Cyclic softening is the progressive reduction of soil stiffness and strength under repeated cyclic loading, such as earthquake shaking. It is related to but distinct from liquefaction, and is especially relevant to silts, clays, and dense sands that may not fully liquefy but still degrade under cyclic loading.
Cyclic softening is the progressive degradation of soil stiffness and strength that occurs under repeated cyclic loading, such as earthquake shaking. Unlike liquefaction, which is characterized by the near-total loss of effective stress and shear strength in saturated cohesionless soils, cyclic softening involves a more gradual reduction in strength and stiffness. It occurs in soils that do not fully liquefy β silts, clays, dense sands, and cemented soils β but that still experience significant loss of strength and stiffness under cyclic loading due to the accumulation of plastic strains, the generation of pore water pressure, and the rearrangement of soil particles. The phenomenon is sometimes called cyclic mobility (for the case where the soil is dense enough to dilate) or cyclic degradation (for the general case).
Cyclic softening is important for several reasons. First, it can trigger lateral spreading, ground settlement, and foundation failure even when full liquefaction does not occur β extending the range of soils and conditions that pose seismic risk. Second, it affects the dynamic response of soil-structure systems: as the soil softens, the effective period of the soil-structure system lengthens, which can move it toward or away from resonance with the ground motion. Third, it affects the performance of earth structures (embankments, dams, levees) that may undergo large deformations under cyclic loading without a clear liquefaction trigger. Case histories such as the 1989 Loma Prieta, 1995 Kobe, and 2010β2011 Christchurch earthquakes have documented cyclic softening in a range of soil types, including silts and clays that were historically considered non-liquefiable. The phenomenon is particularly relevant in Iran, where silty and clayey soils are common in many urban areas and where seismic hazard is significant.
Cyclic softening is evaluated through a combination of laboratory testing and field correlations. Laboratory tests β cyclic triaxial, cyclic simple shear, and resonant column tests β are used to characterize the cyclic strength and deformation behavior of soils under controlled conditions. The results are typically expressed as a cyclic resistance ratio (CRR) or a cyclic softening curve that relates the number of cycles to failure (or to a specified strain level) to the applied cyclic stress ratio (CSR). Field correlations β based on SPT, CPT, or shear wave velocity measurements β are used to estimate cyclic softening potential for design purposes, following frameworks similar to those used for liquefaction. In recent years, the recognition that cyclic softening can occur in a wider range of soils than previously thought has led to updated procedures in codes and standards, and to increased research on the constitutive modeling of softening soils. Mitigation measures are similar to those for liquefaction: ground improvement (densification, drainage, mixing), structural measures (deep foundations, base isolation), and geometric measures (slope modification, buttressing).