number, and size criteria to simulate suffusion-induced fines loss more accurately than previous methods. This algorithm is coupled with an advanced undrained servo mechanism capable of applying arbitrary loading paths to three-dimensional representative volume elements (RVEs).
Simulation results reveal that increasing fine particle loss significantly weakens the specimen’s liquefaction resistance. A critical transition in failure mode from cyclic mobility to flow failure is observed as the removal mass fraction of fine particles increases from 1% to 2%, indicating a shift from a medium-dense to a loose state. This substantial reduction in liquefaction resistance occurs despite the minimal contribution of fine particles to the overall effective stress and the fact that most removed particles are suspended within the soil skeleton voids. The study examines various multidirectional loading paths in the deviatoric plane, including unidirectional line, bidirectional line, circular, and figure-8 stress trajectories. Among these, the figure-8 path is most prone to liquefaction failure, while the circular path exhibits the strongest resistance to liquefaction, regardless of the degree of erosion. With the increase of fine particle loss, the effective stress of fine particles after liquefaction becomes less. The impact of fines loss on the evolution of the average and mechanical coordination numbers is apparent; specimens with greater fines loss exhibit larger average coordination number values, while their mechanical coordination number values show minimal variation. Through the contacts analysis of different kinds of particles, it is found that fine particle loss mainly affects the contacts between fine particles. The loss primarily affects fine particles not in contact with coarse particles, which are loosely suspended within void spaces. As the number of fine particles decreases, the remaining fine particles come into closer contact with each other. As cyclic shearing progresses, all coordination numbers decrease significantly due to the increasing degree of specimen liquefaction. The specimen’s microstructure is quantified and analyzed using a contact-normal-based fabric tensor. Fabric norms exhibit similar evolutionary patterns for different fines loss values, fluctuating within a small range before liquefaction and increasing abruptly upon liquefaction, indicating the formation of a highly anisotropic internal structure. Continuous changes in loading direction relative to fabric contribute to significant liquefaction in most specimens by the end of the tests. The non-eroded Bi-C path specimen shows some differences in fabric evolution due to the alignment of loading and fabric directions during partial cyclic liquefaction, which inhibits liquefaction.
To sum up, this research contributes to understanding the impact of extreme climate events on geotechnical structure stability. The findings have important implications for the design, construction, and maintenance of geo-structures in areas prone to both seismic activity and internal erosion.