Numerous earth fill water-retaining dams in power generating stations that were constructed in the 1950’s are still in active service today. Despite satisfactory performance for over fifty years, an aging earth dam (CBBD2) began to show signs of instability by exhibiting sudden movement in its upstream side. Although remedial measures to increase stability has been put into place, it was imperative to understand what caused the sudden movement. There was a need to develop and calibrate a numerical model based on the observed conditions in CBBD2 using parameters determined from extensive laboratory tests conducted on collected soil samples. The calibrated model was used to evaluate the long-term perform performance of other aging earth fill dams to assess if they still meet current dam safety standards.

The calibrated model that represented the expected deformation and stability conditions of CBBD2 included creep deformation analyses using clay strength values between the post peak and residual shear strengths. Results indicated that the reduction of shear strength in fissured overconsolidated clays from a post peak value to the average between post peak and residual shear strengths with creep movement, predominantly occurring at the upstream slope, led to the delayed instability in CBBD2.

Stability assessment of the remaining earth dams, considering time-dependent creep deformation and using clay strength values between the post peak and residual shear strengths, indicated that these dams were still stable with factors of safety greater than unity. Particular attention must be given to CBBD4 as the factor of safety was at unity which could be an indication of impending failure.

Based on the shear creep test results, a factor of safety higher than 1.3 would be needed to avoid long-term failure due to creep. Only two of the remaining seven earth fill dams passed this criteria, implying that there is a need to closely monitor the other dams with regards to creep deformation to avoid creep rupture in the future. In addition, none of the dams had safety factors greater than 1.5 which meant that these dams are still subject to remedial measures to meet this safety requirement.