Simplified Isotache Method for Back-Analyzing Long-Term Settlement in Clay

This study presents a simplified isotache-based method for back-analyzing consolidation, delayed settlements, and the compression ratio (CR) in clays. The method employs two complementary models: a multilinear total strain–logarithmic normalized preconsolidation pressure (ε–log p) model and a linear ε–log p compression path model, the slope of which represents the back-analyzed CR. The approach was validated using long-term consolidation test data from Osaka Bay clay and field-monitored settlement data from two Canadian clay sites (Olga B and Berthierville). Depending on the ratio of preconsolidation pressure to effective overburden stress, the models yield either bilinear or trilinear compression paths representing recompression, virgin compression, and secondary compression. Despite its segmented formulation, the proposed method closely reproduces measured strain–time (ε–t) relationships and captures secondary compression more accurately than existing methods. An important finding is that, regardless of the magnitude of CR, creep strain that developed during primary consolidation consistently accounts for approximately 10% of the primary compression strain. For incompletely monitored data sets, a semiempirical procedure was applied to estimate CR. The back-analyzed CR values were consistent with laboratory results at Olga B but were approximately twice the laboratory range at Berthierville, likely reflecting strain-rate effects and sample disturbance.