Unified Fluid Approach Based on Extension–Compression Apparent Viscosity for Liquefaction Triggering

Existing liquefaction triggering methods often struggle to capture the complex behavior of soils under cyclic loading, emphasizing the need for a unified fluid approach to enhance liquefaction assessment accuracy. In this paper, a series of undrained cyclic triaxial tests were performed on calcareous sand, Fujian silica sand, and Nanjing sand under various loading conditions to demonstrate that the fluid approach is effective in liquefaction evaluation. The extension–compression apparent viscosity ( η c t ) is proposed to evaluate the variation in soil fluidity with the number of cycles. The results demonstrate that different types of sand all exhibit nonNewtonian fluid behavior under different influencing factors. Additionally, the relationship between normalized η c t and excess pore pressure ratio ( r u ) is independent of D r , σ c ′ , cyclic stress ratio (CSR), and f . Another significant finding is that the relationship between normalized η c t and double amplitude shear strain is independent of D r , σ c ′ , CSR, f , and sand types, which can be used to predict the development of shear strain. In addition, the relationship between degradation of η c t and energy dissipation only depends on the value of CSR. Based on the relationship between η c t and r u , the cyclic loading process can be divided into solid stage, transition stage, and fluid stage. The proposed flow criterion successfully distinguishes between cyclic mobility and flow liquefaction failure patterns. Also, the liquefaction resistance of three different sands under flow failure criterion is discussed and compared with pore pressure-based and strain-based criteria. Finally, a unified fluid constitutive model, based on extension–compression apparent viscosity, is proposed to predict the pore pressure development in saturated sand.