Objective Liquefaction risk in saturated loess on the Loess Plateau has become increasingly prominent under earthquakes, monotonic loading, and static loading, whereas the liquefaction mechanism of high-water-content undisturbed loess remains insufficiently understood.

Methods Undisturbed Q3 _Malan loess from Jishishan was tested using unsaturated consolidated drained (CD) and saturated consolidated undrained (CU) triaxial shear tests. The stress-strain response, pore-water-pressure evolution, stress paths, shear-induced volumetric response, and liquefaction susceptibility were examined under different water contents (14%, 18%, 22%, and saturated) and confining pressures (100, 150, and 200 kPa). On this basis, a new liquefaction evaluation method based on the shear-induced volumetric response of unsaturated loess was proposed.

Results Unsaturated loess exhibited strain hardening and persistent shear contraction, and volumetric contraction increased markedly with increasing water content and confining pressure. In contrast, saturated loess showed strain softening, with pore water pressure evolving through three stages: rapid initial increase, slower development, and approach to a critical stable state. Liquefaction susceptibility was positively correlated with both confining pressure and water content.

Conclusions In the critical-state plane, the unsaturated critical state line (CSL) shifted downward as water content increased, whereas its slope remained nearly unchanged. The saturated CSL extrapolated to the saturation water content (wsat = 36.6%) shifted markedly downward, indicating that higher water content drives the soil toward a denser state after large deformation. Based on critical state theory, a method was established to predict the saturated CSL from the unsaturated CSL and was verified through comparative liquefaction evaluations using the pore pressure ratio (Δu/σ₃′) and liquefaction potential index (LPI). All three evaluation methods indicate that the loess has liquefaction potential within the tested confining-pressure range, with higher liquefaction susceptibility under higher confining pressure. These findings provide a theoretical basis for loess-liquefaction risk assessment; in engineering practice, field monitoring and numerical simulation should be integrated for comprehensive evaluation.