Global climate change has caused frequent extreme weather events, leading to the degradation of soil engineering properties. Eco-friendly biopolymer has been considered for soil reinforcement under extreme climate. This study investigates the effects of biopolymer amendment on soil mechanical properties under freeze-thaw (F-T) cycles. Direct shear tests were conducted on plain soil (PS) and biopolymer reinforced soil (BRS) under varying water contents (5 %, 15 %, and 25 %) and F-T cycles. Microstructural analysis and numerical simulation were carried out to reveal the influence of biopolymer on the evolutions of microstructure, shear band and particle interaction. The results showed that biopolymer significantly enhanced soil strength, particularly at lower water contents, with strength increases of up to 3.6 times as water content decreased from 25 % to 5 %. BRS exhibited better resistance to strength deterioration under F-T cycles, with an average strength loss of 25.5 % compared to 35 % for PS after 10 cycles. SEM and MIP analyses demonstrated that biopolymer reduced porosity and pore size by filling voids and cementing particles while mitigating F-T damage. DEM simulations revealed that F-T cycles increase the shear band area and reduce the average contact force. However, the addition of biopolymer effectively mitigates the adverse effects of F-T cycles. Biopolymer is demonstrated to be effective in enhancing soil strength and durability in seasonally frozen ground region.
