Effect of unloading duration on unconfined compressive strength

This note examines the effect of unloading duration on unconfined compressi on test results. Artificial clayey specimens were prepared using the slurry consolidation technique. Extracted specimens were loaded vertically under K-0 conditions, and the load was kept constant until the end of primary con solidation, Specimens were unloaded and unconfined compression tests were c arried out at different times after unloading. It is observed that the long er the unloading duration, the lower the measured unconfined strength. This behavior is attributed to the presence of negative excess pore pressure th at dissipates with time, reducing the strength. Using the measured coeffici ent of consolidation, the degree of excess pore pressure dissipation and th erefore the average mean effective stress near the failure zone can be calc ulated at the time of failure. Mohr circles are drawn tangential to the tot al shear envelope, using the calculated mean effective stresses. Reasonable agreement between predicted and measured unconfined compressive strengths has been observed, suggesting that consolidation theory can be adopted to a ssess the effect of unloading duration on unconfined compressive strength. Finally, engineering applications using a similar concept are briefly discu ssed.