Mechanism of ground improvement using biomass-derived polymers based on soil mechanics principles

• An intermediate soil was improved using biomass-derived carboxymethyl cellulose (CMC) and chitosan (CS). • Polymer-improved soils exhibited pronounced hardening behavior with plastic expansion, increasing peak strength. • Consolidation yield stress and compression index increased. • The addition of polymers altered not only the soil state but also the soil type. • An increase in the overconsolidation ratio due to an upward-right shift of the NCL was identified as the mechanism of soil improvement. Recently, biomass-derived polymers have been increasingly considered as eco-friendly additives and modifiers for various construction materials; however, the mechanical behavior of such systems is poorly understood. This study investigates the effectiveness of soil improvement and its mechanical behaviour using biopolymers, specifically chitosan (CS) and carboxymethyl cellulose (CMC). Laboratory tests were conducted on specimens prepared with varying degrees of compaction ( D c = 75 % and 90 %), CMC concentrations (0.5 % and 1.0 %), and CS: CMC mixing ratios (1:1 and 2:1). The effects of biopolymer treatment were examined based on soil mechanics principles. In the oedometer tests, specimens with higher CS: CMC ratios exhibited greater consolidation yield stress and compressibility, whereas permeability decreased with the addition of biopolymer. Specifically, at a CS: CMC ratio of 2:1, the consolidation yield stress was approximately 2.5 times higher, and the compression index increased by about 1.3 times compared to unimproved soil. Additionally, the permeability coefficient was reduced to approximately 0.2–0.5 times that of unimproved soil due to the addition of biopolymer. In the triaxial compression tests, a higher CS: CMC ratio led to more pronounced hardening with plastic expansion and improved shear strength. For example, at CMC concentration of 1 % and D c = 75 %, the maximum deviator stress for CS: CMC = 2:1 was approximately three times greater than that of the unimproved soil. These mechanical behaviours suggest an increase in overconsolidation ratio (OCR). Furthermore, the addition of biopolymers altered not only the soil skeleton structure but also fundamental physical properties such as the liquid limit and compression index, indicating a change in the “type” of soil itself. The upward shift of the normal consolidation line (NCL) caused by biopolymer treatment contributes to an increase in the OCR. These findings provide important insights into the mechanisms of biopolymer-based soil improvement from a geotechnical perspective, enabling the design of more efficient and sustainable soil reinforcement systems.