Critical state framework for liquefaction of fine grained soils

A consistent methodology based on the critical state framework to characterize the different regimes of fine-grained soil behavior under earthquake loads is put forward. Shear strength and deformation behavior of soils depend in a major way on the combination of volume and confining stress. Depending on their combination, a soil aggregate may fracture into clastic debris, fail with fault planes, or yield plastically. This characterization of the class of limiting soil behavior is used to analyze the potential for large deformation and liquefaction in fine grained soils. The central piece of the proposed characterization is the (η, LI₅) stability diagram where η = q/p′ and LI₅ = LI + 0.5 log (p′/5). This diagram captures the effects of soil plasticity through liquidity index LI, confinement through mean normal effective stress p′, and shear stress q through the stress ratio η. The three regions of behavior; fracture, fault, and fold/yield are identified. Soils become susceptible to liquefaction when they shift into the fracture zone (LI₅ ≤ 0.4), or if they plot outside of the stable yielding region.Under earthquake loading, the initial soil states will migrate into different regions in the stability diagram depending on their initial location, shear stress increment, and, pore pressure response. The final position of the soil state would dictate the type of limiting behavior expected in the field; fracture, rupture or yield. The final states which fall into the fracture region have the potential for catastrophic failures including “liquefaction”; the ones which fall onto the rupture region would experience the attainment of a peak stress ratio followed by softening along failure planes; the ones in the yield region would continue to yield in a stable manner. The latter two types of deformations while resulting in large deformation may not be of a catastrophic nature. The proposed characterization is used to examine the liquefaction susceptibility of fine grained soils from China, Taiwan, and Turkey. Use of simplified empirical criteria based on parameters such as plasticity index and fines contents may not capture the true nature of the type of undrained limiting behavior of fine grains soils in the field including liquefaction.