Influence of Particle Shape on the Shear Behavior of Biopolymer-Treated Soil under Monotonic and Cyclic Loading

This study aims to explore the role of particle shape on shear behavior of biopolymer-treated soils considering different states (phases) of biopolymer bonds. A series of monotonic and cyclic direct simple shear tests are carried out on two distinctly different shape granules: angular sand and glass beads treated with xanthan gum (XG) biopolymer. Two different conditions of biopolymer treatment—wet hydrogel and dry cemented states—are considered with reference to the nontreatment case, enabling the evolution of cohesive and frictional contacts to be thoroughly characterized. The results show a significant role of the particle shape on the shear behavior of biopolymer-treated soils, but this influence changes considerably with different states of biopolymer (hydrogel and cemented) and normal pressures. In the hydrogel state, biopolymer causes weak cohesive bonds between particles while decreasing friction through a lubricating effect, especially when particles are rounder. When fully cemented, the cohesion increased by a factor of 3.3 and 2.3 compared to the wet state for angular and rounded granules, respectively. Adverse cyclic shear can accelerate sliding and rolling between particles, especially for nontreated and wet hydrogel glass beads, leading to excessive cumulative deformation. Increasing particle angularity and biopolymer cementation, on the other hand, can significantly promote soil resistance to cyclic loading; for example, the cemented rounded granules can sustain cyclic load with a stress ratio exceeding 0.7. Particle-scale observations show larger interparticle contact area through rising multiple-point and planar contacts when particles are more angular, promoting their interlocking and shear resistance. This study brings novel understanding of the interplay between particle shape and biopolymer bonding considering its wet–dry state transition on monotonic and cyclic shear behavior of soils.