The mixing of fly ash with clay or clayey soil has a positive effect on the mechanical behavior of the soil due to its free lime content and pore refinement characteristic. The increase in soil strength may partially be attributed to the desiccation that has occured during the curing process. This study represents an attempt to classify fly ash treated clay so as to characterize time-dependent unconfined strength. The database was collected from previously performed unconfined compression tests on highly plastic clay specimens including zero, 5%, 10%, 15% and 20% fly ash at 1, 7, 28 and 90 days curing periods. Utilizing the normalized unconfined compressive strength (N-q(u)) versus water content of soil, fuzzy c-means, hard k-means and Kohonen's self organizing map methodologies are applied in order to investigate whether the time-dependent strength of the clay is in the same cluster or not. Results revealed that, relatively independent from inclusion level, the strengths belong to different clusters at diverse ages due to water content variations. The consequences are important for the determination of the minimum strength gains for different inclusion levels and curing periods. In addition, this investigation presents an approach for the classification time-dependent soil strength which can also be applied for similar material characterization studies. (c) 2006 Elsevier Ltd. All rights reserved.