One of the earth fill dams in Canada, which was performing satisfactorily for over 50 years, exhibited significant deformation in the upstream side. Investigation revealed that the loss of soil shear strength was brought by creep movement occurring over the service life of the dam. However, analysis from an earlier study was unable to simulate the strain softening of the soil coupled with time-dependent creep movement. To address this, the Time-dependent Model for Structured soils (TMS) was proposed to be used. Necessary laboratory works and calibrations were done to determine all the parameters required in TMS. Full-scale models of earth fill dams CBBD2 and CBBD4 were calibrated based on the observed deformations at the site using TMS to simulate the soil behavior. The results of the calibration illustrated the strain softening of the clay material, as well as the accumulation of significant strains which defined the slip surfaces. Two slip surfaces were observed to form, both on the upstream and downstream side of the clay core at the interface with the rockfill. Results of a separate earlier study generally agree with the existence of the shallow upstream slip surface in the structure. The development of the secondary slip surface at the downstream side of the clay core provides new knowledge on the creeping behavior of the material. Assessment of CBBD4 indicated that there is potentially significant presence of shear strains at the clay core at the present time which could lead to instability of the structure. Field measurements are needed to be conducted to validate the findings, and if needed, mitigating measures must be done to prevent excessive movement of the dam. The results of the analyses demonstrated that TMS is capable of simulating the creep behavior over long periods of time. However, the post-failure deformations were not reproduced well in the model which indicates the limitations of the constitutive model. Even so, the results were able to shed light on the clay material behavior, in terms of strain accumulation, leading up to slope instability.